The interaction of cytochrome b5 with four cytochrome P-450 enzymes from the untreated rat.

The capacity of four native P-450 enzymes to interact with cytochrome b5 was compared and discussed in terms of spin shift and metabolism. Two-dimensional electrophoresis was employed as a tool to aid in characterization of the different enzymes isolated from liver microsomes of the untreated rat. RLM5 had a pl of 7.4 and RLM3 had a pl of 7.1. Two new forms isolated by this laboratory, RLM2 and RLM5a, differed from other forms of cytochrome P-450 characterized to date. The pl values of these forms were 7.35 and 7.6, respectively. The interaction of all four enzymes with cytochrome b5 differed. Cytochrome b5 caused a major low to high spin transition when added to RLM5. The latter hemoprotein was 28% high spin at 25 degrees C and was shifted to 55% high spin by cytochrome b5. RLM5a shifted from 4% high spin to 15% high spin under comparable conditions. In contrast, RLM2 and RLM3 were both minimally influenced by cytochrome b5, reaching only 8% high spin. Cytochrome b5 did not appreciably influence the rates of metabolism of aminopyrine, benzphetamine, testosterone, or p-nitroanisole with RLM2 or RLM3. However, with RLM5 and RLM5a, rates of aminopyrine and benzphetamine demethylation and testosterone hydroxylation were increased to about 130% with RLM5 and up to 200% with RLM5a. The demethylation of p-nitroanisole was stimulated by cytochrome b5, 3.5-fold with RLM5 and 14-fold with RLM5a. In no case was the ratio of monohydroxy metabolites of testosterone altered by the addition of cytochrome b5, indicating an effect on Vmax rather than Km.(ABSTRACT TRUNCATED AT 250 WORDS)