Methene bridge carbon atom elimination in oxidative heme degradation catalyzed by heme oxygenase and NADPH-cytochrome P-450 reductase

Physiological heme degradation is mediated by the heme oxygenase system consisting of heme oxygenase and NADPH-cytochrome P-450 reductase. Biliverdin IX alpha is formed by elimination of one methene bridge carbon atom as CO. Purified NADPH-cytochrome P-450 reductase alone will also degrade heme but biliverdin is a minor product (15%). The enzymatic mechanisms of heme degradation in the presence and absence of heme oxygenase were compared by analyzing the recovery of 14CO from the degradation of [14C]heme. 14CO recovery from purified NADPH-cytochrome P-450 reductase-catalyzed degradation of [14C]methemalbumin was 15% of the predicted value for one molecule of CO liberated per mole of heme degraded. 14CO2 and [14C]formic acid were formed in amounts (18 and 98%, respectively), suggesting oxidative cleavage of more than one methene bridge per heme degraded, similar to heme degradation by hydrogen peroxide. The reaction was strongly inhibited by catalase, but superoxide dismutase had no effect. [14C]Heme degradation by the reconstituted heme oxygenase system yielded 33% 14CO. Near-stoichiometric recovery of 14CO was achieved after addition of catalase to eliminate side reactions. Near-quantitative recovery of 14CO was also achieved using spleen microsomal preparations. Heme degradation by purified NADPH-cytochrome P-450 reductase appeared to be mediated by hydrogen peroxide. The major products were not bile pigments, and only small amounts of CO were formed. The presence of heme oxygenase, and possibly an intact membrane structure, were essential for efficient heme degradation to bile pigments, possibly by protecting the heme from indiscriminate attack by active oxygen species.

The oxidation of hemins by microsomal heme oxygenase

The substrate specificity of microsomal heme oxygenase from rat liver was studied by introducing systematic structural changes in the array of substituents of the protohemin IX rings. Replacement of the vinyls by methyl groups resulted in hemins which were excellent substrates of the heme oxygenase. Replacement of the 4-vinyl group by a propionic acid chain (harderohemin), decreased substrate activity to 40%. The replacement of the vinyls by formyl residues strongly decreased substrate activity but the hemins were still substrates of heme oxygenase. The oxidation rates of Spirographis hemin and of 2,4-diformyldeuterohemin IX showed a time lag which was absent when isoSpirographis hemin was used as a substrate. This lag could be attributed to the formation of a transient hemiacetal between the 2-formyl group and the alpha-mesohydroxy residue. The isomeric protohemins I, XI, and XIV (Fischer's notation) were examined as possible substrates of microsomal heme oxygenase. In these protohemins the array of substituents of rings A and B was the same as in protohemin IX, but the methyl and propionic acid residues of rings C and D were at different positions from those of protohemin IX. None of them had substrate activity, indicating that the presence of two vicinal propionic acid side-chains at C6 and C7 was necessary for substrate activity. A hemin with only one propionic acid residue at C5 was not a substrate of the enzyme, either. When the propionic acid residues of protohemin IX were replaced by butyric acid residues, substrate activity decreased to 50% (as compared to protohemin IX), while when they were replaced by acetic acid residues, the substrate activity was entirely suppressed. The addition of dimethyl sulfoxide (25 mM) to the incubation mixture enhanced the oxidation of hemins with non-polar substituents in rings A and B by about 35%, while it was without effect on hemins with polar substituents in the same rings.

Cyclophosphamide-impaired regulation of hepatic heme metabolism

In male rats hepatic cytochromes b5 and P-450 were reduced at different times after treatment with cyclophosphamide (CP) (200 mg/kg i.p. for 3 days). In contrast, microsomal heme did not change until 48 h after the last dose of CP, leading to accumulation of heme in a 'non-cytochromal' form. Parallel to the above changes the heme metabolism showed derangement: delta-aminolaevulinate synthase, the rate-limiting enzyme in heme synthesis, was depressed and heme oxygenase, the enzyme which catalyzes the oxidative degradation of heme, was increased.

Hemoglobin-degrading enzymes in experimental subcutaneous hematomas

The activities of heme oxygenase and biliverdin reductase were determined in subcutaneous (s.c.) hematomas of rats after different periods of vital time. The postmortem stabilities of the heme-degrading enzymes were also studied by keeping the rats with vital hematomas for 1-5 days at temperatures of +4 degrees C and +22 degrees C. A tenfold increase of heme oxygenase activity over the starting level was observed in 2-9-day-old vital hematomas, when the specimens were taken immediately after death. Biliverdin reductase showed only negligible changes. Postmortally, heme oxygenase activity started to decrease in hematomas immediately at +22 degrees C and from day 2 on at +4 degrees C.

Mechanism of action of heme oxygenase. A study of heme degradation to bile pigment by 18O labeling

The formation of bile pigment from heme by a reconstituted heme oxygenase system containing purified bovine spleen heme oxygenase, NADPH-cytochrome P-450 reductase, and biliverdin reductase was studied under an atmosphere containing 18,18O2. The product, bilirubin, was isolated and subjected to mass spectrometry, which revealed incorporation of 18O consistent with a two-molecule mechanism, whereby the product bile pigment contains oxygen atoms derived from two different oxygen molecules.

Heme metabolism in liver and spleen of 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU)-treated rats

The effect of 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU), an anticancer alkylating agent of the nitrosourea group, on liver and spleen enzymes involved in the control of heme metabolism was studied. A single oral dose of 50 or 100 mg/kg CCNU caused a time-dependent loss in weight of both spleen and liver. Seven days after CCNU treatment (100 mg/kg) the weights were at 45 and 65% of controls respectively. The activity of delta-aminolevulinic acid synthetase (ALA-S), the rate-limiting enzyme in heme biosynthesis, declined in spleen and liver to 11 and 24% of control values, respectively, 7 days after CCNU treatment. Heme oxygenase activity, the rate-limiting enzyme of heme breakdown, was moderately increased in liver and spleen following CCNU administration. In liver, heme oxygenase activity was 142% of control values at 24 hr, and in spleen the activity was 180% of controls at 1 week. Pretreatment of the animals with phenobarbital (PB) (40 mg/kg/day, i.p.) for 4 days caused a reversal in the decline of liver weight with no effect on the decline in spleen weight following CCNU treatment. Similarly, PB pretreatment reversed the decline in hepatic ALA-S activity after CCNU administration but had no effect on the decline in splenic ALA-S activity. This study indicates that CCNU causes significant decreases in the activity of enzymes of heme biosynthesis in spleen and liver. The CCNU hemotoxicity in the liver was reversed by PB pretreatment whereas the splenic hemotoxicity was unchanged.

