Schiff Base Adducts Research Articles

Vinylogous Amadori rearrangement: Implications in food and biological systems

The 4-hydroxy-alkenals are important lipid peroxidation products and are known to play a major role both in the development of degenerative diseases in biological systems and off-flavors, or rancidity in food systems. The 4-hydroxy-alkenals can also be formed in nonlipid systems from 2-deoxy-sugar moieties such as 2-deoxy-ribose. FTIR spectroscopic evidence was provided for such a transformation catalyzed by amino acids through monitoring the decrease in intensity of the aldehydic band centered at 1716 cm(-1) of the open form of 2-deoxy-ribose and increase in the intensity of the formed conjugated aldehydic band centered at 1672 cm(-1). Furthermore, 4-hydroxy-alkenals can react with nitrogen nucleophiles such as amino acids and proteins to form Schiff base adducts that are able to undergo vinylogous Amadori rearrangement (vARP) and subsequently cyclize to generate a pyrrole moiety. This cyclization is prevented in the case of secondary amino acids such as proline to form a stable vinylogous Amadori rearrangement product (vARP). Monitoring this reaction of proline with 4-hydroxy-2-nonenal (HNE) has indicated that within 15 min at 28 degrees C the 1685 cm(-1) band of HNE completely disappears and that at 50 degrees C, vARP is formed within 5 min, as indicated by the formation of a characteristic band at 1709 cm(-1).

Unravelling the mechanism of the interactions of oxazolidine A and E with collagens in ovine skin

Cross-linking agents play an important part in the physical properties of collagen based biomaterials. Oxazolidines are novel aldehydic tanning agents that are widely used to stabilise collagens in the leather industry. The exact mechanism through which they cross-link collagens is, however, not well understood. When they are combined with vegetable tannins, it is thought that oxazolidines form carbocationic intermediates through ring opening, which are then able to interact with the amino acid side chains of collagens and flavonoid ring systems of vegetable tannins. In this study, the interactions of oxazolidines, with collagens, have been investigated using a number of analytical techniques. High pressure liquid chromatography (HPLC) analysis of oxazolidine tanned collagen samples showed that there is an irreversible reaction with tyrosine side chains. Mass spectrometry (MS) revealed the formation of a Schiff's base adduct with lysine residues, which was reversible in nature. MS analysis of reaction of oxazolidines with a model peptide Suc-Ala-Phe-Lys-AMC in presence of NaCNBH 3, indicated the formation of a product with an increase in molecular weight of 28 kD characteristic of the addition of two methyl groups to lysine. Differential scanning calorimetry showed a synergistic effect for combination tannage, with best results being obtained when vegetable tan was added prior to the aldehydic tanning agents. Circular dichroism (CD) studies of collagen in presence of the more reactive oxazolidine A showed that there was a loss in ellipticity simply because of aggregation of collagen molecules rather than a change in the secondary structure. Based on the results obtained, a scheme has been proposed to explain the possible mechanism of action of oxazolidines with the collagen amino acid side chains.

Skraup−Doebner−Von Miller Quinoline Synthesis Revisited: Reversal of the Regiochemistry for γ-Aryl-β,γ-unsaturated α-Ketoesters

A reversal of the standard regiochemistry of the Skraup-Doebner-Von Miller quinoline synthesis was observed when anilines were condensed with gamma-aryl-beta,gamma-unsaturated alpha-ketoesters in refluxing TFA. The reaction is proposed to involve 1,2-addition of the anilines to gamma-aryl-beta,gamma-unsaturated alpha-ketoesters to form Schiff's base adducts, followed by cyclization and oxidation. The products were unambiguously shown to the 2-carboxy-4-arylquinolines by spectroscopy and X-ray crystallographic analysis.

