2,5-Diphenyloxazole as a probe for microsomal mono-oxygenation in human and rat liver.

2,5-Diphenyloxazole (PPO) is metabolized to one major fluorescent product by human liver and rat liver microsomes. PPO metabolism by human-liver microsomes involves more than one cytochrome P-450 isozyme, termed low-affinity and high-affinity components. At a substrate concentration of 0.1 microM, 95% of activity is due to the high-affinity component whereas at 100 microM 69% of activity is due to the low-affinity component. Inhibition studies with metyrapone and alpha-naphthoflavone at 0.1 microM and 100 microM suggest that the high-affinity component may reflect a 3-methylcholanthrene-inducible form of cytochrome. Therefore studies at low substrate concentrations may be a useful tool for cytochrome P-450 studies in man. Rat liver microsomes show linear kinetics indicating the involvement of one major form of cytochrome P-450.