Stimulation of lipid peroxidation and heme oxygenase activity with inhibition of cytochrome P-450 mono-oxygenase in the liver of rats repeatedly exposed to cadmium

Exposure of rats to 0.1 and 0.5 mg Cd/kg subcutaneously (s.c.) thrice weekly for 5 weeks resulted in an accumulation of cadmium in the liver in concentrations of 40 and 95 micrograms/g tissue, respectively, and a microsomal burden of Cd amounting to approx. 2-3% of the retained cadmium. The cytoplasm contained about 80% of the cadmium. At an exposure dose of 0.1 mg Cd/kg, stimulation of lipid peroxidation by 22% and inhibition of ALA synthetase by 16% in the liver were observed. The higher exposure of 0.5 mg Cd/kg caused an inhibition of microsomal monooxygenase with depression of cytochrome P-450 and cytochrome b5 by 20% (over 2-fold prolongation of hexobarbital sleeping time and statistically significant decrease of activity of aniline p-hydroxylase). The loss of cytochrome P-450 probably was due to an intensified lipid peroxidation and induction of heme oxygenase (30% and 60% over control, respectively). Sequestration of cadmium by cytoplasm (metallothionein) does not protect microsomes against cadmium accumulation and specific biochemical disturbances.

Features of intermediary steps around the 688-nm substance in the heme oxygenase reaction

The degradation of protoheme in the heme oxygenase reaction involves three oxidation steps: from protoheme to hydroxyheme, from hydroxyheme to a 688-nm substance, a protein-bound intermediate, and from the 688-nm substance to a biliverdin-iron complex. The 688-nm substance has a ferrous iron and it readily binds carbon monoxide to form a CO-complex, called the 638-nm substance (Yoshida, T., Noguchi, M., & Kikuchi, G. (1980) J. Biochem. 88, 557-563). The ferric 688-nm substance was prepared from the 638-nm substance by the addition of potassium ferricyanide together with aspiration to eliminate CO. The ferric 688-nm substance did not show any distinct absorption maximum in the red region of the absorption spectrum. The ferric 688-nm substance was readily reduced on the addition of the NADPH-cytochrome P-450 reductase system, but the ferric 688-nm substance could also be reduced spontaneously though at a very low rate. The ferrous 688-nm substance free from excess reducing agents was prepared by passing the 638-nm substance through a column of Sephadex G-25. The ferrous 688-nm substance was degraded to a biliverdin-iron complex much more rapidly in the presence of the NADPH-cytochrome P-450 reductase system than in its absence, indicating that a reducing equivalent is essential for the initiation of heme degradation even when starting from the ferrous 688-nm substance. Cyanide was found to bind to the ferrous 688-nm substance to form a stable compound; the cyanide compound formed could revert to neither the ferrous 688-nm substance nor the 638-nm substance.(ABSTRACT TRUNCATED AT 250 WORDS)

Control of heme and cytochrome P-450 metabolism by inorganic metals, organometals and synthetic metalloporphyrins

The heme-cytochrome P-450 complexes represent sensitive metabolic systems for examining the biological impact of metals on important cellular functions. Many metals, both in the inorganic form and bound to organic moieties, potently induce heme oxygenase, the rate limiting enzyme of heme degradation. The resulting increase in the rate of heme breakdown is reflected in a marked depression of cellular cytochrome P-450 content and impairment of the oxidative metabolism of natural and foreign chemicals dependent on this hemeprotein. Organometal complexes do not mimic in all their aspects the actions of the inorganic elements which they contain. For example, organotins, in contrast to inorganic tin, produce a prolonged induction response of heme oxygenase in the liver but not in the kidney. Co-protoporphyrin is a much more potent inducer of heme oxygenase in liver than is inorganic cobalt; and Sn-protoporphyrin inhibits heme oxygenase activity nearly completely, whereas inorganic tin is a powerful inducer of the renal enzyme. Contrasting effects on heme metabolism exist as well within the metalloporphyrin species as demonstrated by the effects in vivo of Co-protoporphyrin and Sn-protoporphyrin on heme oxygenase activity; the former induces the enzyme whereas the latter potently inhibits it. In vitro, however, both compounds competitively inhibit heme oxidation activity. These differences, among others which characterize metal actions in vivo and in vitro attest to the importance of pharmacokinetic, adaptive and other host factors in defining the responses of the heme-cytochrome P-450 systems to the impact of metals in the whole animal.

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.

Hepatic heme metabolism: Possible role of biliverdin in the regulation of heme oxygenase activity

Treatment of rats with biliverdin (48 h) resulted in an increase in microsomal heme oxygenase activity in the liver. This was accompanied by decreases in the microsomal heme and cytochrome P-450 contents. In these respects cellular responses elicited by biliverdin resembled those produced by hematin (48 h). When rats were treated with biliverdin for a short interval (3 h) an inhibition of the activities of heme oxygenase and biliverdin reductase, concomitant with an increase in microsomal heme and cytochrome P-450 contents, were observed. Hematin was ineffective in altering these parameters under similar conditions. Biliverdin, in a concentration-dependent manner, inhibited the activities of purified heme oxygenase and biliverdin reductase. The activity of purified rat liver heme oxygenase was refractory to bilirubin, whereas that of purified biliverdin reductase was severely inhibited. It is suggested that biliverdin regulates cellular heme degradation processes by occupying the heme binding site on heme oxygenase, thus hindering the access of the substrate to the catalytic site of the enzyme.

