Adult male gonads of the cricket(Gryllus domesticus) contain a 17β-hydroxysteroid dehydrogenase system, transformingtestosterone to androstenedione.4 Theenzymewas localized in the 105,000 Xg supernatantfraction of gonad homogenates and requiredNADP for maximal activity at a pH of 9.1. Thesystem had a Km of 1.9X10–7M, a Q10 of 1.91(optimal temperature: 38 C), and the solublefraction originating from 0.6 mg of gonad tissue(wet wt) was saturated by 1 n mole of testosterone.The system showed specificity for the C-17β-oriented secondary alcohol group. The presenceof Na+, K+, Ca++ or Mg++ was not requiredfor maximal activity. On a tissue-weightbasis, cricket leg muscle, intestine, fatty tissueand cuticle homogenates showed 10.6, 58.6, 22.3and 48.2% of the dehydrogenase activity exhibitedby gonad homogenates under identicalexperimental conditions. However, these data donot prove the synthesis of steroid hormones byinsect tissues or that testosterone is the preferentialsubstrate of the enzyme described.(Endocrino...