Metabolism of Myeloperoxidase-derived 2-Chlorohexadecanal

Numerous studies have suggested relationships between myeloperoxidase (MPO), inflammation, and atherosclerosis. MPO-derived reactive chlorinating species attack membrane plasmalogens releasing alpha-chloro fatty aldehydes including 2-chlorohexadecanal (2-ClHDA), which have been found to accumulate in activated neutrophils, activated monocytes, infarcted myocardium and human atheromas. The present study employed synthetically prepared 2-Cl-[3H]-HDA as well as stable isotope-labeled 2-ClHDA to elucidate the metabolism of 2-ClHDA. The results herein demonstrate that human coronary artery endothelial cells oxidize and reduce 2-ClHDA to its respective chlorinated fatty acid (alpha-ClFA) and chlorinated fatty alcohol (alpha-ClFOH). Within the first hour of incubations of human coronary artery endothelial cells with 2-Cl-[3H]-HDA, the label was incorporated into the alpha-ClFOH and alpha-ClFA pools. After 1 h, the radiolabel was predominantly found in the alpha-ClFOH pool. Cell-derived alpha-ClFOH and alpha-ClFA were also released into the cell culture medium. Additionally, chlorinated fatty acid was incorporated into complex endothelial cell glycerolipids, including monoglycerides, triglycerides, phosphatidylcholine, and phosphatidylethanolamine. The oxidation and reduction of 2-ClHDA to alpha-ClFA and alpha-ClFOH, respectively, was further supported by mass spectrometric analyses of human coronary artery endothelial cells incubated with either 2-ClHDA or stable isotope-labeled 2-ClHDA (2-Cl-[d4]-HDA). 2-ClHDA was also oxidized to alpha-ClFA and reduced to alpha-ClFOH in both control and phorbol 12-myristate 13-acetate-stimulated neutrophils. Taken together, these results show that a family of chlorinated lipidic metabolites is produced from alpha-chloro fatty aldehydes derived from reactive chlorinating species targeting of plasmalogens. These metabolites are incorporated into complex lipids and their biological roles may provide new insights into MPO-mediated disease.

Synthesis and characterization of organotin(IV) compounds with Schiff base of o-vanillin-2-thiophenoylhydrazone

Seven Schiff base adducts of organotin(IV), RSnLCl 2, which L is o-vanillin-2-thiophenoylhydrazone, and R is n-C 4H 9 ( 1), Me ( 2), Ph ( 3), and [R 2SnL], which L is o-vanillin-2-thiophenoylhydrazone, R is n-C 4H 9 ( 4), Me ( 5), Ph ( 6), PhCH 2 ( 7) have been synthesized. Those products were characterized by elemental analysis, IR, 1H, 13C and 119Sn NMR spectra. The crystal and molecular structures of compounds 1, 4, and 6 have been determined by X-ray single crystal diffraction. In the crystal of compound 1 the tin atom is rendered six-coordinate in a distorted octahedral configuration by coordinating with the N atom of the Schiff base ligand, in compounds 4 and 6 the central tin atoms are five-coordinate in distorted trigonal-bipyramidal geometry and the comparison of the IR spectra reveal that disappearance of the bands assigned to carboxyl unambiguously conforms the ligand coordinate with the tin atom in enol form.

NMR Determination of Lysine pKa Values in the Pol λ Lyase Domain: Mechanistic Implications

The base excision repair (BER) process requires removal of an abasic deoxyribose-5-phosphate group, a catalytic activity that has been demonstrated for the N-terminal 8 kDa domain of DNA polymerase beta (Pol beta), and for the homologous domain of DNA polymerase lambda (Pol lambda). Previous studies have demonstrated that this activity results from formation of a Schiff base adduct of the abasic deoxyribose C-1' with a lysine residue (K312 in the case of Pol lambda), followed by a beta-elimination reaction. To better understand the underlying chemistry, we have determined pKa values for the lysine residues in the Pol lambda lyase domain labeled with [epsilon-13C]lysine. At neutral pH, the H(epsilon) protons on 3 of the 10 lysine residues in this domain, K287, K291, and K312, exhibit chemical shift inequivalence that results from immobilization of the lysyl side chains. For K287 and K291, this results from the K287-E261 and K291-E298 salt bridge interactions, while for K312, immobilization apparently results from steric and hydrogen-bonding interactions that constrain the position of the lysine side chain. The pKa value of K312 is depressed to 9.58, a value indicating that at physiological pH K312 will exist predominantly in the protonated form. Titration of the domain with hairpin DNA containing a 5'-tetrahydrofuran terminus to model the abasic site produced shifts of the labeled lysine resonances that were in fast exchange but appeared to be complete at a stoichiometry of approximately 1:1.3, consistent with a dissociation constant of approximately 1 microM. The epsilon-proton shifts of K273 were the most sensitive to the addition of the DNA, apparently due to changes in the relative orientation between K273 and W274 in the DNA complex. The average pKa values increased by 0.55, consistent with the formation of some DNA-lysine salt bridges and with the general pH increase expected to result from a reduction in the net positive charge of the complex. A general increase in the Hill coefficients observed in the complex is consistent with the screening of the interacting lysine residues by the DNA. The pKa of K312 residue increased to 10.58 in the complex, probably due to salt bridge formation with the 5'-phosphate group of the DNA. The pKa values obtained for the lyase domain of Pol lambda in the present study are consistent with recent crystallographic studies of Pol beta complexed with 5-phosphorylated abasic sugar analogues in nicked DNA which reveal an open site with no obvious interactions that would significantly depress the pK value for the active site lysine residue. It is suggested that due to the heterogeneity of the damaged DNA substrates with which Pol lambda as well as other related polymerases may be required to bind, the unexpectedly poor optimization of the lyase catalytic site may reflect a compromise of flexibility with catalytic efficiency.