Pharmacologic toxicity of cimetidine on hepatic and renal drug metabolism

We studied the effect of cimetidine on liver and kidney heme metabolism and on the activity of the cytochrome P-450 drug metabolizing enzyme system. Results show that the induction of a heme biosynthetic enzyme and the activities of two drug metabolizing enzymes are impaired when cimetidine is given in combination with phenobarbital (PB). When rats were given four 33 mg doses of cimetidine IP per day for 2 days and sacrificed, we found no significant effect on kidney or liver delta-aminolevulinic acid (ALA) synthase activity. Heme oxygenase and cytochrome P-450 levels were also unchanged in these tissues. In contrast, when we measured activities of certain liver drug metabolizing enzymes, it was found that cimetidine significantly inhibited aniline hydroxylase and aminopyrine-N-demethylase by 43% and 65%, respectively. The observed changes in the activities of these drug metabolizing enzymes led us to study cimetidine in combination with other drugs. When the porphyric inducing agent allylisoprophyacetamide (AIA) was administered alone, we found a 326% increase in hepatic ALA synthase activity at 16 hours. Cimetidine given together with AIA increases hepatic ALA synthase 291 and 300% at 16 and 20 hour intervals respectively. Cytochrome P-450 levels in AIA treated rats with or without cimetidine were decreased to 52-65% of control values without a significant change in heme oxygenase levels. When PB was given alone, we found an increase in hepatic ALA synthase activity by 223 and 400% at 16 and 20 hour intervals, respectively. Cimetidine in combination with PB at the same time intervals showed a slightly diminished increase of hepatic ALA synthase activity by 146 and 238%, respectively. When PB was given alone, hepatic cytochrome P-450 was increased 96% at 16 hours, whereas when combined with cimetidine a similar increase of hepatic cytochrome P-450 was observed. In conclusion cimetidine does not significantly alter the action of the porphyric agents PB and ALA on cytochrome P-450; however, combined administration of PB plus cimetidine does impair the induction of ALA synthase. Additionally, cimetidine markedly decreased the drug metabolizing enzymes aniline hydroxylase and aminopyrine-N-demethylase in vivo, and subsequently may interfere with the endogenous metabolism of other drugs.

Induction of hepatic heme oxygenase and its effect on drug metabolizing enzyme by DL-, D- and L-ethionine administration to rats

The chronic administration of ethionine, an analogue of methionine and alkylating hepatocarcinogen, to rats resulted in the decrease of hepatic drug metabolizing enzyme activities and cytochrome P-450 content. In contrast, the activity of hepatic heme oxygenase, a first enzyme of heme degradative pathway, was increased following ethionine administration. delta-Aminolevulinic acid (ALA) synthetase, a key enzyme of heme biosynthesis pathway, was also increased except during 9 to 15 weeks of the experimental period. Changes of cytochrome P-450 content and heme oxygenase activity were proportional to the carcinogen provided period. Further, ethionine induced neoplastic nodules showed more profound changes in heme oxygenase, ALA synthetase and drug metabolizing enzyme activities than the surrounding liver. Similar changes in the enzyme activities were also obtained by a single administration of ethionine to rats and these effects were dependent upon dose levels of the carcinogen except ALA synthetase activity. These results revealed that ethionine is a mild inducer of hepatic heme oxygenase. It is thus likely that induction of heme oxygenase produced by ethionine could lead to the decrease of cytochrome P-450 and drug metabolizing enzyme activities in the liver. The decrease of heme availability due to induction of heme oxygenase by ethionine may be one of the explanations for the decrease of cytochrome P-450 content under the experimental conditions.

Trinitrotoluene-induced effects on rat heme metabolism

A single injection (100 mg/kg body wt) of trinitrotoluene in rats caused decreased delta-aminolevulinic acid synthase activity in reticulocytes and decreased erythrocyte coproporphyrin concentration 48 hr after the intraperitoneal dosage. For comparison, liver delta-aminolevulinic acid synthase was unaffected while heme synthase activity was below the control range at the same time. Heme oxygenase activity increased simultaneously. These effects are likely to lead to a negative heme balance and may be developed for a biological exposure test to trinitrotoluene in occupational health service.

Biosynthesis of the chromophore of phycobiliproteins. A study of mesohaem and mesobiliverdin as possible intermediates and further evidence for an algal haem oxygenase

The possible roles of mesohaem and mesobiliverdin as metabolic precursors of phycocyanobilin, the chromophore of phycocyanin, were studied in the unicellular rhodophyte Cyanidium caldarium. Dark-grown cells of this organism, which had been exposed to mesohaem, were either incubated in the dark with 5-aminolaevulinate, which results in excretion of bilins into the suspending medium, or incubated in the light, which results in synthesis of phycocyanin within the cells. By using 14C-labelling, either in the mesohaem or in the 5-aminolaevulinate administered, it was shown that mesohaem is not a precursor of phycocyanobilin in either dark or light systems. However, mesohaem was converted into mesobiliverdin in both systems, a phenomenon that is further evidence for the existence of an algal haem oxygenase. The data also showed that mesobiliverdin is not a precursor of phycocyanobilin. These results suggest that algal bilins are formed via haem degradation to biliverdin in the same way as mammalian bile pigments.

Altered regulation of hepatic heme metabolism in cadmium exposed chick embryo

An investigation on the process of heme metabolism with special emphasis on ALA synthetase, heme synthetase and heme oxygenase was studied in cadmium exposed chick embryo to enlighten the mechanism of cadmium embryotoxicity. Cadmium chloride injection (2.5-10 mumole/kg) to chick embryo increases the activity of ALA synthetase by 5-7 folds, however, it inhibits the activity of heme synthetase significantly. The activity of heme oxygenase is further shown to be enhanced by cadmium chloride treatment. These changes are accompanied by a marked reduction in hepatic heme content. The induction of ALA synthetase and heme oxygenase was dependent on the initial concentration of exogenous cadmium. Pretreatment with actinomycin D completely blocks the cadmium mediated induction of both ALA synthetase and heme oxygenase. Time course studies on the stimulation of these two enzymes show that cadmium enhances the activity of heme oxygenase to its maximum level after 24 h. of injection, whereas ALA synthetase activity reaches its highest value only by 48 h. and both the enzymes remain elevated at least upto 96 h. This observation can be correlated with the hepatic heme level at different time intervals after cadmium exposure. These observations suggest the presence of regulatory process for heme metabolism which is susceptible to alteration of 'regulatory heme pool' caused by cadmium.

