Genetic contributions to the variation among rabbits of liver microsomal deoxycorticosterone synthesis

Outbred New Zealand white rabbits exhibit two phenotypes, 21H and 21L, corresponding to rates greater than or less than 1 nmol/min/mg, respectively, for liver microsomal progesterone 21-hydroxylase activity. In contrast, the inbred strain III/J exhibits only the 21L phenotype. Two 21H male New Zealand white rabbits were mated with several female III/J rabbits to produce a total 46 progeny. Both the 21H and 21L phenotypes were evident among male and female offspring in roughly equal numbers. Backcrosses between 21L progeny and III/J rabbits exhibit only the 21L phenotype, whereas 21H offspring yield both 21H and 21L progeny when backcrossed to the 21L inbred strain III/J. These results are consistent with autosomal dominant inheritance of the 21H phenotype. Analysis of Southern blots of genomic DNA digested with the restriction endonuclease KpnI reveals 20-, 13-, and 9-kb fragments that hybridize with a probe derived from the 3'-untranslated region of the 21-hydroxylase cDNA. The 13-kb band is not observed for strain III/J or 21L progeny of strain III/J crossed with 21H rabbits, but it is detected for both 21H fathers and 21H progeny indicating that the genetically determined difference of 21-hydroxylase expression is inherited cis to the gene for P450IIC5, the hepatic progesterone 21-hydroxylase. Electrophoretic analysis of P450IIC5 synthesized in vitro from mRNA isolated from 21L and 21H rabbits reveals that little or no P450IIC5 is synthesized from 21L mRNAs. A second immunoreactive, electrophoretically distinct protein is synthesized from both 21L and 21H mRNAs to a similar extent but in lesser amounts than P450IIC5. The second protein could represent either an allozymic form of the enzyme or the product of a distinct locus. Thus, it is likely that distinct structural genes for P450IIC5 contribute to the differences in P450-mediated metabolism in 21L as compared to 21H rabbits.