Myeloperoxidase-derived 2-chlorohexadecanal forms Schiff bases with primary amines of ethanolamine glycerophospholipids and lysine

Numerous studies have suggested relationships between myeloperoxidase, inflammation, and atherosclerosis. MPO-derived reactive chlorinating species (RCS) attack membrane plasmalogens releasing α-chloro-fatty aldehydes (α-Cl-FALDs) including 2-chlorohexadecanal (2-ClHDA). The molecular targets of α-Cl-FALDs are not known. The current study demonstrates 2-ClHDA adducts with ethanolamine glycerophospholipids and Fmoc-lysine. Utilizing electrospray ionization mass spectrometry, chlorinated adducts were observed that are apparent Schiff base adducts. Reduction of these Schiff base adducts with sodium cyanoborohydride resulted in a novel, stable adduct produced by the elimination of HCl. NMR further confirmed this structure. 2-ClHDA adducts with ethanolamine glycerophospholipids were also substrates for phospholipase D (PLD). The hydrolysis products were derivatized to pentafluorobenzoyl esters, and further structurally confirmed by GC–MS. Multiple molecular species of 2-ClHDA- N-modified ethanolamine glycerophospholipids were observed in endothelial cells treated with 2-ClHDA. These results show novel Schiff base adducts of α-Cl-FALDs with primary amines, which may represent an important fate of α-Cl-FALDs.

Protein Adducts Generated from Products of Lipid Oxidation: Focus on HNE and ONE

Modification of proteins in conditions of oxidative stress can contribute to protein dysfunction or tissue damage and disease progression. Bifunctional, most often α,β-unsaturated carbonyl compounds such as 4-hydroxy-2-nonenal (HNE), 4-oxo-2-nonenal (ONE), and acrolein, generated from oxidation of polyunsaturated fatty acids (PUFAs), readily bind to protein nucleophiles. Modification by bifunctional aldehydes can also lead to intramolecular or intermolecular protein crosslinking. Model studies are revealing the structure of adducts that can then be more readily identified in mass spectrometric studies on proteins exposed to the various pure aldehydes or to peroxidized PUFAs. Although simple Michael and Schiff base adducts are often formed initially, only some of these adducts, such as the HNE- and ONE-derived Michael adducts on Cys and His residues, are found to survive the conditions of proteolysis and HPLC-MS analysis. Reversibly formed adducts, such as the HNE-Lys Michael adduct, can be found on proteolytic peptides only if a NaBH4-reduction step is used prior to proteolysis. Initial adducts can evolve by tautomerization, oxidation, cyclization, dehydration, and sometimes condensation with a second aldehyde molecule (the same or different), to give stable advanced lipoxidation end products (ALEs) that can be found by mass spectrometry. These include the HNE-Lys-derived 2-pentylpyrrole, the ONE-Lys-derived 4-ketoamide, the ONE-derived His-Lys pyrrole crosslink, and a Lys-derived 3-formyl-4-pentylpyrrole that results from combined action of ONE and acrolein. Michael adducts of α,β-unsaturated aldehydes such as HNE and ONE can be derivatized by 2,4-dinitrophenylhydrazine (DNPH) and can thus constitute significant DNPH-detectable protein-bound carbonyl activity that serves as a key indicator of oxidative stress in tissues. It appears that lipid oxidation is a more important contributor to such activity than metal-catalyzed oxidation of protein side-chains.

In Vitro Antisickling Effects of Novel Pyridyl Derivatives with Enhanced Potency.