Protective effect of interferon inducers against hyperoxic pulmonary damage

Interferon inducers, poly I:poly C, endotoxin, hepatic RNA, and Tilorone, were administered to rats at different time points in relation to the onset of hyperoxic exposure (O2 greater than 97%). All interferon inducers tested significantly reduced the mortality of rats when compared with the control groups. In hyperoxia alone, malondialdehyde, a product of lipid peroxidation, was significantly increased and the microsomal enzyme NADPH cytochrome c reductase decreased as measured in the whole lung. With the administration of either endotoxin or poly I:poly C these two parameters remained within the range of control values. These data suggest that the administration of interferon inducers protects against hyperoxic microsomal damage. After the administration of these interferon inducers with or without hyperoxia the increased activity of heme oxygenase and marked reduction of the heme content of microsomes were demonstrated. Since cytochrome P-450 and b5 are the major hemoproteins of microsomes and the known source of oxygen-free radical generation, the results obtained in this study appear to indicate that the depression of the hemoprotein of microsomes by the administration of interferon inducers may be largely responsible for the protective effects of these agents against hyperoxia.

Diabetes-induced metabolic alterations in heme synthesis and degradation and various heme-containing enzymes in female rats

Diabetes-induced alterations in heme and hemoproteins, as well as its relationship to drug-mediated induction of ALA Synthase (ALA-S), were examined in female Sprague-Dawley rats. Animals were rendered diabetic by a single i.v. injection of streptozotocin (STZ, 65 mg/kg) and measurements were made at various times after treatment. The basal levels of the key enzymes involved in heme synthesis, ALA-S and ALA-dehydratase (ALA-D), were decreased about 36% and 54%, respectively, 44-46 days after diabetes induction. Furthermore, the catabolism of heme that occurs via microsomal heme oxygenase progressively decreases in activity during the course of diabetes, and reaches 69% of control in 90-day diabetic animals. The basal levels of heme, cytochromes P-450 and b5 were elevated about twofold in diabetic rats as compared with their corresponding control values. The activity of benzo(a)pyrene hydroxylase in diabetic rats was also increased in proportion to the microsomal content of cytochrome P-450. In contrast, delta 4-hydrogenase, the rate-limiting enzyme in corticosterone metabolism, exhibited a 35-65% decrease in activity throughout the experimental period. Tryptophan pyrrolase activity (total, holo-, and apoenzyme) was elevated about 2.5-fold in STZ diabetic rats. In vivo insulin therapy of diabetic animals antagonized the effect of the diabetic state on the above measured parameters. Treatment with aminoglutethimide resulted in about a twofold elevation in ALA-S activity in control as well as chronically diabetic rats. However, a similar stimulatory response in ALA-S activity to CoCl2 administration was observed only in control or insulin-treated diabetic rats.(ABSTRACT TRUNCATED AT 250 WORDS)

New developments in the regulation of heme metabolism and their implications

Various endogenous and exogenous chemicals, such as hormones, drugs, and carcinogens and other environmental pollutants are enzymatically converted to polar metabolites as a result of their oxidative metabolism by the mixed-function oxidase system. This enzyme complex constitutes the major detoxifying system of man and utilizes the hemoprotein--cytochrome P-450--as the terminal oxidase. Recent studies with trace metals have revealed the potent ability of these elements to alter the synthesis and to enhance the degradation of heme moiety of cytochrome P-450. An important consequence of these metal actions is to greatly impair the ability of cells to oxidatively metabolize chemicals because of the heme dependence of this metabolic process. In this report the effects of exposure to trace metals on drug oxidations is reviewed within the framework of metal alterations of heme metabolism, including both its synthesis and degradation, since these newly discovered properties of metals have made it possible to define a major dimension of metal toxicity in terms of a unified cellular mechanism of action.

Alterations of hepatic .DELTA.-aminolevulinic acid synthetase, heme oxygenase, microsomal cytochrome content and drug metabolism in rats bearing ascitic tumors AH13,AH66 and AH414 and a 3-methylcholanthrene induced tumor

Hepatic microsomal drug-metabolizing enzyme activities, cytochrome content, delta-aminolevulinic acid (ALA) synthetase and heme oxygenase activities were studied in rats bearing ascitic tumors AH 13, AH 66 and AH 414 and a primary, 3-methylcholanthrene (3-MC)-induced, tumor. Hepatic microsomal drug-metabolizing enzyme activities and cytochrome content were decreased in rats transplanted intraperitoneally with 1-2 x 10(6) cells of ascitic tumor cell lines AH 13, AH 66 and AH 414. The extent of the decrease of the microsomal cytochrome content and enzyme activities were dependent on the tumor-bearing periods after inoculation. Hepatic microsomal heme oxygenase activity was significantly increased concurrently with the decrease of microsomal drug-metabolizing enzyme activities and cytochrome content. Hepatic ALA synthetase was not changed appreciably in these tumor-bearing rats. Similar alterations of microsomal enzyme content and activities were observed in the livers of rats transplanted subcutaneously with AH 66 tumor cells and in rats bearing a primary tumor initiated by the subcutaneous injection of 3-MC. When the tumor was surgically removed from the rats bearing AH 66 subcutaneously, these hepatic microsomal parameters returned to normal levels. Microsomal drug-metabolizing enzyme activities and cytochrome content in these ascitic tumor cells were found to be at very low levels. From these results, if appears that there is an inverse relationship between the increase of microsomal heme oxygenase activity and the decrease of cytochrome P-450 and b5 as well as drug-metabolizing enzymes in the liver of tumor bearing rats.

