Inactivation of S-Adenosylhomocysteine hydrolase by 5′-deoxy-5′-methylthioadenosine

S-Adenosylhomocysteine hydrolase from human red blood cells is inactivated by the naturally occurring metabolite, 5′-deoxy-5′-methylthioadenosine. Utilizing an 1896-fold purified enzyme preparation, the kinetics of inactivation by 5′-deoxy-5′-methylthioadenosine were examined and a K_I value of 36 μM determined. Neither 5-methylthioribose nor adenine, degradation products of 5′-deoxy-5′-methylthioadenosine, inactivated the hydrolase, while both S-adenosylhomocysteine and adenosine protected the enzyme from inactivation by 5′-deoxy-5′-methylthioadenosine. The inactivation of S-adenosylhomocysteine hydrolase by 5′-deoxy-5′-methylthioadenosine may explain the cell growth inhibitory properties of this nucleoside.