Chemical modification of sickle hemoglobin (Hb S) to form stable high affinity Schiff-base adducts has been an attractive approach towards finding a potential therapeutic option for sickle cell disease (SCD). An ideal candidate drug should rapidly enter the bloodstream, permeate red blood cell membrane, bind specifically with intracellular Hb S and inhibit cell sickling with minimal adverse effect. In an effort to find drugs that satisfy these criteria, we recently designed, synthesized and studied three novel benzaldehydes (INN-296, INN-298 and INN-312) with enhanced potency. The compounds are pyridyl derivatives of benzaldehyde, and hence, combine structural features of two previously determined antisickling agents: vanillin and pyridoxal. All three compounds shifted the allosteric equilibrium of Hb S toward the oxy- or R-state by destabilizing the deoxy- or T-state. The results of in vitro studies of the antisickling effects of a representative compound (INN-312) are reported. Upon incubation of suspensions of sickle erythrocytes (SS cells) with 0.5, 1 or 2 mM of INN-312 under hypoxia (4% O2 :96% N2) at 37°C, sickling of SS cells was inhibited in a dose-dependent manner (15 ± 2, 44 ± 10 and 81 ± 8% inhibition, respectively). Cation-exchange HPLC analysis of lysates from the pre-incubated SS cells revealed a new peak in addition to the original Hb S peak, indicative of formation of Schiff-base adducts of Hb. Oxygen equilibrium curves (OECs) of SS-cell suspensions and lysates were shifted toward the left in a dose-dependent manner. X-ray crystal structures of these derivatives revealed their symmetric binding to the two N-terminal αVal1 of Hb S, and seem to indicate that their superior antisickling activity may arise from effector-induced interference with Hb S polymerization, as well as shifting the OEC to the high affinity state. In vitro studies on INN-296 and INN-298 showed similar results.Studies in vivo were performed using transgenic sickle mice (3 mice per group). The mice were treated intraperitoneally with single doses of 50, 100 or 150 mg/kg of INN-312. To study pharmacokinetic profiles of INN-312 in treated mice, blood samples (~20 μl each) were collected under anesthesia via retro-orbital venipuncture into EDTA tubes at 30 min, 1 h and every hour afterwards for 5 hours. Plasma from each sample was de-proteinized and analyzed by reversed-phase HPLC for quantification of INN-312 present in the blood. A non-compartmental pharmacokinetic model with first-order elimination rate was used to determine the plasma concentration-time data using PK Solutions 2.0 software (SUMMIT Research Services, Montrose, CO, USA). The area under the plasma concentration curve (AUC) increased in a dose-dependent manner (314 ± 22 μg/ml/min, 648 ± 33 μg/ml/min and 1044 ± 63 μg/ml/min in mice treated with 50, 100 and 150 mg/kg, respectively). The terminal half-life (T1/2= 0.75 ± 0.15 h), peak concentration time (Tmax= 0.5 h), and mean resident time (MRT= 1.2 ± 0.2 h) values were consistent for all three dosage groups. The observed maximum plasma concentration (Cmax)was also increased in a dose-dependent manner. These novel pyridyl derivatives of benzaldehyde shifted the position of Hb OEC toward the left most strongly among various compounds reported to date. Further detailed studies are necessary to validate this approach to developing better antisickling agents.

Induction of covalent binding antibodies

Covalent interactions between antibody combining site residues and substrates have been implicated in the catalytic activity of abzymes elicited by design or occurring naturally in autoimmune disease. In this study, the potential for covalent binding by antibodies (Abs) was investigated by the induction of immune responses against molecules presenting chemically reactive haptenic groups. Immunogenic conjugates containing a phosphonate diester or a pyruvate carbonyl group were used to elicit antibodies that could specifically react with the electrophilic moieties. Products formed by covalent binding were detected by a western blot technique or by differential ELISA on reduced or unreduced carbonyl haptens. Antisera to the diphenylphosphonate contained antibodies with covalent reactivity, which increased with immunization. The reactivity was specific to the anti-phosphonate response and not to control immune sera induced against the unmodified carrier protein. Reactivity was focused on the antibody light (L) chain. Antisera to the phenylpyruvate hapten appeared to bind strongly to proteins modified by the carbonyl group hapten. However, anti-carrier antisera and non-immune sera had similar reactivity, indicating that the pyruvate moiety reacts nonspecifically with immunoglobulins. This suggested that affinity maturation of antibodies for reversible binding through hemiacetal or Schiff base adducts with antigens requires a less reactive carbonyl in the antigen. On the other hand, the induction of antibodies with enhanced nucleophilic reactivity toward phosphonate esters implies that irreversible binding to the B cell receptor can drive clonal expansion and antibody selection. These results support a designer strategy for generating nucleophilic abzymes and could also account for the occurrence of chemically reactive or catalytic antibodies in natural immunity or autoimmunity.

