MnCl2 Concentrations Research Articles

In situ quantitation of the index of mixing in a single-screw extruder by magnetic resonance imaging

Magnetic resonance imaging (MRI) has been used to quantitate in situ the goodness of laminar mixing of two streams of 1% aqueous sodium carboxymethylcellulose with different MnCl2 concentrations, flowing through a single-screw extruder (0.04 m diameter, length to diameter ratio of 4.575). Two-dimensional MR images (0.0002 × 0.0002 × 0.003 m3 resolution) were acquired using gradient echo MRI protocols which had been calibrated to map MnCl2 concentrations. The resultant distributions were then used to calculate indices of mixing (normalized variances of concentration) in the part of the extruder under study. The smallest index (0.34) was measured near the outlet at the faster speed (30 rpm) and slower flow rate (47 ml min−1); and the largest index (0.73) was near the inlet for 20 rpm and 76 ml min−1.

Mutagenesis of β-Glu-195 of the Rhodospirillum rubrum F1-ATPase and Its Role in Divalent Cation-dependent Catalysis

We introduced mutations at the fully conserved residue Glu-195 in subunit beta of Rhodospirillum rubrum F1-ATPase. The activities of the expressed wild type (WT) and mutant beta subunits were assayed by following their capacity to assemble into the earlier prepared beta-depleted, membrane-bound R. rubrum enzyme (Philosoph, S., Binder, A., and Gromet-Elhanan, Z. (1977) J. Biol. Chem. 252, 8742-8747) and to restore ATP synthesis and/or hydrolysis activity. All three mutations, beta-E195K, beta-E195Q, and beta-E195G, were found to bind as the WTbeta into the beta-depleted enzyme. They restored between 30 and 60% of the WT restored photophosphorylation activity and 16, 45, and 105%, respectively of the CaATPase activity. The mutants required, however, much higher concentrations of divalent cations and could not restore any significant MgATPase or MnATPase activities. Only beta-E195G could restore some of these activities when assayed in the presence of 100 mM sulfite and high MgCl2 or MnCl2 concentrations. These results suggest that the observed difference in restoration of ATP synthesis and CaATPase, as compared with MgATPase and MnATPase, can be due to the tight regulation of the last two activities, resulting in their inhibition at cation/ATP ratios above 0.5. The R. rubrum F1beta-E195 is equivalent to the mitochondrial F1beta-E199, which points into the tunnel leading to the F1 catalytic nucleotide binding sites (Abrahams, J. P., Leslie, A. G. W., Lutter, R., and Walker, J. E. (1994) Nature 370, 621-628). Our findings indicate that this residue, although not an integral part of the F1 catalytic sites, affects divalent cation binding and release of inhibitory MgADP, suggesting its participation in the interconversion of the F1 catalytic sites between different conformational states.

Effects of guanine nucleotides and divalent cations on forskolin activation of rabbit luteal adenylyl cyclase: evidence for the existence of an inhibitory guanine nucleotide-binding regulatory component.