Effect of heme on allylisopropylacetamide-induced changes in heme and drug metabolism in the rhesus monkey (Macaca mulatta)

In rhesus monkeys, in which porphyria was induced by the administration of allylisopropylacetamide (AIA), hepatic delta-aminolevulinic acid synthase (ALA-S) was increased. Cytochrome P-450 and associated monooxygenase activities and microsomal heme oxygenase activity were decreased in these animals. Administration of heme for 4 days concurrently with AIA prevented the induction of hepatic ALA-S but produced further decreases in cytochrome P-450 and monooxygenase activities. The decrease in heme oxygenase activity elicited by AIA alone was partially reversed. Administration of heme alone caused an impairment of hepatic drug metabolism but had no significant effect on heme metabolism. The porphyric monkeys showed elevation of porphyrin levels in blood and urine. When heme was administered concurrently with AIA, blood porphyrin levels were further elevated, while the urinary excretion of porphyrins was lower than that following treatment of monkeys with AIA. Following the administration of heme alone, blood and urinary porphyrin levels were minimally affected. These results suggest that repeated heme administration in the primate may adversely affect drug metabolism by the liver.

Differential response of testicular and ovarian heme oxygenase activity to metal ions

The response of the microsomal heme oxygenase in the testis to metal ions distinctly differed from that of the ovarian source. The activity of the ovarian enzyme in rats treated with Co2+ (250 mumol/kg, 24 h) responded in consonance with that of the liver and the kidney, i.e., heme oxygenase activity was elevated. In contrast, similar treatments did not increase the activity of testicular heme oxygenase. In addition, other metal ions, such as Cu2+, Sn2+, Pb2+, and Hg2+, known for their potency to increase heme oxygenase activity, were ineffective in increasing the enzyme activity in the testis. The unprecedented response of heme oxygenase in the testis to metal ions did not reflect an unusual nature of the enzyme protein insofar as it displayed a similar cofactor requirement and inhibition by known inhibitors of the enzyme activity, such as KCN and NaN3. Moreover, the apparent Km's for oxidation of hematoheme by the testicular and ovarian microsomal fractions were comparable and measured 2.3 and 1.4 microM, respectively. In the testis of Co2+-treated rats, the concentration of cytochrome P-450 in the rough and smooth endoplasmic reticular fractions was significantly decreased. The decrease in the hemoprotein level, however, did not reciprocate the activity of heme oxygenase in the fractions. The inability of metal ions to induce heme oxygenase activity in the testis did not represent the general refractory nature of the enzymes of heme metabolism to metal ions in this organ, since in rats treated with Co2+ the activity of delta-aminolevulinate synthetase was significantly decreased 24 h after treatment. However, the activities of uroporphyrinogen-I synthetase, delta-aminolevulinate dehydratase, and ferrochelatase and the content of porphyrins were not altered in the testis of rats treated with Co2+. The response of delta-aminolevulinate synthetase in the ovarian tissue to Co2+ treatment contrasted that of the testis. In the ovary, the enzyme activity significantly decreased 6 h after treatment. This decrease was followed by a rebound increase at 24 h after administration of Co2+. The presently described inability of metal ions to induce testicular heme oxygenase activity suggests that the activity of the enzyme in the testis is controlled by factor(s) which differ from those regulating the enzyme activity in other organs, including another steroidogenic organ, the ovary.

Heme oxygenase activity is decreased by D-penicillamine in neonates

A 3-day D-penicillamine treatment of neonatal rats caused a significant decrease in heme oxygenase activity. This change was not observed in adult rats. The data indicate age-related differences in the effects of D-penicillamine.

Changes in 25-hydroxyvitamin D3 alpha- and 24-hydroxylase activities of kidney cells isolated from rats with either unilateral kidney damage or acute renal insufficiency

25-Hydroxyvitamin D3 1 alpha- and 24-hydroxylase, NADPH-cytochrome c reductase, heme oxygenase, and ATPase activities were studied in viable kidney cells isolated from rats submitted to unilateral kidney damage (cortical electrocoagulation) and during the development of acute renal failure subsequent to excision of the contralateral undamaged kidney. Measurements of blood pH, plasma total and ionized calcium, phosphorus, creatinine, kidney histology, and phosphorus nuclear magnetic resonance spectroscopy determinations of phosphorus-containing compounds in kidney tissue were also performed. Seventy-two hours after unilateral kidney damage, no significant changes were observed in blood pH or in the plasma parameters studied. During this period, a significant increase in the activity of the 25-hydroxyvitamin D3 hydroxylases could be demonstrated in the cells of the contralateral undamaged kidney. A similar pattern of compensatory rise in the activity of the other enzymes studied was not detected. However, in the damaged kidney viable cells, the hydroxylase activities remained unchanged relative to those in sham-operated controls, despite a 5-fold increase in the inorganic phosphate content and a marked decrease in the organophosphorus and ATP content of this tissue. During the development of acute renal failure, a significant decrease in the activity of the hydroxylases occurred only when the rise in plasma creatinine concentration suggested severe renal insufficiency.

Heme and hemoproteins in streptozotocin-diabetic female rats

Alterations in heme biosynthetic and degradative capabilities and in the activities of several heme-containing enzymes were examined in hepatic tissues of streptozotocin (STZ)-diabetic female Sprague-Dawley rats. Activities were measured 10, 30 and 90 days following the administration of STZ (65 mg/kg, i.v.). The activities of the key enzymes involved in heme synthesis, delta-aminolevulinic acid (ALA) synthase, ALA dehydratase, and uroporphyrinogen synthase, were decreased markedly in STZ-diabetic rats as compared to sham-operated animals. Furthermore, the catabolism of heme which occurs via microsomal heme oxygenase (MHO) remained unaltered in these animals. Microsomal content of heme and cytochrome P-450, and the activities of tryptophan pyrrolase and the drug-metabolizing enzymes benzo[a]pyrene (BP) hydroxylase and aniline hydroxylase, were increased in the livers of diabetic rats. By contrast, the activity of the heme-containing enzyme catalase was decreased in these animals. Cobalt chloride produced a marked increase in MHO with a concomitant decrease in microsomal content of cytochrome P-450 and its associated BP hydroxylase activity in normal as well as chronically diabetic rats. It was of interest, however, that the increase in ALA synthase that is normally produced by this metal was not seen in chronic diabetic animals. Thus, chronic diabetes produced subtle and important disruptions in cellular metabolism, which may have been the result of long-term alterations in key enzymes involved in heme synthesis.