Differences in Lysine Adduction by Acrolein and Methyl Vinyl Ketone: Implications for Cytotoxicity in Cultured Hepatocytes

Acrolein is a highly toxic environmental pollutant that readily alkylates the epsilon-amino group of lysine residues in proteins. In model systems, such chemistry involves sequential addition of two acrolein molecules to a given nitrogen, forming bis-Michael-adducted species that undergo aldol condensation and dehydration to form Nepsilon-(3-formyl-3,4-dehydropiperidino)lysine. Whether this ability to form cyclic adducts participates in the toxicity of acrolein is unknown. To address this issue, we compared the chemistry of protein adduction by acrolein to that of its close structural analogue methyl vinyl ketone, expecting that the alpha-methyl group would hinder the intramolecular cyclization of any bis-adducted species formed by methyl vinyl ketone. Both acrolein and methyl vinyl ketone displayed comparable protein carbonylating activity during in vitro studies with the model protein bovine serum albumin, confirming the alpha,beta,-unsaturated bond of both compounds is an efficient Michael acceptor for protein nucleophiles. However, differences in adduction chemistry became apparent during the use of electrospray ionization-MS to monitor reaction products in a lysine-containing peptide after modification by each compound. For example, although a Schiff base adduct was detected following reaction of the peptide with acrolein, an analogous species was not formed by methyl vinyl ketone. Furthermore, while ions corresponding to mono- and bis-Michael adducts were detected at the N-terminus and lysine residues following peptide modification by both carbonyls, only acrolein modification generated ions attributable to cyclic adducts. Despite these differences in adduction chemistry, in mouse hepatocytes, the two compounds exhibited very comparable abilities to induce rapid, concentration-dependent cell death as well as protein carbonylation. These findings suggest that the acute toxicity of short-chain alpha,beta-unsaturated carbonyl compounds involves their ability to form acyclic Michael addition adducts rather than Schiff conjugates or heterocyclic adducts.

Estimation of daily exposure to 4-hydroxy-2-alkenals in Korean foods containing n-3 and n-6 polyunsaturated fatty acids

4-Hydroxy-2-alkenals are cytotoxic aldehydes generated from the oxidation of n-3 and n-6 polyunsaturated fatty acids. The compounds have shown various biological effects via Schiff base adducts or Michael addition adducts at levels higher than physiological ones. To assess human exposure to 4-hydroxy-2-alkenals in the diet, 4-hydroxy-2-alkenals in vegetable oils, fish and shellfish were monitored using GC/MS/SIM. 2001 National Health and Nutrition Survey (2002) data were employed for the dietary intake pattern. The Korean daily exposure to 4-hydroxy-2-alkenals, excluding a possible one from fried food, was 4.3 µg day−1, constituted of 1.6 µg 4-hydroxy-2-hexenal (HHE) and 2.7 µg 4-hydroxy-2-nonenal (HNE). It was calculated that Koreans could be additionally exposed to more than 11.8 µg day−1 4-hydroxy-2-alkenal from fried foods. Thus, the combined exposure would be 16.1 µg day−1, which corresponds to 0.3 µg kg−1 body weight day−1 for a 60 kg Korean adult. In spite of the biological toxicity of 4-hydroxy-2-alkenals, the risk for human could not be quantified due to the lack of a virtually safe dose of the compounds. However, considering the basal level of 4-hydroxy-2-alkenals in many tissues, the present value from the diet may not pose a significant risk for human health.

5‐hydroxymethyl‐2‐furfural modifies intracellular sickle haemoglobin and inhibits sickling of red blood cells†,‡

In an attempt to find new types of anti-sickling agents that specifically bind to intracellular sickle haemoglobin (HbS) without inhibition by plasma and tissue proteins or other undesirable consequences, we identified 5-hydroxymethyl-2-furfural (5HMF), a naturally occurring aromatic aldehyde, as an agent that fulfils this criterion. Preliminary studies in vitro showed that 5HMF forms a high-affinity Schiff-base adduct with HbS and inhibits red cell sickling by allosterically shifting oxygen equilibrium curves towards the left. Further studies with transgenic (Tg) sickle mice showed that orally administered 5HMF was rapidly absorbed into the bloodstream from the gastrointestinal tract without being destroyed, traversed the red blood cell membrane and specifically bound with, and modified, HbS molecules at levels as high as 90%. Pretreatment of Tg sickle mice with 5HMF inhibited the formation of sickle cells and significantly prolonged survival time under severe hypoxia, compared with untreated mice, which died within 15 min because of sickling-dependent pulmonary sequestration. These results indicate the feasibility of 5HMF as an attractive potential candidate for therapy of sickle cell disease.

