MECHANISM OF METOPIRONE INHIBITION OF A SOLUBLE ADRENAL STEROID 11β-HYDROXYLASE

The effect of Metopirone (2-methyl-1,2-bis(3-pyridyl)-1-propanone) (Su-4885) on the 11β-hydroxylation of 11-deoxycorticosterone (DOC) and NADPH oxidation has been examined in a soluble enzyme system extracted from an acetone powder of bovine adrenal mitochondria. Addition of Metopirone in the absence of DOC stimulated the oxidation of reduced nicotinamide–adenine dinucleotide phosphate (NADPH) by the enzyme system. Under the experimental conditions employed, 11β-hydroxylation was inhibited 50% by Metopirone at a final concentration of 4.2 μmoles/l, a quantity 27 times less than that required to produce a twofold increase in NADPH oxidation in the absence of DOC. Low levels of Metopirone that effectively inhibited corticosterone formation produced a similar decrease in NADPH oxidation associated with the hydroxylation reaction, indicating the interrelationship of 11β-hydroxylation and NADPH oxidation. Evaluation of kinetic data showed that the inhibition of 11β-hydroxylation produced by Metopirone was competitive. The Km for the 11β-hydroxylation of DOC was 1.8 × 10−5 mole/l and the Ki for Metopirone was 3.2 × 10−7 mole/l. The ratio Km:Ki indicated that the affinity of Metopirone for the 11β-hydroxylase was 56 times greater than that of DOC.