Mutant forms of cytochrome P-450 controlling both 18- and 11beta-steroid hydroxylation in the rat

A reciprocal relationship between steroid 18- and 11beta-hydroxylase activities in the salt susceptible (S) and the salt resistant (R) strains of rats was previously shown to be controlled by a single genetic locus with two alleles and inheritance by co-dominance (Rapp, J. P., and Dahl, L. K. (1972), Endocrinology 90, 1435). The strain specific steroidogenic patterns, characterized by the relative magnitudes of 18- and 11beta-hydroxylase activities, were found to be determined by adrenal mitochondrial cytochrome P-450 particles. Carbon monoxide inhibition of 18- and 11beta-hydroxylation of deoxycorticosterone in these strains showed that the CO/O2 ratio causing 50% inhibition (i.e., Warburg's partition constant, K) was identical for 18- and 11beta-hydroxylation within a strain, but different for both 18- and 11 beta hydroxylation between strains. (K values were: S rats, 18-hydroxylation = 11.4 +/- 1.4; S rats, 11beta-hydroxylation = 11.0 +/- 1.2; R rats, 18-hydroxylation = 56.4 +/- 13.7; R rats, 11beta-hydroxylation = 46.7 +/- 11.7). This between-strain difference was unique for 18- and 11beta-hydroxylation; i.e., it was not seen with cholesterol side-chain cleavage or 21-hydroxylation. Moreover, the strain-specific K values for 18- and 11beta-hydroxylase and the strain-specific steroidogenic patterns due to the relative magnitudes of 18- and 11beta-hydroxylase activities segregated together in an F2 population. These data strongly suggest the same cytochrome P-450 is involved in both 18- and 11beta-hydroxylation and that this cytochrome is mutated between S and R rats. K values for the reaction corticosterone leads to 18-hydroxycorticosterone were different between S and R strains, indicating that the mutant cytochrome was also involved in this hydroxylation, but K values for the conversion corticosterone leads to aldosterone were not different between strains. This was interpreted to mean that each step in the sequence corticosterone leads to 18-hydroxycorticosterone leads to aldosterone was mediated by a different cytochrome, the K value for the second step being the lower and dominating the overall reaction. It was speculated that the second step could be a second hydroxylation at position 18 to yield 18,18-dihydroxycorticosterone which could be unstable and decompose into aldosterone and water.