Potential therapeutic implication of nifedipine, a dihydropyridine-based calcium antagonist, in advanced glycation end product (AGE)-related disorders

A non-enzymatic reaction between ketones or aldehydes and the amino groups of proteins contributes to the aging of proteins and to pathological complications of diabetes. Under hyperglycemic conditions in diabetes, this process begins with the conversion of reversible Schiff base adducts, and then to more stable, covalently-bound Amadori rearrangement products. Over a course of days to weeks, these early glycation products undergo further reactions and rearrangements to become irreversible crossed-linked, fluorescent protein derivatives termed advanced glycation end products (AGEs). Recent understanding of this process has confirmed that AGE-their receptor (RAGE) interaction-elicited oxidative stress generation was implicated in the pathogenesis of diabetic vascular complication, melanoma growth, expansion and metastasis. We have recently found that nifedipine, one of the most widely used dihydropyridine-based calcium antagonists (DHPs) for treatments of patients with angina pectoris and hypertension, inhibited RAGE overexpression in AGE-exposed endothelial cells by suppressing reactive oxygen species generation. Since RAGE is a signal-transducing receptor for AGEs and subsequently evokes inflammatory responses in various types of cells, thus eliciting angiogenesis and thrombogenesis, we hypothesize here that blockade of RAGE expression by nifedipine may have therapeutic potentials in treatment of patients with various AGE-related disorders. In this paper, we would like to propose the possible ways of testing our hypothesis. Does nifedipine treatment reduce the development and progression of diabetic vascular complications? If the answer is yes, is this beneficial effect of nifedipine superior than that of other DHPs with equihypotensive properties? Does nifedipine treatment decrease the incidence of melanoma and/or prolong the survival of patients with this devastating disorder? These prospective studies will provide further valuable information whether blockade by nifedipine of the AGE-RAGE signaling could be clinically relevant.

Finding of 8 New Therapeutic Agents by the Use of New High-Throughput Drug Screening Methods Established by the Nhlbi Sicke Cell Disease Reference Laboratory.

The NHLBI Sickle Cell Disease (SCD) Reference Laboratory (Ref Lab) was established in 1997 to facilitate rapid, efficient, accurate and economical evaluation of agents or therapies that may be beneficial for the treatment of patients with sickle cell disease. On the basis of the assumption that the unsuccessful finding of antisickling agents in the past was attributed to insufficient pre-clinical drug evaluation, the SCD Ref Lab proposed to screen drugs using more than 10 in vitro tests to study if a drug inhibits or promotes sickling, hydrates or dehydrates SS cells, prolongs or shortens the delay time prior to deoxy-Hb S polymerization, induces Hb F, increases or decreases the solubility of deoxy-Hb S; Hb oxygen affinity; membrane ion transport, adhesion of SS cells to endothelial cells; met-Hb S formation; hemolysis and the amount of intracellular denatured Hb S. In addition, drugs that showed a beneficial effect without significant adverse effects in these tests were further studied using transgenic mice that produce human sickle hemoglobin. To perform many in vitro tests efficiently, the SCD Ref Lab established a new high-throughput drug evaluation protocol by which many tests are conducted simultaneously using a small volume of SS blood samples or Hb S. To date, we have found 8 new antisickling agents. They are NS3623 (Blood 97:1451–7, 2001); MX-1520 (Brit. J. Haematol. 125:788–795, 2004); NIPRISAN [Brit. J. Haematol. 118: 337–343, 2002 and 122: 1001–8 (2003)]; FLOCOR (manuscript in preparation); and 5HMF which forms a Schiff base adduct with Hb S. In vivo studies using transgenic sickle mice showed that pretreatment with each of these drugs prevented the formation of sickled cells in the blood and prolonged the survival time under severe hypoxic conditions. We also found that Amidox, Didox and Trimidox increased the synthesis of Hb F in K562 cells (Am J. Hematol. 63: 176–183, 2000).

MSDD1, a Prodrug of 5-Hydroxymethyl-2-Furfural (5HMF), Prolongs the Antisickling Effect of 5HMF in Transgenic Sickle Mice.

