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

Regioselective Porphyrin Bridge Cleavage Controlled by Electronic Effects. Coupled Oxidation of 3-Demethyl-3-(trifluoromethyl)mesohemin IX and Identification of Its Four Biliverdin Derivatives

This report describes the nonenzymatic oxidative cleavage of the title porphyrin (2) performed with oxygen and ascorbic acid in aqueous pyridine at 37 degrees C (coupled oxidation), via hydrolysis of the corresponding verdoheme intermediates, followed by esterification of the resulting free acid mesobiliverdin analogues to their dimethyl esters 4 (alpha isomer), 5 (beta isomer), 6 (gamma isomer), and 7 (delta isomer). The four biliverdin derivatives were purified by HPLC, and their structures were confirmed by FAB MS and also by UV-vis and (1)H NMR spectroscopies. The purity of each compound was checked by (19)F NMR, and the four regioisomers were assigned through their 2D-NMR ROESY spectra and confirmed by UV-vis spectroscopy. The ratio of regioisomers was determined by (19)F NMR spectroscopy before any purification of single compounds was attempted: alpha:beta:gamma:delta 11:6:26:57 (%). This unusually high regioselectivity was attributed to the electron-withdrawing effect of the CF(3) group on the electronic structure of porphyrin as shown considering the ab initio calculations of an iron(II) beta-substituted (trifluoromethyl)porphyrin used as a model compound. In porphyrin 2, the oxidation clearly takes place at the electron richest meso positions, the order of reactivity strictly following that of electron density, pointing out that the regiospecificity of the bridge cleavage can be effectively controlled by the electronic effects of some strategic substituents in the chromophore. The relevance of all these results in the study of the mechanism of the reactions involved in the natural catabolism of heme, catalyzed by heme oxygenase, is discussed. The advantages of 2 derived from this work, which make it a suitable model compound for the enzymatic reaction, are also discussed.

Biliverdin reductase: substrate specificity and kinetics

The substrate specificity of the different forms of rat liver biliverdin reductase was examined using synthetic biliverdins. Biliverdins carrying methyl, ethyl and one propionate residue in their structure were not substrates of biliverdin reductase. Biliverdins with one propionate and one acetate residue or with two acetate residues were not reduced by the enzyme either. The presence of two propionates in the biliverdin structure gave a biliverdin with substrate activity. Increasing the number of propionates to four, as in coprobiliverdins, did not affect substrate activity, while the octaacid urobiliverdins were also good substrates of the enzymes. The beta isomer of urobiliverdin III and coprobiliverdin III were reduced at much higher rates by molecular form 3 of the enzyme as compared to molecular form 1, a fact which had already been observed with the beta isomer of biliverdins IX, XIII and hematobiliverdin. All the biliverdins mentioned above were readily reduced to bilirubins by sodium borohydride. The purified molecular forms 1 and 3 displayed sigmoidal kinetics with most of the biliverdins tested. The data were analyzed by nonlinear regression in a microcomputer and it was found that they fitted a model of a moderate cooperative dimer where both ES and ES2 are catalytically active. The Vm, Ks and the Hill numbers, nH, for biliverdin IX alpha and beta, hematobiliverdin IX alpha and beta, and several synthetic biliverdin isomers are given. Molecular form 2 showed classical Michaelian kinetics.

The chemical and enzymatic oxidation of hematohemin IX. Identification of hematobiliverdin IX alpha as the sole product of the enzymatic oxidation

Oxidative cleavage of hematohemin IX in pyridine solution in the presence of ascorbic acid (coupled oxidation), followed by esterification of the products with boron trifluoride/methanol produced the four possible hematobiliverdin dimethyl esters in 11.1% overall yield. Transetherifications took place simultaneously with the esterification reaction and resulted in the formation of the dimethyl ester of hematobiliverdin IX gamma 8a,13a-dimethyl ether (1.8%), the dimethyl ester of hematobiliverdin IX beta 13a,18a-dimethyl ether (1.9%), the dimethyl ester of hematobiliverdin IX delta 8a-monomethyl ether (1.4%), and the dimethyl ester of hematobiliverdin IX alpha 18a-monomethyl ether (0.4%). The latter was the sole product obtained after the enzymatic oxidation of hematohemin with heme oxygenase, after esterification of the reaction product with boron trifluoride/methanol. When the esterification step was omitted hematobiliverdin IX alpha was obtained from the enzymatic oxidation. The structures of the hematobiliverdin derivatives were secured by their NMR and mass spectra data. Saponification of the dimethyl esters afforded the hematobiliverdin methyl ethers, which were excellent substrates of biliverdin reductase and were readily reduced to the corresponding bilirubins. Hematobiliverdin IX alpha was also a good substrate of biliverdin reductase. It is concluded that the enzymatic oxidation of hematohemin IX by heme oxygenase is alpha-selective, while biliverdin reductase shows no selectivity in the reduction of the four hematobiliverdin isomers.

The specificity of biliverdin reductase. A study with different biliverdin types

The specificity of rat liver biliverdin reductase was examined with the help of a series of synthetic biliverdins. The mixture of the four biliverdin isomers obtained by the chemical oxidation of protohemin I, protohemin XI, protohemin XIV and harderohemin were used as substrates of biliverdin reductase and were compared with the mixture of biliverdins IX alpha-delta. Biliverdin reductase (molecular form 1) from rat liver efficiently reduced the isomer mixtures of biliverdins I, XI, XIV and harderobiliverdins to the bilirubins in the presence of NADPH. The enzymatic reduction of the different biliverdin types was studied in the presence of different NADPH analogues. NADPH could be replaced by NADH, 3-acetyl NADPH and deamino-NADPH with retention of a good substrate activity only in the case of biliverdins of types I and IX and harderobiliverdins. Biliverdins XI and XIV were efficiently reduced only in the presence of NADPH and an excess of NADH. Bactobilin III-alpha was also very efficiently reduced by biliverdin reductase in the presence of both NADPH and NADH but not in the presence of the other analogues. These results indicate that biliverdin reductase reduced bilitriene acids substituted with non-polar and polar residues.

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.

The specificity of biliverdin reductase. The reduction of biliverdin XIII isomers

The substrate specificity of the different molecular forms of biliverdin reductase (bilirubin:NAD(P)+ oxidoreductase, EC 1.3.1.24) using biliverdin XIII alpha, XIII beta and XIII gamma was examined. It was found that molecular form 1 (the major form in normal rat liver) reduced biliverdin XIII alpha at a much higher rate than the other two isomers. Molecular form 2 (the minor form) reduced isomers XIII alpha and XIII beta at similar rates, while molecular form 3 (the major form induced by CoCl2 treatment) reduced the XIII beta isomer at a slightly higher rate than the XIII alpha isomer. Molecular forms 2 and 3, both reduced isomer XIII gamma more slowly than they reduced the XIII alpha and XIII beta isomers. These results are similar to those obtained previously using biliverdins IX alpha, IX beta and IX gamma, suggesting that biliverdin reductase specificity is related to the type of the isomer rather than to the series (IX or XIII) of the isomer.

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.