22-Ketocholesterol. A potent competitive inhibitor of cytochrome P- 450scc-dependent side-chain cleavage of cholesterol

JD Lambeth

22-Ketocholesterol binds with high affinity to purified, phospholipid vesicle-reconstituted cytochrome P-450scc. Binding, quantitated using reversal of cholesterol-induced absorbance changes in the Soret region of the enzyme, indicates an affinity 3-5 times greater than that for the normal substrate cholesterol. The ketosteroid cannot be hydroxylated at position 22 and thus acts as a potent inhibitor of cholesterol side-chain cleavage. Steady-state kinetics demonstrate competitive inhibition by this steroid and provide a KI value several- fold lower than the cholesterol Km. On the basis of recently proposed mechanisms for hydroxylation by cytochromes P-450, 22-ketocholesterol may exert its inhibitory effect by acting as a tightly bound analogue that resembles the enzyme-bound cholesterol from which a hydrogen has been abstracted from position 22.

Volume 23, Issue 3, pp. 743-747, 05/01/1983
Copyright © 1983 by American Society for Pharmacology and Experimental Therapeutics