Synthetic Heme Analogue Research Articles

Utility of heme analogues to intentionally modify heme–globin interactions in myoglobin

Myoglobin reconstitution with various synthetic heme analogues was reviewed to follow the consequences of modified heme–globin interactions. Utility of dimethyl sulfoxide as the solvent for water-insoluble hemes was emphasized.Proton NMR spectroscopy revealed that loose heme–globin contacts in the heme pocket eventually caused the dynamic heme rotation around the iron–histidine bond. The full rotational rate was estimated to be about 1400s−1 at room temperature for 1,4,5,8-tetramethylhemin. The X-ray analysis of the myoglobin containing iron porphine, the smallest heme without any side chains, showed that the original globin fold was well conserved despite the serious disruption of native heme–globin contacts. Comparison between the two myoglobins with static and rotatory prosthetic groups indicated that the oxygen and carbon monoxide binding profiles were almost unaffected by the heme motion. On the other hand, altered tetrapyrrole array of porphyrin dramatically changed the dissociation constant of oxygen from 0.0005mmHg of porphycene-myoglobin to ∞ in oxypyriporphyrin-myoglobin.Heme–globin interactions in myoglobin were also monitored with circular dichroism spectroscopy. The observation on several reconstituted protein revealed an unrecognized role of the propionate groups in protoheme. Shortening of heme 6,7-propionates to carboxylates resulted in almost complete disappearance of the positive circular dichroism band in the Soret region. The theoretical analysis suggested that the disappeared circular dichroism band reflected the cancellation effects between different conformers of the carboxyl groups directly attached to heme periphery. The above techniques were proposed to be applicable to other hemoproteins to create new biocatalysts. This article is part of a Special Issue entitled Biodesign for Bioenergetics – the design and engineering of electronic transfer cofactors, proteins and protein networks, edited by Ronald L. Koder and J.L. Ross Anderson.

Electronic ménages a trois: A molecular orbital perspective of protonated ferryl intermediates and synthetic models

Presented here is a molecular orbital perspective of various S = 1 iron(IV)-hydroxo compound II intermediates as well as of synthetic heme and nonheme analogues. A key conceptual issue concerns how the iron(IV) center in these species coexists with highly reducing alkoxide, thiolate, phenolate, and hydroperoxide ligands. We suggest that a clue to this conundrum involves a three-way splitting of the spin density among the iron and two π-basic ligands, which effectively delocalizes the high positive charge away from the iron.

Comparative Pharmacology of Zinc Mesoporphyrin and Tin Mesoporphyrin: Toxic Actions of Zinc Mesoporphyrin on Hematopoiesis and Progenitor Cell Mobilization

The effects of two synthetic heme analogues, zinc mesoporphyrin (ZnMP) and tin mesoporphyrin (SnMP), on in vivo hematopoietic progenitor cell mobilization and in vitro hematopoiesis were examined in rabbit bone marrow. Rabbits received granulocyte colony-stimulating factor (rhG-CSF) for 7 days in order to mobilize increased numbers of erythroid (BFU-E) and myeloid (CFU-GM) progenitors in peripheral blood. Concurrent treatment of rhG-CSF-treated rabbits with ZnMP reduced mobilization of the numbers of BFU-E (76% inhibition, p < 0.0001) and CFU-GM (70% inhibition, p < 0.005) in peripheral blood. In contrast, SnMP administered at the same concentration had no significant suppressive effect on BFU-E and CFU-GM recruitment. Both metalloporphyrins inhibited bone marrow heme oxygenase activity equally in vivo, thus indicating that both compounds enter bone marrow cells. Direct in vitro addition of ZnMP to normal rabbit bone marrow cultures suppressed BFU-E and CFU-GM growth, whereas SnMP had no such effect. These results confirm, in an in vivo system, our earlier in vitro studies and demonstrate that, at the concentrations studied, ZnMP, in contrast to SnMP, displays toxicity for hematopoietic growth and progenitor cell production.

