| Summary: | Sterols and steroids are relatively complex molecule^ which contain a number of asymmetric centres; numerous species are able to synthesise such molecules and transform them to a wide variety of related compounds. Thus many of the enzymes concerned in steroid hormone metabolisin offer unique opportunities as model systems for the study of substrate specificity. Although steroid hormones share certain basic structural characters the possession of specific conformations and configurations provides a physico-chemical basis for the observed differences in physiological action. In this discussion only certain aspects of the enzymic transformations of l?a-oestradiol, 17P-oestradiol and oestrone will be considered. These phenolic steroids are closely related by simple oxidation-reduction reactions, viz. 17a-oestradiol oestrone 17P-oestradiol The isolation and partial purification of a human placental 17P-oestradiol dehydrogenase by Langer and Sngel (1956) and subsequent kinetic studies by Langer, Alexander and Engel (1959) showed that this enzyme 1. had an absolute steric requirement for the 17P-hydroxyl group 2. required that the steroid substrate must possess a highly planar ring A or B or both for significant reactivity 3. interacted with the entire steroid surface. These findings were largely confirmed by Adams, Jarabak and Talalay (1962) using enzyme preparations of much higher specific activity. It was therefore of interest to extend these observations using a 17a-oestradiol dehydrogenase, thereby providing a more complete system for the investigtion of effects of substrate structure upon reaction kinetics. Such a model would also facilitate the experimental approach to studies of the mechanisrn(s) of these enzyme reactions.
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