Affinity Labeling of Arginyl Residues at the Catalytic Region of Estradiol 17β‐Dehydrogenase from Human Placenta by 16‐Oxoestrone

Abstract

16-Oxoestrone inhibited competitively the activity of estradiol 17β-dehydrogenase from human placenta against estradiol in phosphate buffer (pH 7.2), suggesting reversible binding of 16-oxoestrone to the substrate-binding site. 16-Oxoestrone irreversible inactivated the estradiol 17β-dehydrogenase in borate buffer (pH 8.5) in a time-dependent manner, following pseudo-first-order kinetics. The rate constant (k₃) obtained for the inactivation by 16-oxoestrone was 8.3 × 10⁻⁴ s⁻¹. The rate of inactivation was significantly decreased by addition of estrone, estradiol, estriol, NAD(H) and NADP⁺. Also, the rate was reduced markedly by 2′AMP, 5′ATP and 2′,5′ADP, but not by NMN(H) and 3-pyridinealdehyde adeninediphospho nucleotide. The inactivation by 16-oxoestrone was neither prevented by sodium azide nor influenced by light. From these data, 16-oxoestrone, an χ-dicarbonyl steroid, was suggested to inactive estradiol 17β-dehydrogenase by modification of arginyl residues located around the substrate-binding site of the enzyme. Biphasic inactivation of the enzyme by 16-oxoestrone was observed with an increase of modified arginyl residues. The first phase of the inactivation was regarded as an affinity labeling of the arginyl residues at or near the substrate-binding site of the enzyme. Stoichiometry of the inactivation indicated that two arginyl residues were essential for maintenance of the enzyme activity. The second phase was considered as chemical modification of the arginyl residues outside of the catalytic region of the enzyme.