Tin-protoporphyrin suppression of hyperbilirubinemia in mutant mice with severe hemolytic anemia

Tin-protoporphyrin is a potent competitive inhibitor of heme oxygenase both in vivo in animals and in vitro in isolated enzyme preparations, and when administered to neonatal rats, prevents the development of postnatal hyperbilirubinemia. In this study we examined the effect of the metalloporphyrin on the activity of heme oxygenase in liver, kidney, and spleen, and on the level of bilirubin in plasma in three types of anemic mutant mice with severe hemolytic diseases. We report that the administration of tin-protoporphyrin to anemic mutants homozygous for severe hemolytic disease results in substantial inhibition of heme oxidation in liver, spleen, and kidney and in significant reduction of plasma bilirubin levels. Tin-protoporphyrin thus has the capacity to significantly inhibit in vivo heme degradation and to concurrently diminish plasma bilirubin levels in severe chronic hemolytic disorders.

A stoichiometric study of heme degradation catalyzed by the reconstituted heme oxygenase system with special consideration of the production of hydrogen peroxide during the reaction

In heme degradation catalyzed by the reconstituted heme oxygenase system, 8 to 9 mol of dioxygen and 11 to 12 mol of NADPH were consumed per mol of hemin lost, and about half the amount of dioxygen consumed could be accounted for by the production of hydrogen peroxide, which accumulated in the reaction mixture. Production of hydrogen peroxide in the heme oxygenase reaction did not appear to be due to the bimolecular dismutation of superoxide anions but rather seemed to be due to dissociation of a "peroxo" species formed on heme or intermediates of heme degradation. The hydrogen peroxide produced appeared to cause a considerable degree of non-specific degradation of heme (not leading to the formation of biliverdin) and also caused an inactivation of heme oxygenase. By taking into account the amount of dioxygen incorporated into hydrogen peroxide and some other factors, it could be deduced that 3 mol of dioxygen is consumed for the formation of 1 mol of biliverdin in the heme oxygenase reaction.

The role of 5-aminolaevulinate synthase, haem oxygenase and ligand formation in the mechanism of maintenance of cytochrome P-450 concentration in hepatocyte culture

The present work shows that the ability of pyridines e.g. metyrapone, to maintain the cytochrome P-450 concentration in cultured hepatocytes is not due to their ability to alter the 5-aminolaevulinate synthase and haem oxygenase activities of the hepatocytes. Since ligands such as metyrapone will prevent the cobalt-mediated loss of hepatic cytochrome P-450 in rats, the hypothesis that ligand formation is the mechanism of maintenance of the cytochrome in hepatocyte culture was tested. The observation that non-pyridine ligands will maintain the cytochrome P-450 concentration supports this hypothesis.

A novel method for continuous monitoring of bilirubin production in unstressed rats

We describe a device for continuous infusion and monitoring of exhaled 14CO as a test of hepatic bilirubin production in rats. A Silastic catheter, implanted into a jugular vein under light ether anesthesia, was protected with a spring shield and a cannula swivel. The animals were kept in a modified Bollman cage. delta-[5-14C]aminolevulinic acid, a heme precursor yielding 14CO upon breakdown of heme to bilirubin, was infused at a constant rate. Exhaled 14CO was oxidized to 14CO2 and collected in ethanolamine. The efficiency of the system averaged 97.8%. In untreated animals 14CO production reached a plateau within 12 h; thereafter, it increased by 2.8% per day. The responsiveness of the system was tested by fasting the animals, which stimulated hepatic bilirubin production. Fasting increased 14CO production by 32.8 +/- 8% (mean +/- SD, P less than 0.005) after 72 h. This was associated with an increase in hepatic heme oxygenase activity (+48%, P less than 0.05) and a decrease in microsomal cytochrome P-450 content (-45%, P less than 0.05). Thus, our approach permits continuous monitoring of hepatic bilirubin production without subjecting the animals to the stress of handling, restraint, or anesthesia. The method can easily be applied to other breath tests involving formation of 14CO2.

Developmental changes in bilirubin production in the rat

Total bilirubin production and relative rates of early labeling of bilirubin (ELB) were determined in rats during postnatal development. Total production was estimated by measuring endogenous rate of excretion of carbon monoxide (VeCO), while ELB was determined by measuring the incorporation of glycine-2-14C and delta-aminolevulinic acid-5-14C(delta-ALA-5-14C) into expired 14CO over a 30-h period after isotope injection. VeCO was considerably higher in 1- and 4-day-old rat pups than in adults, but fell rapidly toward adult values by 9 days of age. 14CO excretion from both isotopic precursors of bilirubin was significantly greater in suckling animals than in postweanling and young adult animals when expressed as a percent of the administered radioactivity. The activity of hepatic heme oxygenase showed a similar pattern of postnatal change to 14CO excretion from both glycine-2-14C and delta-ALA-5-14C. However, phenobarbital administration to young weanling animals significantly increased 14CO excretion from glycine-2-14C, but did not result in a change in the activity of hepatic heme oxygenase. The activity of hepatic heme oxygenase does not always reflect in vivo ELB.

Effect of cholestasis produced by bile duct ligation on hepatic heme and hemoprotein metabolism in rats

Cholestasis produced by bile duct ligation has been associated with decreased concentrations of hepatic microsomal cytochrome P450 and decreased hepatic microsomal oxidative drug metabolism. Bile duct ligation producing cholestasis results in a marked increase in hepatic microsomal heme oxygenase activity, with corresponding decreases in hepatic microsomal cytochrome P450 concentration, reduced form of nicotinamide adenine dinucleotide phosphate-cytochrome c reductase activity, and hepatic delta-aminolevulinic acid synthetase activity. As sham-operated rats also demonstrate a less prolonged decrease in cytochrome P450 concentration and reduced form of nicotinamide adenine dinucleotide phosphate-cytochrome c reductase activity, the metabolic effects of surgery and anesthesia must also be involved in these alterations in microsomal oxidative drug metabolism. The relative rate of hepatic cytochrome P450 synthesis and of degradation are both decreased after bile duct ligation. These data suggest that decreased hepatic microsomal cytochrome P450 concentrations in cholestasis are partly the result of decreased cytochrome P450 synthesis. Increased levels of heme oxygenase activity are not related to increased cytochrome P450 turnover, but may instead reflect enlargement and increased catabolism of a free heme pool resulting from decreased hemoprotein (cytochrome P450) synthesis.

