Human 17 beta-hydroxysteroid dehydrogenase (17-HSD) type 1 catalyzes the conversion of the low activity estrogen, estrone, into highly active estradiol, both in the gonads and in target tissues. The present study was carried out to characterize the dimerization, microheterogeneity, and phosphorylation of human 17-HSD type 1 and to evaluate the current model of hydride transfer and substrate recognition of the enzyme, based on its x-ray structure. 17-HSD type 1 is a homodimer consisting of noncovalently bound subunits, and the data in the present study indicate an exceptionally strong association between the monomers [dissociation constant (Kd) < 5 pmol/monomers liter]. Furthermore, substitutions constructed at the hydrophobic dimer interface always resulted in inactive aggregates of the protein. The enzyme was shown to be phosphorylated by protein kinase A exclusively at Ser134 only in vitro. However, in contrast to previous suggestions, phosphorylation of Ser134 was shown to play no role in the activity or microheterogeneity of human 17-HSD type 1. The presence of microheterogeneity in the recombinant enzyme also indicates that it does not result from the frequent protein polymorphism previously found for the enzyme. In line with the x-ray structure and the proposed catalytic mechanism of the enzyme, our results indicate that Ser142, Tyr155, and Lys159 are all critical for hydride transfer in human 17-HSD type 1. In contrast, the proposed interaction between His221, Glu282, and the 3-OH group of the steroid at the substrate recognition helix could not be shown to exist. Neither of these residues plays a critical role in the catalytic action of the enzyme in cultured cells.