The effects of guanine nucleotides and divalent cations on the activation of rabbit luteal adenylyl cyclase by the diterpene forskolin were investigated. Saturating concentrations of forskolin elicited 10- to 15-fold stimulation of adenylyl cyclase activity in the absence of added guanine nucleotide. No lag was observed in the time course of forskolin-induced activation. Addition of 10 microM guanosine triphosphate (GTP) and guanyl-5'-yl imidodiphosphate [GMP-P(NH)P] inhibited forskolin activation by 10-15% and 30-40%, respectively, in the presence of 3.0 mM MgCl2. GMP-P(NH)P was more potent than GTP in inhibiting forskolin activation of adenylyl cyclase having an IC50 of 36 nM compared to 610 nM for GTP. In contrast, the Kact for stimulating adenylyl cyclase activity by both GMP-P(NH)P and GTP were similar, 1.00 and 0.86 microM, respectively. GMP-P(NH)P-induced inhibition of the forskolin-activated enzyme was not due to the hysteretic nature of GMP-P(NH)P activation of luteal adenylyl cyclase, as addition of GMP-P(NH)P to an enzyme that had been treated 5 min earlier with forskolin resulted in the immediate inhibition of enzymatic activity. Addition of GMP-P(NH)P to a concentration-effect curve for forskolin increased the Kact value for forskolin form 7.18 to 26.8 microM. There was a different MgCl2 concentration requirement for maximal stimulation of luteal cyclase by GMP-P(NH)P (8 mM MgCl2) and maximal inhibition of forskolin-stimulated activity by GMP-P(NH)P (0.5-0.6 mM MgCl2). Further, MnCl2 concentrations above 1.0 mM completely abolished the inhibitory action of GMP-P(NH)P on forskolin activation of luteal cyclase. In fact, at 2.0 mM MnCl2, adenylyl cyclase activity in the presence of GMP-P(NH)P plus forskolin was greater than that of forskolin alone. Thus taken together, these findings suggest that the rabbit corpus luteum contains an inhibitory guanine nucleotide-binding regulatory component in addition to a stimulatory regulatory component. Further, these components demonstrate differing requirements for both guanine nucleotides and divalent cations in order to interact with the catalytic moiety of adenylyl cyclase. The existence of such an inhibitory component suggests the presence of an inhibitory receptor in the corpus luteum which could negatively regulate adenylyl cyclase resulting in the inhibition of cAMP production and reduced progesterone output from the corpus luteum under normal physiological conditions.

Rat liver microsomes catalyse mannosyl transfer from GDP-D-mannose to retinyl phosphate with high efficiency in the absence of detergents.

In the absence of detergent, the transfer of mannose from GDP-mannose to rat liver microsomal vesicles was highly stimulated by exogenous retinyl phosphate in incubations containing bovine serum albumin, as measured in a filter binding assay. Under these conditions 65% of mannose 6-phosphatase activity was latent. The transfer process was linear with time up to 5min and with protein concentration up to 1.5mg/0.2ml. It was also temperature-dependent. The microsomal uptake of mannose was highly dependent on retinyl phosphate and was saturable against increasing amounts of retinyl phosphate, a concentration of 15mum giving half-maximal transfer. The uptake system was also saturated by increasing concentrations of GDP-mannose, with an apparent K(m) of 18mum. Neither exogenous dolichyl phosphate nor non-phosphorylated retinoids were active in this process in the absence of detergent. Phosphatidylethanolamine and synthetic dipalmitoylglycerophosphocholine were also without activity. Several water-soluble organic phosphates (1.5mm), such as phenyl phosphate, 4-nitrophenyl phosphate, phosphoserine and phosphocholine, did not inhibit the retinyl phosphate-stimulated mannosyl transfer to microsomes. This mannosyl-transfer activity was highest in microsomes and marginal in mitochondria, plasma and nuclear membranes. It was specific for mannose residues from GDP-mannose and did not occur with UDP-[(3)H]galactose, UDP- or GDP-[(14)C]glucose, UDP-N-acetyl[(14)C]-glucosamine and UDP-N-acetyl[(14)C]galactosamine, all at 24mum. The mannosyl transfer was inhibited 85% by 3mm-EDTA and 93% by 0.8mm-amphomycin. At 2min, 90% of the radioactivity retained on the filter could be extracted with chloroform/methanol (2:1, v/v) and mainly co-migrated with retinyl phosphate mannose by t.l.c. This mannolipid was shown to bind to immunoglobulin G fraction of anti-(vitamin A) serum and was displaced by a large excess of retinoic acid, thus confirming the presence of the beta-ionone ring in the mannolipid. The amount of retinyl phosphate mannose formed in the bovine serum albumin/retinyl phosphate incubation is about 100-fold greater than in incubations containing 0.5% Triton X-100. In contrast with the lack of activity as a mannosyl acceptor for exogenous dolichyl phosphate in the present assay system, endogenous dolichyl phosphate clearly functions as an acceptor. Moreover in the same incubations a mannolipid with chromatographic properties of retinyl phosphate mannose was also synthesized from endogenous lipid acceptor. The biosynthesis of this mannolipid (retinyl phosphate mannose) was optimal at MnCl(2) concentrations between 5 and 10mm and could not be detected below 0.6mm-MnCl(2), when synthesis of dolichyl phosphate mannose from endogenous dolichyl phosphate was about 80% of optimal synthesis. Under optimal conditions (5mm-MnCl(2)) endogenous retinyl phosphate mannose represented about 20% of dolichyl phosphate mannose at 15min of incubation at 37 degrees C.