We recently reported that 5-hydroxymethyl-2-furfural (5HMF), a 5-membered heterocyclic aldehyde, had a potent anti-sickling effect in both in vitro tests and in vivo studies using transgenic (Tg) sickle mice. 5HMF is quickly absorbed and covalently binds with sickle hemoglobin (Hb S) to form a high-affinity reversible Schiff-base adduct. Although 5HMF is readily bioavailable after oral administration to Tg sickle mice, preliminary pharmacokinetic data suggest that its terminal half-life (T1/2 = 1.5 ± 0.6 hr) may be too short to exert a sustained anti-sickling effect in vivo for a sufficient length of time. We also determined that although 100mg/kg inhibited sickling and prevented the development of pulmonary sequestration in Tg sickle mice exposed to severe hypoxic conditions, high levels of modified Hb in the blood were sustained for only about 4hrs. Since a significant anti-sickling effect is observed by modifying 50% of Hb S molecules, a reduction in dosage combined with less frequent intake seems to be a more attractive and acceptable option.Based on this, we designed a slow-release prodrug of 5HMF by protecting the aldehyde group with L-cysteine to form the thiazolidine complex (MSDD1), expecting that the prodrug would slowly release active 5HMF into the bloodstream over a longer period of time, resulting in a longer half-life for 5HMF.To compare the pharmacokinetic profiles of these two compounds, 2 groups of Tg sickle mice (6 mice per group) were treated orally with a single 100 mg/kg dose of either 5HMF or its slow-release form, MSDD1. Blood samples (~20 μl each) were collected via retro-orbital venipuncture under anesthesia into EDTA tubes at 20 min, 40 min, 1 hr and every hour afterwards for 7 hours. Plasma from each sample was deproteinized, 10 μl of each sample was diluted 20-fold and analyzed by reversed-phase HPLC for quantification of 5HMF present in the blood. A non-compartmental pharmacokinetic model with first-order elimination rate was used to determine the plasma concentration-time data. The area under the plasma concentration curve (AUC), terminal half-life (T1/2 ), peak concentration time (Tmax), maximum plasma concentration (Cmax), and the mean resident time (MRT) were determined using PK Solutions 2.0 software (SUMMIT Research Services, Montrose, CO, USA).The AUC of the group that received MSDD1 did not significantly differ from the AUC of those that received 5HMF (430 ± 32 μg/ml/min vs. 410 ± 24 μg/ml/min). However, in the group that received MSDD1, both the T1/2 and Tmax increased 2-fold (2.9 ± 0.32 hr vs. 1.5 ± 0.6 hr, and 1hr vs. 0.5 hr, respectively). The MRT also increased from 2.3 ± 0.0.16 hr in the 5HMF-treated group to 5.8 ± 0.26 hr in the MSDD1-treated group, although the Cmax was reduced almost 3-fold in the MSDD1-treated mice at 70 ± 12 μg/ml, compared to 180.7 ± 32.6 μg/ml in 5HMF-treated miceWe conclude that MSDD1, a slow-release prodrug of 5HMF seems to provide the possibility of administering a single oral daily dose, and may be a suitable treatment option for sickle cell disease patients in the future.

Selective covalent binding of a positively charged water-soluble benzoheterocycle triosmium cluster to single- and double-stranded DNA

The water-soluble triosmium cluster [Os 3(CO) 9(μ-η 2-(4-CHO)C 9H 5N)(μ-H)(P(OCH 2CH 2N(CH 3) 3I) 3)] ( 4) was tested for its reactivity with plasmid DNA. In contrast to the band retardation previously observed with a related series of positively charged clusters, an intensification and retardation of three discrete bands was observed with increasing cluster concentration. In order to further investigate the apparent modification of DNA by 4, its interaction with a 22-oligomer (sequence 5′-AGT TGT GGT GAC TTT CCC AGG C-3′) was examined. Incubation with this oligonucleotide (pH 7.4 in Tris–HCl buffer and 100 mM NaCl) followed by HPLC analysis revealed the formation of three dose dependent products assigned as covalent modifications at three sites of the oligonucleotide. Incubation of 4 with 32P-ATP labeled oligonucleotide at the 5′-end followed by treatment with piperidine and comparison with the standard Maxam–Gilbert sequencing protocol products revealed only general background cleavage, indicating that the modification products are piperidine labile and suggesting that the modification involved formation of a Schiff base. An alternative approach was then pursued which involved annealing the 4-oligonucleotide products with their complementary strand and treatment of the resulting duplex DNAwith the exonuclease, Exo III. This assay indicated three exonuclease stops, consistent with the three products observed by HPLC whose electrophoretic mobility approximately matched guanine containing fragments when compared with the Maxam–Gilbert sequencing lanes. Reduction of the 4-oligonucleotide products with borohydride reducing agents, followed by treatment with piperidine, resulted in the formation of one product (by HPLC) with the same electrophoretic mobility as the AGTT fragment based on comparison with the Maxam–Gilbert sequencing lanes. This product most likely results from reduction of an initially formed Schiff base adduct (to the corresponding amine) with the guanine of the TGT fragment of the oligonucleotide, and corresponds to the most stable of the three Schiff base adducts detected by HPLC and by incubation with the exonuclease. The other two products are less stable and competitive reduction of the free aldehyde functionality on the cluster in equilibrium with these adducts precludes their detection after treatment with the reducing agents. The formation of the Schiff base adduct is further corroborated by the model reaction of [Os 3(CO) 10(μ-η 2-(4-CHO)C 9H 5N)(μ-H)] ( 4′) with acetylated guanine in nonaqueous solvents where disappearance of the aldehyde resonance and the appearance of several new resonances in the 6–9 ppm region of the 1H NMR of the reaction mixture is noted.

