Cytochrome P-450 K of rat kidney cortex microsomes: Further studies on its interaction with fatty acids

Abstract

The cytochrome P-450 K containing monooxygenase system of rat kidney cortex microsomes catalyzes the hydroxylation of various saturated fatty acids of medium chain length to the corresponding ω- and (ω-1)-hydroxy derivatives. The hydroxylation activity, as well as the ratio between the two hydroxylated products, vary with the carbon chain length of the fatty acid. Optimal hydroxylation activity is observed with myristic acid which yields the 13- and 14-hydroxylated products at a ratio of about 1. The ω/(ω-1)-hydroxylation ratio decreases with increasing carbon chain length of the fatty acid. On the other hand, with lauric acid as a substrate the ratio between ω- and (ω-1)-hydroxylation does not change significantly with varying time of incubation or substrate concentration, or incubation in a medium containing D 2O or after induction of enhanced hydroxylation activity by starvation of the animals. Furthermore, 12-hydroxylauric acid and capric acid—which is almost exclusively ω-hydroxylated by rat kidney cortex microsomes—inhibit both 11- and 12-hydroxylation of lauric acid to a similar extent whereas 11-hydroxylauric acid does not seem to inhibit either 11- or 12-hydroxylation. C 10-C 16 fatty acids produce the type I spectral change upon addition to rat kidney cortex microsomes and seem to interact with similar amounts of the cytochrome P-450 K present in these particles. In agreement with the metabolic studies, 12-hydroxylauric acid interacts with cytochrome P-450 K giving rise to a reverse type I spectral change, whereas 11-hydroxylauric acid does not produce an observable spectral change. Finally, results of binding experiments with a series of derivatives of dodecane suggest that type I binding to cytochrome P-450 K requires, besides a proper chain length, the presence of a carbonyl group together with an electron pair on a neighboring atom at the end of the carbon chain. A chain length of 14 carbon atoms seems to be optimal and it is suggested that this chain length may correspond to the distance between a possible binding site and the catalytic site of cytochrome P-450 K