Oxidation of corticosteroids to steroidal 20-hydroxy-21-oic acids by mouse liver

We have studied the enzyme catalyzed oxidation of 11-deoxycorticosterone to 20-hydroxy-3-oxo-4-pregnen-21-oic acid (pregnolic acid) in mouse liver. Enzyme activity, though variable, was higher than that of other species. Pregnolic acid was identified as the free acid, as the methyl ester by thin layer chromatography, and as the p-bromophenacyl ester by high performance liquid chromatography. With [4-14C, 21-3H]-DOC as substrate, exchange of tritium with water (interpreted as due to the reversible isomerization of the ketol to aldol form by the side chain) and the overall conversion of the ketol side chain to hydroxy acid was catalyzed by the post-microsomal supernatant fraction. Although we could not physically separate tritium exchange and acid production, pregnolic acid formation could be decreased or eliminated while tritium exchange was retained, consistent with our previous conclusion that isomerization to aldol was a precondition for acid formation. In preparations that made no acid, [4-14C]-DOC was recovered, depleted of tritium. The rate of exchange of [21S, 21-3H]-DOC with water was faster than [21R, 21-3H]-DOC. The stereochemistry of pregnolic acid at C-20 was 85-90% R (i.e.. 20 alpha-hydroxy-21-oic acid). The Km for isomerase with [21RS-21(3) H-DOC was 4.3 x 10(-5); Km for pregnolic acid formation was 8.0 x 10(-5) M. Corticosterone was oxidized to acid metabolites at 20% the rate of DOC.