Sn-mesoporphyrin Suppression of Hyperbilirubinemia in EHBR/Eis Rats, an Animal Model of Dubin-Johnson Syndrome

Sn-mesoporphyrin (SnMP), a potent inhibitor of heme oxygenase (HO), controlled hyperbilirubinemia in rats of the mutant strain EHBR/Eis. This mutant strain displays pronounced conjugated hyperbilirubinuria and dark pigmentation of hepatocytes, characteristics of the Dubin-Johnson syndrome. SnMP administered at a dose of 10 µmol/kg body weight produced an immediate decrease in plasma bilirubin concentrations which could be maintained by weekly injections of this synthetic heme analogue. Marked inhibition of HO activity was demonstrated in liver, kidney and spleen but not in brain. These results demonstrate that SnMP can lower plasma bilirubin concentrations for extended periods in a new mutant rat model of Dubin-Johnson syndrome.

Effects of a series of metalloporphyrins on adrenal, testicular and thyroid function in rats.

We have extended our earlier studies [Pharmacology 1986;34:9-16] on the effects of certain synthetic heme analogues and cobalt chloride (CoCl2) on endocrine functions mediated by the hypothalamic-pituitary axis to examine specifically the ability of Sn-protoporphyrin (SnPP) and Sn-mesoporphyrin (SnMP) to perturb adrenal, testicular and thyroid function since there is interest in the use of Sn(tin)-porphyrins in the treatment of hyperbilirubinemia of the newborn. SnPP and SnMP when administered to adult male rats did not alter serum corticosterone, testosterone, thyroxine or triiodothyronine levels when compared to control animals. In addition, administration of exogenous adrenocorticotrophic hormone produced an increase in serum corticosterone levels that was comparable in placebo-treated and SnPP- and SnMP-treated animals. These studies involved doses of both compounds substantially greater than those used clinically. The results clearly indicate that SnMP, presently the compound of choice for use in newborns, and SnPP do not in the doses studied impair adrenal, testicular and thyroid function in vivo.

Tissue distribution of synthetic heme analogues: studies with tin, chromium, and zinc mesoporphyrins.

The uptake in tissue of Sn-mesoporphyrin (SnMP), Cr-mesoporphyrin (CrMP) and Zn-mesoporphyrin (ZnMP) administered at doses ranging from 1 to 10 mu mol/kg BW and the effects of these compounds on heme oxygenase activity were examined in both adult and neonatal rats. SnMP and CrMP, but not ZnMP, were rapidly cleared from blood and taken up by liver, spleen and kidney where marked inhibition of heme oxygenase activity was demonstrated. None of the metalloporphyrins were detectable in brain, and no inhibition of heme oxygenase activity was demonstrable in this tissue after administration of the compounds to both adult and neonatal rats. These results demonstrate that SnMP, CrMP and ZnMP do not cross the blood brain barrier, a fact of interest in relation to the potential use of these compounds clinically.

Chemistry and biology of heme. Effect of metal salts, organometals, and metalloporphyrins on heme synthesis and catabolism, with special reference to clinical implications and interactions with cytochrome P-450.

