Studies on the biochemical actions of 6-selenoguanine and 6-selenoguanosine

Survival studies were performed in mice bearing Sarcoma 180 ascites tumor treated with 6-thio and 6-seleno analogs of guanine and guanosine. The selenium-containing analogs were somewhat superior to the sulfur-containing compounds in antitumor activity and therapeutic index. The formation of 6-SeGMP from 6-seleno-guanine (6-SeG) was demonstrated in Sarcoma 180 ascites cells. Guanine, 6-thioguanine (6-TG) and 6-SeG show comparable substrate activity whereas 8-azaguanine is a much poorer substrate for hypoxanthine-guanine phosphoribosyl transferase from Sarcoma 180 cells. Both 6-TG and 6-SeG are good substrates for purine nucleoside phosphorylase from Sarcoma 180 cells. Chemically and enzymatically synthesized 6-SeGMP behaved as a competitive inhibitor (K_i 1 × 10^?4 M) of erythrocytic and Sarcoma 180 guanylate kinases. Weak substrate activity was demonstrated in the presence of large amounts of erythrocytic guanylate kinase.