Association of cytochrome b5 with 16-androstene steroid synthesis in the testis and accumulation in the fat of male pigs.

The 16-androstene steroids, one of the principal causes of boar taint, are synthesized in the testis by the andien-beta synthase enzyme system. This system has been shown in vitro to involve both cytochrome P450c17 and cytochrome b5. The objective of this work was to investigate the relationship between the levels of cytochrome b5 in the testis, in vitro steroidogenesis, and the accumulation of 16-androstene steroids in the fat of pubertal boars. We found that the in vitro rate of 16-androstene steroidogenesis in testis microsomes was correlated with 16-androstene steroid concentrations in fat (r = .66, P < .01). Western blots were used to determine the amounts of cytochrome b5 and cytochrome P450c17 protein in testis, and two immunoreactive cytochrome b5 proteins of approximately 12 and 16 kDa were found. Levels of cytochrome P450c17 or the high molecular weight cytochrome b5 in testis were not significantly correlated to levels of 16-androstene steroids in fat. However, levels of total cytochrome b5 immunoreactive protein and levels of the low molecular weight immunoreactive cytochrome b5 were correlated to fat 16-androstene steroid concentrations (r = .59, P < .001; r = .72, P = .0001, respectively). Levels of the low molecular weight immunoreactive cytochrome b5 were also correlated to 16-androstene steroid synthesis rates in vitro (r = .62, P < .05). These results indicate that increased levels of a low molecular weight immunoreactive cytochrome b5 protein, and not of cytochrome P450c17, are related to increased testicular 16-androstene steroid production and accumulation in fat. These results support the hypothesis that selection for reduced levels of this low molecular weight immunoreactive cytochrome b5 protein in the testis may result in decreased levels of 16-androstene steroids in fat and reduced boar taint in uncastrated male pigs.