Although free porphyrins occur in nature in small quantities, no known function has been assigned to them. In contrast, heme and cobalamin, which are Fe and Co chelates of porphyrins or porphyrin derivatives, respectively, carry out crucial biological functions. Heme is the prosthetic group for a number of hemoproteins. These include myoglobin and hemoglobin, which carry out oxygen binding or transport; mitochondrial cytochromes aa3, b, c, and c3, which are important in transferring electrons; microsomal cytochrome P-450, which catalyzes mixed-function oxidations; catalase, which decomposes H2O2; peroxidase, which activates H2O2; and tryptophan pyrrolase, which catalyzes the oxidation of tryptophan. Recently, heme has also been shown to be the prosthetic group of prostaglandin and peroxide synthetase and indoleamine dioxygenase. The elegant studies of the biochemical pathway for the formation of heme demonstrated the arrangement in the porphyrin macrocycle of the carbon and nitrogen atoms originating from the eight glycine and the succinic acid molecule that are the precursors of porphyrins. There are eight enzymes involved in the synthesis of heme. The first and last three of these enzymes are localized in mitochondria, while the intermediate enzymes are localized in cytosol. The catalytic site of HMOX recognizes metalloporphyrins with central metal atoms other than iron; it favors some of these metalloporphyrins over heme as a potential substrate, sometimes by a large factor, permitting the synthetic heme analogue to serve as a potent competitive inhibitor of HMOX reaction. Since these synthetic metalloporphyrins do not bind molecular oxygen, they are not metabolically degraded by ring rupture and do not add to the body pool of bile pigment. One possible consequence of this competitive inhibition of heme degradation is suppression of bile pigment formation to such a degree that excessive plasma levels of bilirubin may be diminished. The studies of Drummond and Kappas (1981) and later studies in rats, mice, monkeys, and man, and also our studies have proved the latter phenomenon. The compound does not appear to affect the metabolic disposition of preformed bilirubin but inhibits biliary bilirubin excretion derived from the metabolism of endogenous or exogenous heme. Whether some of the effect of Sn-PP on naturally occurring or experimentally induced jaundice in animals reflects diversion of heme to nonheme to oxygenase-dependent pathways of heme metabolism, or whether a pathway which is normally latent becomes activated concurrent with HMOX inhibition is not known.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of food intake and body weight in rats by the synthetic heme analogue cobalt protoporphyrin

Cobalt protoporphyrin (CoPP) in single subcutaneous doses produces prolonged weight loss in adult and aged male rats. The altered body weight level in treated animals is actively defended against starvation or overfeeding over prolonged time periods (greater than 50-100 days). The actions of CoPP on appetite and body weight regulation are biphasic, comprising an initial period of hypophagia, probably mediated centrally, until a particular body weight level is attained. Resumption of normal calorie intake follows, although lowered body weight levels are sustained, suggesting an additional action of the compound on peripheral substrate metabolism. Controlled decrements in body weight can be produced by repetitive low-dose CoPP treatment (e.g., 1 mumol/kg body wt weekly); low-dose regimens do not elicit altered hormonal homeostasis or aberrations in heme/cytochrome P-450 regulation that have been observed following larger doses (25-50 mumol/kg body wt) of the compound. CoPP may be a valuable probe with which to explore the role of heme-related molecules in the regulation of appetite and body weight.

Intracerebroventricular administration of cobalt protoporphyrin elicits prolonged weight reduction in rats

Intracerebroventricular administration of the synthetic heme analogue cobalt protoporphyrin (CoPP) results in a dose-dependent decrease in body weight in rats. Among several other metalloporphyrins tested, this effect was found to be specific to cobalt chelated in native or minimally modified protoporphyrin rings. Metabolic balance studies showed that the initial effect of intracerebroventricular treatment with CoPP (0.4 mumol/kg body wt) was a transient reduction in food intake. However, following resumption of normal food intake and growth rate, absolute body weights remained reduced for prolonged periods of time (200-300 days) in both male and female rats. Heme oxygenase activity was induced and cytochrome P-450 activities were reduced in both brain and hypothalamus following intracerebroventricular administration of CoPP. The proximate mechanism of action of this synthetic heme analogue is not known at present, but it appears to act in the central nervous system, probably in the vicinity of the hypothalamus, to reduce the body weight set point of treated animals.

Regulation of food intake and body weight by cobalt porphyrins in animals.