Covalent binding of isoketals to ethanolamine phospholipids

Free radicals have been strongly implicated in the pathogenesis of many human diseases. We previously identified the formation of highly reactive gamma-ketoaldehydes, isoketals, in vivo as products of free radical-induced peroxidation of arachidonic acid. Isoketals react with lysine residues on proteins at a rate that far exceeds that of 4-hydroxynonenal and demonstrate a unique proclivity to crosslink proteins. Hydroxynonenal has been shown to react with aminophospholipids, particularly phosphatidylethanolamine. We explored whether isoketals also react with phosphatidylethanolamine. Using liquid chromatography/electrospray mass spectrometry, we found that isoketals form pyrrole and Schiff base adducts with phosphatidylethanolamine. In addition, the ability of isoketals to covalently modify phosphatidylethanolamine is greater than that of 4-hydroxynonenal. These studies identify in vitro novel isoketal adducts. This provides the basis to explore the formation of isoketal-aminophospholipid adducts in vivo and the biological consequences of the formation of these adducts.

Structural basis for the potent antisickling effect of a novel class of five-membered heterocyclic aldehydic compounds.

Naturally occurring five-membered heterocyclic aldehydes, including 5-hydroxymethyl-2-furfural, increase the oxygen affinity of hemoglobin (Hb) and strongly inhibit the sickling of homozygous sickle red blood (SS) cells. X-ray studies of Hb complexed with these compounds indicate that they form Schiff base adducts in a symmetrical fashion with the N-terminal alphaVal1 nitrogens of Hb. Interestingly, two cocrystal types were isolated during crystallization experiments with deoxygenated Hb (deoxyHb): one crystal type was composed of the low-affinity or tense (T) state Hb quaternary structure; the other crystal type was composed of high-affinity or relaxed state Hb (with a R2 quaternary structure). The R2 crystal appears to be formed as a result of the aldehydes binding to fully or partially ligated Hb in the deoxyHb solution. Repeated attempts to crystallize the compounds with liganded Hb failed, except on rare occasions when very few R state crystals were obtained. Oxygen equilibrium, high performance liquid chromatography (HPLC), antisickling, and X-ray studies suggest that the examined heterocyclic aldehydes may be acting to prevent polymerization of sickle hemoglobin (HbS) by binding to and stabilizing liganded Hb in the form of R2 and/or various relaxed state Hbs, as well as binding to and destabilizing unliganded T state Hb. The proposed mechanism may provide a general model for the antisickling effects of aldehyde containing small molecules that bind to N-terminal alphaVal1 nitrogens of Hb. The examined compounds also represent a new class of potentially therapeutic agents for treating sickle cell disease (SCD).

Evidence for the reactivity of fatty aldehydes released from oxidized plasmalogens with phosphatidylethanolamine to form Schiff base adducts in rat brain homogenates

The vinyl ether bond of plasmalogens could be among the first target of free radicals attack. Consequently, because of their location in the membranes of cells, plasmalogens represent a first shield against oxidative damages by protecting other macromolecules and are often considered as antioxidant molecules. However, under oxidative conditions their disruption leads to the release of fatty aldehydes. In this paper, we showed using gas chromatography–mass spectrometry (GC–MS) analyses that fatty aldehydes released from plasmalogens after oxidation (UV irradiation and Fe 2+/ascorbate) of cerebral cortex homogenates can generate covalent modifications of endogenous macromolecules such as phosphatidylethanolamine (PE), like the very reactive and toxic malondialdehyde (MDA) and 4-hydroxynonenal (4-HNE). These newly formed Schiff base adducts could be responsible for deleterious effects on cells thus making the protective role of plasmalogens potentially questionable.