ω-1 and ω-2 hydroxylation of prostaglandins by rabbit hepatic microsomal cytochrome P-450 isozyme 6

Incubation of prostaglandin E1 (PGE1) with liver microsomes from control rabbits and from rabbits treated with ethanol or imidazole yielded 18-, 19-, and 20-hydroxy metabolites, representing hydroxylation at omega-2, omega-1, and omega carbons, respectively. The current investigation demonstrates that rabbit liver P-450 isozyme 6 effectively catalyzes the omega-1 and omega-2 hydroxylation of PGE1 and PGE2. Additionally, a small amount of product with chromatographic characteristics of the corresponding 20-hydroxy metabolite has been detected. The incorporation of cytochrome b5 into the reconstituted system did not enhance the rate of PGE1 hydroxylation and had no effect on the ratio of products formed. The Km value for the omega-1 and omega-2 hydroxylation of PGE1 with P-450 isozyme 6 from imidazole-treated rabbits was approximately 140 microM; the Vmax's (nmol product min-1 nmol P-450-1) were 2.1 and 1.1 for the omega-1 and omega-2 hydroxylations, respectively. These rates represent the highest activities by hepatic P-450 isozymes for hydroxylation of PGs, and suggest that isozyme 6 is responsible for the omega-2 hydroxylation of PGEs observed in rabbit liver microsomes.