Cobalt-substituted protoporphyrin administered subcutaneously to normal adult rats elicited prompt decreases in food intake and sustained decreases in body weight. Repetitive parenteral administration of small doses of this synthetic heme analogue resulted in dose-related diminutions of carcass fat content without changes in carcass protein content. Direct injection of the compound into the third ventricle of the brain produced changes in food intake and body weight that were quantitatively similar to those observed after parenteral treatment but required only 1-2% of the parenteral dose. The effects of intracerebroventricularly administered cobalt protoporphyrin on body weight were dose-related and were not produced by inorganic cobalt, heme, and a number of other metal-substituted protoporphyrins. Differential body weights between control and treated animals persisted for at least 300 days after intracerebroventricular injections of a single dose (0.2 or 0.4 mumol/kg of body weight) of the compound. Similar effects were observed after subcutaneous administration of the metalloporphyrin to genetically obese Zucker (fa/fa) rats and normal and genetically obese (ob/ob) mice as well as chickens and dogs. Cobalt-substituted mesoporphyrin elicited comparable effects on food intake and body weight. The results of these studies define a new biological action of cobalt protoporphyrin and demonstrate that this and certain other cobalt porphyrins can act, at least in part, in the central nervous system to regulate appetite and to produce long-sustained diminutions in body weight and carcass content of fat in animals.

Cobalt-protoporphyrin, a synthetic heme analogue, feminizes hepatic androgen metabolism in the rat

A single injection of cobalt-protoporphyrin (50 μmol/kg) produced marked changes in the metabolism of 14C-labeled testosterone and 4-androstenedione by male rat liver microsomes and this effect was maintained for at least 3 weeks. The rate of 3β- and 5α-reduction was increased to levels observed in untreated adult female animals and cobalt-protoporphyrin altered the metabolic profile of testosterone towards that observed after infusion of growth hormone whereas hypophysectomy produced a more general inhibition of androgen metabolism. The reduction of testosterone or 4-androstenedione by liver microsomes was also increased when cobalt-protoporphyrin (10–30 μM) was added in vitro but a higher concentration (100 μM) led to inhibition of androgen metabolism. The identity of the main androgen metabolites was established by TLC, HPLC and mass spectrometry and the role of 5α-reductase was demonstrated using a specific inhibitor of this enzyme. The possible sites of action of cobalt-protoporphyrin are discussed in relation to its in vivo effects on serum testosterone and LH concentrations.

Cobalt-protoporphyrin suppresses testosterone secretion by multiple interactions within the brain-pituitary-testicular axis.

Administration of large doses of cobalt-protoporphyrin (CoPP), a synthetic heme analogue, to adult male rats leads to a rapid decline in serum concentrations of testosterone and luteinizing hormone (LH). Hypothalamic luteinizing hormone-releasing hormone (LHRH) contents are not affected by CoPP. The pituitary LH response to exogenous LHRH is attenuated by CoPP both in vivo and in vitro in pituitary cultures. Additionally, CoPP directly inhibits synthesis or release of testosterone by the testes. These effects are specific to CoPP and not shared by heme, inorganic cobalt or tin-protoporphyrin. Castration of CoPP-treated rats leads to a normal rise in LH concentrations, but this can be abolished by using testosterone implants to elevate serum testosterone concentrations to the low levels (approximately 0.4 ng/ml) typical of CoPP-treated intact rats. Thus, CoPP appears to be a pharmacological model for testing the mechanisms which underlie an enhanced negative feedback efficacy of testosterone on pituitary LH release.

Properties of human kidney heme oxygenase: Inhibition by synthetic heme analogues and metalloporphyrins

Heme oxygenase activities in human kidney microsomes were found to be from 0.238 to 0.620 nmol of bilirubin/mg/hr (mean 0.375, SD 0.134), which represent approximately 30% of activities determined for human adult liver. There was interindividual variation in heme oxygenase activity of a 2-5-fold difference. Rabbits were immunized with purified human liver heme oxygenase and the resulting antibody preparation was used to examine the species specificity of the enzyme. Microsomal protein with a molecular weight of 32,000 from human kidney was identified on Western blots by its reaction with the anti-heme oxygenase liver antibody similar to the purified enzyme protein. Thus, a homology exists between human hepatic and kidney heme oxygenase. The enzyme activity was sensitive to inhibition by metalloporphyrins, such as tin-protoporphyrin IX and, to a lesser degree, by zinc and cobalt protoporphyrin IX. In a study of different synthetic heme analogues for in vitro inhibition of heme oxygenase, we found that replacement of iron by zinc in deuteroporphyrin IX 2,4 bis glycol dramatically potentiated the inhibition of heme oxygenase activity. This finding demonstrated that zinc deuteroporphyrin IX 2,4 bis glycol is a most potent inhibitor of heme oxygenase activity.