RNA synthesis in isolated nuclei of the dinoflagellate Crypthecodinium cohnii.

Atypical eukaryotic RNA polymerase activity was demonstrated in nuclei of Crypthecodinium cohnii, a eukaryote devoid of histones. Nuclei were isolated from growing cultures of this dinoflagellate and assayed for endogenous RNA polymerase (EC 2.7.7.6) activity. There was a biphasic response to Mg2+ with optima at approximately 0.01 and 0.02 M MgCl2, but in contrast to other eukaryotic RNA polymerases, this enzyme activity was inhibited by low MnCl2 concentrations. In the presence of 0.01 M MgCL2 the optimum (NH4)2SO4 concentration was 0.025 M, a concentration at which the nuclei were lysed. Incorporation of [3H]UMP into RNA was inhibited by actinomycin D and dependent on the presence of undergraded DNA, and the reaction product was sensitive to ribonuclease and KOH digestion. Omission of one or more ribonucleoside triphosphates greatly reduced the incorporation. Only a slight enhancement of RNA polymerase activity resulted from the addition of various amounts of native and denatured calf thymus DNA. Spermine caused a marked inhibition while spermidine had little effect on RNA synthesis in the nuclei. Under the optimum conditions described in the present paper the nuclei incorporated approximately 3 pmoles of [3H]UMP/microgram DNA at 25 C for 15 min, and approximately 80% of this activity was inhibited by the eukaryotic RNA polymerase II inhibitor, alpha-amanitin (20 micrograms/ml). A unique situation therefore exists in C. cohnii nuclei, in which absence of histones (a prokaryotic trait) is combined with alpha-amanitin-sensitive RNA polymerase activity (a eukaryotic trait).

Pulse nuclear magnetic resonance measurements of water exchange across the erythrocyte membrane employing a low Mn concentration

A simple, precise, and rapid pulse nuclear magnetic resonance technique for measuring the rate of water exchange across the erythrocyte membrane is presented. The technique is based upon the nonlinear fit of Carr-Purcell-Meiboom-Gill (CPMG) transverse relaxation time data of blood doped with 1.7 mM MnCl2 to the general two-compartment exchange condition. Previous approaches using CPMG data required high MnCl2 concentrations (25-53 nM), shown in this work to induce systematic errors ranging from 35 to 45%. At 23 degrees C the average residence time of a water molecule inside the erythrocyte (tau a) is 21.0 +/- 0.6 ms (SE). The Arrhenius plot for water exchange is linear over the range of 3 degrees - 37 degrees C and th Arrhenius activation energy is 4.79 +/- 0.03 kcal (SE). This value does not differ significantly from the energy required for bulk water flow. Results are compared with previous determinations, and sources of systematic error in tau a and the activation energy are evaluated.

Plasma high-density lipoprotein cholesterol concentrations determined after removal of other lipoproteins by heparin/manganese precipitation or by ultracentrifugation.

The widely used heparin/MnCl2 precipitation procedure for determination of plasma high-density lipoprotein cholesterol has been re-examined in light of recent reports that isolated preparations of the lipoprotein are only partly precipitated under the test conditions. In the present study, the procedure as applied to plasma tolerated rather wide variations in heparin and MnCl2 concentrations without significant effects on the assayed values in several plasma pools tested. The procedure was further tested on 129 individual samples by comparison with an ultracentrifugal method in which high-density lipoprotein-cholesterol is assumed to be represented by the cholesterol content of the plasma fraction of relative (to water) density greater than 1.063. Our results indicate that high-density lipoprotein is not precipitated under the test conditions when applied to unfractionated plasma.

