Native estradiol 17 beta-dehydrogenase (EC 1.1.1.62) from human placenta was inactivated in time dependent manner by p-nitrobenzenesulfonyl fluoride (NBSF), which is a reagent for chemical modification of tyrosine. The sulfhydryl-blocked enzyme by 5,5'-dithio-bis(2-nitrobenzoic acid) (DTNB) was also reacted with NBSF more slowly in pseudo-first-order kinetics. After the sequential treatments with DTNB, NBSF and dithiothreitol (DTT), the enzyme in which tyrosine residues alone were modified was isolated, and its activity was decreased. These results suggest that tyrosyl residues of the estradiol 17 beta-dehydrogenase from human placenta are located at or near its catalytic site, and play a functional role in the enzyme reaction.
17-Hydroxysteroid Dehydrogenases Binding Sites