Inhibition of Human Adult and Fetal Heme Oxygenase by New Synthetic Heme Analogues

Heme oxygenase (HO) is the rate-limiting enzyme for heme degradation, and elevated levels of HO may be associated with a variety of pathologic disturbances. A limited number of HO inhibitors such as the metalloporphyrins have been proposed as possible chemotherapeutic agents for the treatment of hyperbilirubinemia. We undertook the study of various natural newly synthesized heme analogues as possible inhibitors of HO in human adult and fetal liver microsomes. We investigated two compounds with substitutions at the 2 and 4 position of the porphyrin ring, iron deuteroporphyrin 2,4 disulfonic (1a) and iron deuteroporphyrin 2,4 bis glycol (1b), and two compounds with substitutions of aromatic groups on the methene bridges of the porphyrin molecule, meso-tetra-4-carboxyphenyl-porphine (2a) and meso-tetra-4-sulfonatophenyl-porphine (2b). When these heme analogues were incubated in the reaction media in the presence of heme, two of the analogues (1a) and (1b) inhibited the conversion of heme to bilirubin. This inhibition was 97% and 65% respectively for (1a) and (1b) when both were present in 30 microM concentrations. Both of these compounds exhibited competitive type inhibition. The kI for the more potent inhibitor, (1b), was determined to be 0.30 microM. Porphyrins with aromatic substitutions at the methene bridges (2a, 2b) did not inhibit the conversion of heme to bilirubin, even at relatively high concentrations. Furthermore, the specific activity of HO was significantly greater (5X) in fetal microsomes as contrasted with adult microsomes as contrasted with adult microsomes. Even though fetal microsomes had greater HO activity, 5 microM of compound (1b) caused a similar degree of inhibition in both adult and fetal preparations.(ABSTRACT TRUNCATED AT 250 WORDS)

Tin-protoporphyrin inhibits heme oxygenase and prevents the decline in hepatic heme and cytochrome P-450 contents produced in nude mice by tumor transplantation

Heme and hemeprotein perturbations are present in nude mice bearing transplanted tumors. Hepatic microsomal heme oxygenase activity is increased 50–100% in tumor bearing nu nu mice when compared with normal controls. This elevation in activity of the rate-limiting enzyme of heme degradation is associated with a 50% depletion of microsomal heme and cytochrome P-450 concentrations in liver. The synthetic heme analogue, Sn-protoporphyrin, a potent inhibitor of heme oxygenase, lowers the activity of heme oxygenase in tumor bearing animals to below control levels. This effect is associated with a normalization of hepatic heme and cytochrome P-450 contents. These findings might have implications for protecting normal cells during tumor growth and chemotherapy.

Dual control mechanism for heme oxygenase: tin(IV)-protoporphyrin potently inhibits enzyme activity while markedly increasing content of enzyme protein in liver.

