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Either the enzyme which oxidizes 3β-ol groups in C-19 and C-21 steroids is properly designated a steroid-3β-hydroxy dehydrogenase or there is a multiplicity of such dehydrogenases. Although DHA is a satisfactory substrate for the human ovary, adrenal, and placenta, it is not generally so for the human testis. Pregnenolone is only at times a moderately satisfactory substrate in the ovary and is not satisfactory for the testis. In the present histochemical study two Δ4,3β-ol steroids have been reported as satisfactory substrates for human endocrine tissues as well as liver and intestine. These new substrates make it possible to demonstrate activity in human testicular tissues and at probable sites of androgen production. If these tenets hold, they indicate that it is more satisfactory to use unphysiologic substrates for the histochemical demonstration of enzymes as this will obviate the interplay of homeostatic mechanisms which must constantly be at work to preserve the cellular equilibrium. Although the data might suggest that the human testis, in contrast to the ovary, is deficient in isomerizing capacity and that this step precedes oxidation of the 3β-ol group, the biochemical data do not support this theory. A more probable explanation is that either DHA, its reaction product 5-androstenedione, or both, inhibit the human testicular enzyme system, while progesterone inhibits that of the corpus luteum; thus a difference between the dehydrogenases of these two human tissues appears to exist.

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