Chemical modification of tyrosine residues in active-site of human placental estradiol 17β-dehydrogenase by tetranitromethane

Abstract

The native estradiol 17 beta-dehydrogenase purified from human placenta was rapidly inactivated by tetranitromethane, which is a reagent for chemical modification of tyrosine and cysteine residues. The inactivation followed pseudo-first-order reaction kinetics, and the activity was partially recovered by addition of dithiothreitol. On the other hand, the enzyme sulfhydryl-blocked by 5,5'-dithiobis(2-nitrobenzoic acid) was treated with tetranitromethane, and the activity was assayed in the presence of dithiothreitol. Tetranitromethane inactivated the enzyme in a time-dependent manner, following pseudo-first-order kinetics. The rate of inactivation was significantly decreased by addition of NADP(H), 2'-AMP, 5'-ATP, 2',5'-ADP, 3-pyridine aldehyde-DPN and 3-acetylpyridine-DPN(reduced form). These results suggest that the tyrosyl residues are located at or near the cofactor-binding site of the estradiol 17 beta-dehydrogenase and play an essential role in the catalytic function of the enzyme.