The mechanism of the synthesis of 16-androstenes in human testicular homogenates.

The biochemical pathway leading to the 16-unsaturated C19 steroids--known as sex pheromone (precursors) in pig and man--is still a matter of dispute. In the 16-ene-synthetase process, via which 5,16-androstadien-3 beta-ol (ADL) or 4,16-androstadien-3-one (ADN) are biosynthesized from pregnenolone (P5) or progesterone (P4), a number of 2 or even 3 step conversions have been suggested in porcine tests, including 20 beta-reduction, 21-hydroxylation and 16,17-dehydrogenation. Studying the 16-ene-synthetase reaction in human testicular homogenates, we adduced evidence for the hypothesis that ADL is synthesized from P5 in a single step, not requiring separate intermediates. Our proposal for the 16-ene-synthetase mechanism also explains why, at least in our hands, synthesis of ADL is always accompanied by co-synthesis of its satellite 5-androstene-3 beta,17 alpha-diol (epiA5): both steroids are synthesized as a mere consequence of the fact that the proposed elimination and substitution reactions for the synthesis of ADL and epiA5, respectively, are competitive processes.