Abstract
The effects of cytochrome b5 with manganese-protoporphyrin IX substituted for heme were compared with those of native cytochrome b5 and the apoenzyme on the oxygenation of substrates in the reconstituted system containing liver microsomal cytochrome P-450, NADPH-cytochrome P-450 reductase, and phosphatidylcholine. Mn-b5, unlike b5, remains essentially fully oxidized in the presence of NADPH and NADPH-cytochrome P-450 reductase under aerobic conditions. The effects of various concentrations of b5 and its derivatives were determined at constant P-450 and reductase concentrations. Cytochrome b5 inhibits benzphetamine demethylation by isozyme 2, the effect increasing up to the highest concentrations tested, and stimulates 7-ethoxycoumarin deethylation by isozyme 2 and acetanilide p-hydroxylation by isozyme 4, the optimal b5:P-450 molar ratio being about 2. In contrast, Mn-b5 inhibits all three reactions and apo-b5 is either inactive or slightly inhibitory. The activities of the three substrates as well as testosterone were determined with P-450 isozymes 2, 3b, 3c, and 4 in the reconstituted system with no additions or with b5 or Mn-b5 present. Cytochrome b5 is stimulatory, inhibitory, or without any effect, the result depending on both the substrate and P-450 isozyme present, whereas Mn-b5 is inhibitory in most instances. Both b5 and its manganese derivative alter the rates of testosterone 6 beta- or 16 alpha-hydroxylation by most of the P-450 cytochromes. The activities are influenced by the molar ratio of reductase to P-450. The Km values of benzphetamine, ethoxycoumarin, and acetanilide are, with one exception, significantly decreased in the presence of b5 or Mn-b5. We conclude that some of the effects of b5 on the oxygenase system are not accounted for by its role as an electron donor to cytochrome P-450.
DMD articles become freely available 12 months after publication, and remain freely available for 5 years.Non-open access articles that fall outside this five year window are available only to institutional subscribers and current ASPET members, or through the article purchase feature at the bottom of the page.
|