Tin(IV)-protoporphyrin (Sn-protoporphyrin) potently inhibits heme degradation to bile pigments in vitro and in vivo, a property that confers upon this synthetic compound the ability to suppress a variety of experimentally induced and naturally occurring forms of jaundice in animals and humans. Utilizing rat liver heme oxygenase purified to homogeneity together with appropriate immunoquantitation techniques, we have demonstrated that Sn-protoporphyrin possesses the additional property of potently inducing the synthesis of heme oxygenase protein in liver cells while, concurrently, completely inhibiting the activity of the newly formed enzyme. Substitution of tin for the central iron atom of heme thus leads to the formation of a synthetic heme analogue that regulates heme oxygenase by a dual mechanism, which involves competitive inhibition of the enzyme for the natural substrate heme and simultaneous enhancement of new enzyme synthesis. Cobaltic(III)-protoporphyrin (Co-protoporphyrin) also inhibits heme oxygenase activity in vitro, but unlike Sn-protoporphyrin it greatly enhances the activity of the enzyme in the whole animal. Co-protoporphyrin also acts as an in vivo inhibitor of heme oxygenase; however, its inducing effect on heme oxygenase synthesis is so pronounced as to prevail in vivo over its inhibitory effect on the enzyme. These studies show that certain synthetic heme analogues possess the ability to simultaneously inhibit as well as induce the enzyme heme oxygenase in liver. The net balance between these two actions, as reflected in the rate of heme oxidation activity in the whole animal, appears to be influenced by the nature of the central metal atom of the synthetic metalloporphyrin.

Relationship of suppression of the androgenic axis by cobalt-protoporphyrin to its effects on weight loss and hepatic heme oxygenase induction.

Cobalt-protoporphyrin administration to adult male rats results in an intense induction of hepatic heme oxygenase, a pronounced decline of cytochrome P-450 content in liver and associated metabolic abnormalities, including a dose-dependent decrease in weight gain and a marked decline in serum concentrations of testosterone without a compensatory increase in serum luteinizing hormone levels. These abnormalities persist for at least 5-6 weeks after a single subcutaneous dose of the metalloporphyrin (25 mumol/kg b.w.). Experiments with pair-fed control and metalloporphyrin-treated rats indicated that the androgenic dysfunction produced by cobalt-protoporphyrin is not causally related to the associated weight loss produced by the compound. Hepatic heme oxygenase activity was markedly induced by cobalt-protoporphyrin as expected; the enzyme activity was not altered in hypothalami of treated rats but was elevated (approximately 5-fold) in pooled pituitaries. However, despite the expected decrease in hepatic cytochrome P-450 content, no changes were noted in cytochrome P-450 content of hypothalami or pituitaries. In experiments in which the enhanced heme oxygenase activity produced in liver by cobalt-protoporphyrin was completely antagonized by tin-protoporphyrin, a competitive inhibitor of the enzyme, neither the endocrine suppression nor the weight loss produced by cobalt-protoporphyrin was altered. These phenomena were thus clearly dissociated from the effects of cobalt-protoporphyrin on heme oxygenase. Whether or not cobalt-protoporphyrin acts centrally to impair both appetite and endocrine control mechanisms could not be determined in these experiments, but remains a possible explanation of the novel actions of this synthetic heme analogue.

The liver excretes large amounts of heme into bile when heme oxygenase is inhibited competitively by Sn-protoporphyrin.

TinIV-protoporphyrin IX (Sn-protoporphyrin) potently inhibits heme degradation to bilirubin in vitro and in vivo, and it completely suppresses neonatal hyperbilirubinemia in experimental animals, including primates. It also reduces plasma bilirubin levels in certain naturally occurring or induced forms of jaundice in animals and man. We have examined in this study the fate of that fraction of heme whose degradation to bile pigment is inhibited in vivo by administration of this heme oxygenase (EC 1.14.99.3) inhibitor. In bile-duct-cannulated rats, infused exogenous heme is rapidly converted to biliary bilirubin; a small amount of the infused heme is excreted into bile as well. Sn-protoporphyrin, administered with the exogenous heme, markedly increased (3- to 4-fold) the amount of heme excreted into bile and greatly diminished biliary output of bilirubin. The increase in biliary heme output exceeded the decrease in bilirubin excretion elicited by the inhibitor metalloporphyrin. In the same experimental model, Sn-protoporphyrin substantially decreased the conversion of heme, derived from heat-damaged erythrocytes, to biliary bilirubin. This decrease in biliary bilirubin output was accounted for entirely by a prompt and marked increase in biliary excretion of unmetabolized heme. The enhanced biliary excretion of unmetabolized heme following administration of Sn-protoporphyrin is a newly defined and biologically important response associated with use of this synthetic heme analogue. The features of the action of this compound in vivo--suppression of formation of the potentially neurotoxic metabolite, bilirubin; enhancement of disposal of the untransformed substrate (heme) of the enzyme that it inhibits; and its own elimination without metabolic alteration--define some of the characteristics of a therapeutically useful chemical.

