Kinetic analysis of enzymic activities: Prediction of multiple forms of 17β-hydroxysteroid dehydrogenase

An overview of the application of kinetic methods to the delineation of 17β-hydroxysteroid dehydrogenase (17β-HSD) heterogeneity in mammalian tissues is presented. Early studies of 17β-HSD activity in animal liver and kidney subcellular fractions were suggestive of multiple forms of the enzyme. Subsequently, detailed characterization of activity in cytosol and subcellular membrane fractions of human placenta, with particular emphasis on inhibition kinetics, yielded evidence of two kinetically-differing forms of 17β-HSD in that organ. Gene cloning and transfection experiments have confirmed the identity of these two proteins as products of separate genes. 17β-HSD type 1 is a cytosolic enzyme highly specific for C₁₈ steroids such as 17β-estradiol (E₂) and estrone (E₁). 17β-HSD type 2 is a membrane bound enzyme reactive with testosterone (T) and androstenedione (A), as well as E₂ and E₁. Useful parameters for the detection of multiple forms of 17β-HSD appear to be the E₂/T activity ratio, NAD/NADP activity ratios, steroid inhibitor specificity and inhibition patterns over a wide range of putative inhibitor concentrations. Evaluation of these parameters for microsomes from samples of human breast tissue suggests the presence of 17β-HSD type 2. The 17β-HSD enzymology of human testis microsomes appears to differ from placenta. Analysis of human ovary indicates granulosa cells are particularly enriched in the type 1 enzyme with type 2-like activity in stroma/theca. Mouse ovary appears to contain forms of 17β-HSD which differ from 17β-HSD type 1 and type 2 in their kinetic properties.