Heme oxygenase activity in liver microsomes of the bullfrog, Rana catesbeiana, and its change during the tadpole development

Heme oxygenase in the liver microsomes of tadpole and adult bullfrog, Rana catesbeiana, was studied in relation to hemoglobin metabolism during tadpole development. The results obtained are as follows. 1. The specific activity of the enzyme of premetamorphic tadpole liver was comparable to that of adult frog liver. The apparent Km for methemalbumin was about 25 microM for both tadpole and frog enzymes. 2. The enzyme activity was stimulated in vivo by the injection of methemalbumin, phenylhydrazine and triiodothyronine to the animals. 3. A marked increase in the enzyme activity was found in the liver of tadpole during prometamorphic stage and metamorphic climax.

Erythroid colony development as a function of age: the role of marrow cellular heme

The activities of the heme biosynthetic enzymes ALA synthase (ALAS) and ALA dehydrase (ALAD) and the heme degradative enzyme heme oxygenase were analyzed from bone marrow cells obtained from young, middle-aged, and senescent rats. There was age-related reduced activity of bone marrow ALAS but no age-related difference in the activity of ALAD. In contrast, heme oxygenase activity was 50% greater in the senescent marrow cells. Incorporation of 14C-glycine into heme was 45% less in senescent rat marrow cells, whereas incorporation of 14C-delta-aminolevulinic acid was not related to age. Senescent bone marrow cells demonstrated a marked reduction in 14C-leucine and 3H-uridine incorporated into protein and nucleic acid synthesis, respectively. In vitro erythroid colony (CFUE) growth by senescent bone marrow cells was as much as 40% less compared with young bone marrow cells. The decreased ability to form CFUE by the senescent bone marrow cells may be related to reduced ALAS activity and increased heme oxygenase activity. Thus, part of the aging process appears to involve fluctuations in the enzyme activities and protein synthesis involved with metabolism of heme.

Regulation of rat hepatic delta-aminolevulinic acid synthetase and heme oxygenase activities: evidence for control by heme and against mediation by prosthetic iron

1. The effect of in vivo administration of 6 compounds on the activity of delta-aminolevulinic acid (ALA) synthetase and heme oxygenase were determined. 2. The order of decreasing potency in reducing ALA synthetase activity was heme, bilirubin, protoporphyrin IX, bilirubin dimethyl ester, CoCl2 and FeCl3. 3. The chelating agents EDTA and deferoxamine did not prevent heme's repression of ALA synthetase or induction of heme oxygenase activity. 4. The dose response, time course, enzyme subcellular distribution and chelation antagonism studies all suggest that heme itself, and not iron, regulates the rate limiting enzymatic steps of rat hepatic heme synthesis and degradation.

Thyroid hormone regulation of heme oxidation in the liver

The effects of 3,5,3'-triiodothyronine (T3) on heme oxygenase (EC 1.14.99.3) activity and cytochrome P-450 content in liver were examined in thyroidectomized rats. T3, when administered for 5 days at a dose of 6 micrograms/100 g of body weight, stimulated basal heme oxygenase activity approximately equal to 2-fold compared to diluent-treated animals. The induction of heme oxygenase by cobalt heme also was enhanced approximately equal to 3-fold in T3-treated animals. T3 treatment lowered cytochrome P-450 content by approximately equal to 50% and potentiated the depletion of this heme protein after cobalt heme administration. Reverse T3 had no effect either on cytochrome P-450 content or on heme oxygenase activity in liver. The time course of response to a single dose of T3 (50 micrograms/100 g of body weight) revealed that both basal and cobalt heme-induced heme oxygenase activity peaked at 48 hr and that cytochrome P-450 content declined to approximately equal to 40% of controls at 96 hr. Examination of microsomal proteins by polyacrylamide gel electrophoresis after T3 treatment disclosed that major bands in the Mr approximately equal to 50,000-55,000 region were diminished. The administration of T3 together with SKF-525A, a compound known to complex with the heme prosthetic group of cytochrome P-450, resulted in partial preservation of these proteins. These data indicate that thyroid hormone can regulate heme oxygenase activity and concomitantly can lower cytochrome P-450 content in liver. The hormone also can act in a synergistic fashion to enhance the response of hepatic heme oxygenase to a chemical inducer of the enzyme. Thyroid status thus may be a potentially significant determinant of the rate of heme oxidation in the liver.

Suppression of hyperbilirubinemia in the rat neonate by chromium-protoporphyrin. Interactions of metalloporphyrins with microsomal heme oxygenase of human spleen

The synthetic metalloporphyrin, Cr-protoporphyrin, as a potent competitive inhibitor of heme oxygenase activity in rat spleen, liver, and kidney. When administered to neonatal animals in a single dose immediately after birth, Cr-protoporphyrin suppresses postnatal hyperbilirubinemia and produces a marked and sustained lowering of heme oxidation activity in liver, spleen, and kidney. The metalloporphyrin also potently inhibited the rate of heme degradation to bile pigment in human spleen.

Effect of melphalan on delta-aminolevulinic acid synthetase in the spleen, bone marrow and liver of rats

The effect of melphalan, an antineoplastic alkylating agent, on the activity of delta-aminolevulinic acid synthetase (ALA-S) was studied. When given in a single oral dose (20 mg/kg) to male rats. ALA-S activity in the spleen rapidly declined, reaching 9% of controls 7 days after administration. A return to control values occurred after 10 days, followed by a rebound increase in activity to 180% of controls at day 15. After melphalan administration, a gradual loss in spleen weight was observed until day 10, followed by a rebound increase in spleen weight at day 15. Spleen heme oxygenase activity was induced to 243% of controls 7 days after melphalan administration. However, by 20 days the activity was only 55% above controls. Bone marrow ALA-S activity declined to 38% of controls 3 days after treatment, followed by a return to controls after 7 days. The return to control values at day 15 was accompanied by a shift in the differential white blood cell count to the segmented form. Red blood cell counts and hemoglobin content were decreased to 77% of controls at day 7. Liver ALA-S was not appreciably affected by melphalan except for a decrease to 56% of controls at day 7. It is concluded that melphalan depresses the level of activity of the erythroid form of ALA-S in spleen and bone marrow but has less effect upon the level of activity of the nonerythroid form in the liver.