A study of the heparin‐manganese chloride method for determination of plasma α‐lipoprotein cholesterol concentration

To assess the limits of the heparin MnCl2 precipitation method for quantitation of alpha-lipoprotein cholesterol (C-HDL), effects of varying final McCl2 and heparin concentrations were studied, and the precipitation method was compared to preparative ultracentrifugation. In 65 parallel plasma aliquots, C-HDL (X+/-SE) determined by ultracentrifugation (54.3+/-1.8 mg/dl) correlated significantly (r=0.98, P less than 0.001) with the precipitation method (56.0+/-1.9 mg/dl). C-HDL by ultracentrifugal and precipitation methods were also similar in 16 subjects with triglycerides ranging from 150 to 312 mg/dl (41.4+/-2.6, 43.4+/-2.8, r=0.97, P less than .001). A constant amount of cholesterol in the supernatant was measured over a final McCl2 range of 0.046-0.23 M, and cholesterol values in the supernatant at final McCl2 concentrations of 0.046, 0.05, and 0.055 M did not differ from each other, P greater than 0.1. However, cholesterol levels in the supernatant at final MnCl2 concentration of 0.042 M differed from those at concentrations of 0.046, 0.05, and 0.055 M, P less than 0.05 and the amount of supernatant cholesterol increased as the final McCl2 concentration was reduced from 0.042 to 0.02 M. A constant amount of cholesterol in the supernatant was measured over a heparin concentration range of 92-734 USP units/ml. The final MnCl2 and heparin concentrations of 0.046 M and 184 USP units/ml, which are incorporated in widely used procedures, gave C-HDL values for the precipitation method which were in close agreement with the ultracentrifugal method. There is no evidence for a heparin-Mn++ precipitation of HDL and systematic underestimation of HDL by the precipitation method. However, the final MnCl2 concentration is very near the minimum required for accurate measurement of C-HDL. To preclude incomplete precipitation of low and very low density lipoproteins by insufficient manganese concentration, an increase of the manganese concentration should be considered.

Phosphoenolpyruvate Carboxykinase from Bakers' Yeast: KINETICS OF PHOSPHOENOLPYRUVATE FORMATION

Abstract The kinetic data of phosphoenolpyruvate formation catalyzed by crystalline bakers' yeast P-enolpyruvate carboxykinase were examined with regard to nucleotide and divalent cation absolute requirements. With Mn2+ as cation, the pH profile shows a peak at pH 8.3 with a shoulder in the vicinity of pH 6. There is no evidence to indicate that this is due to the presence of a second enzyme. The enzyme shows a rather high degree of specificity towards ATP (or deoxyadenosine triphosphate). When initial rate data from ATP saturation curve with Mn2+ as cation is analyzed in Lineweaver-Burk double reciprocal form a biphasic plot, indicative of activation either by Mn-ATP2- or ATP4-, is obtained. Excess of ATP over MnCl2 is inhibitory. Regarding cation requirement, Mn2+ is the most effective at pH 8.1. On the other hand, at pH 5.9 Mn2+ and Cd2+ are equally effective. At pH 8.1 the curve for rate plotted against MnCl2 concentration is sigmoidal, and maximal activity is always reached at a 1:1 ratio between ATP and MnCl2 concentrations. Excess of MnCl2 over ATP is inhibitory. Addition of low concentrations of CdCl2 ( l 10-2 mm) shifts the sigmoidal shaped curves to hyperbolic ones. When the ratio of MnCl2 and ATP concentrations is kept constant and equal to one, the plot of rate versus cofactors concentration is hyperbolic both in the presence and absence of CdCl2. At pH 6.7 and 5.9 the kinetics of MnCl2 activation is hyperbolic. Addition of CdCl2 enhances enzyme activity greatly also at those pH values. From these results it is proposed that at least two binding sites, related to cation requirement, are present on the yeast enzyme, one binding a cation-nucleotide complex, the true substrate, and the other a free divalent cation. Binding of the free cation at the latter site greatly enhances the affinity of the former for the cation-nucleotide substrate. Apparently only Mn2+ and Cd+2 can fulfill the free divalent cation requirement.