Erythrocytic Nucleoside Diphosphokinase

III. STUDIES WITH FREE AND PHOSPHORYLATED ENZYME AND EVIDENCE FOR AN ESSENTIAL THIOL GROUP

From the ¹ From the Division of Biological and Medical Sciences, Brown University, Providence, Rhode Island 02912

Continuing studies with erythrocytic nucleoside diphosphokinase suggest that enzyme preparations of specific activity of 100 µM units per mg of protein are near homogeneity, and that the molecular weight of the enzyme is approximately 100,000. Evidence is presented which confirms previous findings that the enzyme becomes phosphorylated by reaction with adenosine triphosphate. The occurrence of one or more essential thiol group(s) in the region of the catalytic site(s) is noted. Inactivation of the thiol(s) may be prevented by the presence of a nucleotide substrate or inhibitor. Nucleoside diphosphokinase inactivated by reaction with mercuribenzoate may be reactivated by incubation with the thiol reagent, dithiothreitol. Erythrocytic nucleoside diphosphokinase catalyzes exchange between ADP and ATP, but not between orthophosphate and ATP.

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