Structural analogs of 5'-methylthioadenosine as substrates and inhibitors of 5'-methylthioadenosine phosphorylase and as inhibitors of human lymphocyte transformation

5'-Deoxy-5'-methylthioadenosine (MTA) phosphorylase was purified 13.4-fold from human peripheral lymphocytes. The enzyme demonstrated normal Michaelis-Menten kinetics with K_m values of 26 μM and 7.5 mM for the two substrates. MTA and phosphate, respectively. The rate of MTA degradation was temperature dependent, 47° being the optimum temperature. Five structural analogs served as alternative substrates with K_m values ranging from 31 to 53 μM while two compounds, 5'-deoxy-5'methylthiotubercidin (MTT) (K_i = 31 μM) and adenine (K_i = 172 μM), were inhibitory. These same analogs were examined as inhibitors of mitogen-induced human lymphocyte blastogenesis. MTT was found to be the most effective inhibitor of lymphocyte transformation with an I₅₀ of 80 μM.