Purification and properties of bovine spleen heme oxygenase. Amino acid composition and sites of action of inhibitors of heme oxidation

Microsomal heme oxygenase has been purified from bovine spleen to homogeneity using DEAE-cellulose chromatography, initial hydroxyapatite chromatography, gel filtration, and repeat hydroxyapatite chromatography. The purified enzyme showed a single protein band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis with a molecular weight of congruent to 31,000. It had a Km for heme of 0.93 microM, a specific activity of 6770 units/mg of protein, and a turnover number of 3.5 mol/mol of enzyme/min. Amino acid analysis of the purified enzyme revealed an abundance of glutamine and glutamic acid residues, a log tryptophan content (2 tryptophan residues/mol of enzyme), and four free sulfhydryl groups, only two of which were accessible to sulfhydryl (--SH) inactivating reagents without unfolding of the protein. Consistent with these findings, the purified enzyme had a relatively low extinction coefficient at 280 nm (E11%cm = 8.12) and was relatively resistant to inactivation by --SH inactivating reagents. NADPH-cytochrome c reductase from bovine liver was purified to homogeneity and biliverdin reductase from bovine spleen was partially purified. The heme oxygenase system was reconstituted from preparations of all three purified enzymes and, utilizing this reconstituted system, the specific sites of the inhibitory actions of --SH inactivating reagents, inorganic metals, and metalloporphyrins on the heme degrading sequence of reactions were examined. Sn-, Co-, Zn-, and Mn-protoporphyrin strongly inhibited heme degradation in a competitive manner. The Ki values for Sn-, Co-, and Zn-protoporphyrin were determined to be 0;033, 0.082, and 0.13 microM, respectively. Mg-, Ni-, and Cu-protoporphyrin had little effect on heme degradation by the reconstituted system. Metals such as Pt2+ and Hg2+ strongly inhibited the activity of the reconstituted heme oxygenase system, but the principal site of action of these metals was at the level of NADPH-cytochrome c reductase or biliverdin reductase. Similarly --SH inactivating reagents, such as p-chloromercuribenzoate, 5,5'-dithiobis-(2-nitrobenzoate), or N-ethylmaleimide inhibited the reaction catalyzed by the reconstituted heme oxygenase system principally by inhibiting the activity of NADPH-cytochrome c reductase.

Effect of endotoxin and its degradation products on hepatic mixed-function oxidase and heme enzyme systems in mice

Effect of Salmonella minnesota R595 endotoxin (R595 GL) and its chemical degradation products on hepatic cytochrome P-450, delta-aminolevulinic acid (ALA) synthetase and heme oxygenase activity in mice was examined. The levels of cytochrome P-450 and ALA synthetase activity were significantly decreased following the administration of R595 GL, whereas heme oxygenase activity was markedly increased. The effects of endotoxin were dose-dependent for the range of about 1 to 2 micrograms per mouse. The result of a comparative study using R595 GL and its degradation products suggested that intact lipid A moiety plays an important rôle in the activity of endotoxin. In addition, it was found that the depressant effect of endotoxin on the hepatic mixed-function oxidase system in mice was abrogated by treatment with cortisone.

Thyroid hormone-induced changes in the hepatic monooxygenase system, heme oxygenase activity and epoxide hydrolase activity in adult male, female and immature rats

In 8-day-old rat pups, pretreatment with a single injection of L-triiodothyronine or L-thyroxine decreased hepatic cytochrome P-450 content, aminopyrine N-demethylase activity and epoxide hydrolase activity but increased hepatic microsomal cytochrome c reductase, 7-ethoxyresorufin O-deethylase and heme oxygenase activities without significantly altering UDP-glucuronosyltransferase activity (towards o-aminophenol) or the microsomal yield. In adult rats of either sex such single injections of L-triiodothyronine failed to significantly alter these enzyme activities. However, multiple injections evoked changes similar to those observed in the pups, in all these enzyme activities, except that 7-ethoxyresorufin O-deethylase activity was slightly decreased rather than increased. These findings demonstrate that: (1) The hepatic monooxygenase system in the rat pup is more responsive to thyroid hormones than that in adult. (2) Thyroid hormones can decrease rat liver cytochrome P-450 content and its dependent monooxygenase activity independently of sexual maturity. (3) Thyroid hormones also decrease hepatic epoxide hydrolase activity in both pups and adults. Thus, hyperthyroidism could render the rat pup more susceptible to hepatotoxicity from electrophilic epoxides which utilize microsomal epoxide hydrolase as the major detoxication pathway.

Specific alpha-bridge cleavage by heme oxygenase of [alpha-14C]deuterohemin IX, [alpha-14C]hematohemin IX and 2,4-diacetyl[alpha-14C]deuterohemin IX

The enzymatic oxidations of [alpha-14C]deuterohemin IX, [alpha-14C]hematohemin IX and 2,4-diacetyl[alpha-14C]deuterohemin IX were carried out by using a microsomal heme oxygenase system from rat liver in combination with the biliverdin reductase from the same origin. In every case the bilirubins formed were devoid of radioactivity, indicating the alpha-selective oxidation of the three hemins. Hematohemin IX was oxidized at the highest rate, followed by deuterohemin IX and 2,4-diacetyldeuterohemin. When the three hemins were preincubated with microsomal heme oxygenase in the absence of NADPH, and the latter was added after the preincubation period, it was found that the enzymatic oxidation of the hemins was inhibited. Therefore, for the maximal rate of oxidation both hemin and NADPH must be present simultaneously. In the presence of hemin IX (the natural substrate), the enzymatic oxidation of the synthetic hemins was inhibited. The oxidation of 2,4-diacetyldeuterohemin IX was the most inhibited, while the oxidation of hematohemin IX was affected to a much lesser degree. These results are in agreement with the higher affinity (Km = 150 microM) of hematohemin IX for the enzyme, as compared to 2,4-diacetyldeuterohemin IX (Km = 660 microM) and deuterohemin IX (Km = 330 microM).