An experimental model of postnatal jaundice in the suckling rat. Suppression of induced hyperbilirubinemia by Sn-protoporphyrin.

A model of experimental postnatal hyperbilirubinemia in the rat has been developed utilizing the heme precursor delta-aminolevulinic acid (ALA) to produce jaundice during a selective time period after birth. This time period is defined as that between 7 d postnatally, when the initial postpartum alterations of serum bilirubin and heme metabolism in the neonate have subsided, and 21 d, when the hepatic conjugation mechanism for the bile pigment appears fully developed. Administration of ALA in this time period led to a rapid, consistent, and significant dose-dependent increase in serum bilirubin levels in the newborn animals. Heme administration produced a qualitatively similar but enhanced effect. Both compounds, in addition, induced a dose-dependent increase in hepatic heme oxygenase activity concomitant with the increase in serum bilirubin levels. Neither compound increased serum bilirubin levels significantly when administered at or after 21 d postnatally. Administration of the synthetic metalloporphyrin, Sn-protoporphyrin, to ALA-treated neonates resulted in a dose-dependent decrease in serum bilirubin levels and hepatic heme oxygenase activity. Mn- and Zn-protoporphyrin in comparable doses did not significantly inhibit ALA-induced hyperbilirubinemia. Sn-protoporphyrin also inhibited the hyperbilirubinemia produced by heme in the suckling animals. ALA administration to newborn rats during the specific postnatal period described provides a simple and convenient model of experimental jaundice in the developing neonate which permits an examination of the potential ability of synthetic metalloporphyrins or other compounds to suppress induced hyperbilirubinemia in the newborn animal. The ability to induce a consistent and significant degree of jaundice in the postnatal rat by the method described may also be useful for other types of studies concerned with the biological disposition and effects of endogenously formed bilirubin in the neonate. The results of this study confirm in another model system the potent ability of Sn-protoporphyrin to suppress jaundice in the neonate, and suggest that suppression of heme oxidation by synthetic heme analogues may represent a useful therapeutic approach to the problem of severe hyperbilirubinemia in human premature newborn.

Control of heme oxygenase and plasma levels of bilirubin by a synthetic heme analogue, tin-protoporphyrin.

The catalytic site of heme oxygenase recognizes metalloporphyrins with central metal atoms other than iron; it favors some of these metalloporphyrins over heme as a potential substrate sometimes by a large factor, permitting the synthetic heme analogue to serve as a potent competitive inhibitor of the heme oxygenase reaction. Since these synthetic metalloporphyrins do not bind molecular oxygen, they are not metabolically degraded by ring rupture and do not add to the body pool of bile pigment. One possible consequence of this competitive inhibition of heme degradation is suppression of bile pigment formation to such a degree that excessive plasma levels of bilirubin may be diminished. That the latter phenomenon occurs was shown for the first time by our study in 1981 (6), and by subsequent investigations in rats, mice, monkeys and man. The compound does not appear to affect the metabolic disposition of preformed bilirubin but inhibits biliary bilirubin excretion derived from the metabolism of endogenous or exogenous heme. Whether some of the effect of Sn-protoporphyrin on naturally occurring or experimentally induced jaundice in animals reflects diversion of heme to nonheme oxygenase-dependent pathways of heme metabolism or whether a pathway which is normally latent becomes activated concurrent with heme oxygenase inhibition is not known. Sn-protoporphyrin is remarkably innocuous in the newborn rat and may prove so in man.(ABSTRACT TRUNCATED AT 250 WORDS)