MECHANISMS OF TOXICITY OF 3'-AZIDO-3'-DEOXYTHYMIDINE

Recent experiments from our laboratory have indicated that the inhibit ory effect of 3'-azido-3'-deoxythymidine (AZT) on oxidative phosphoryl ation may occur directly in addition to being brought about by its inh ibition of mtDNA replication. We report here studies on the effect of AZT on adenylate kinase, an enzyme crucial to oxidative phosphorylatio n. AZT decreased the aromatic residues fluorescence of rabbit muscle a denylate kinase, indicating binding of AZT to the enzyme. Of three oth er enzymes studied as controls, AZT bound only to those that possessed ATP/ADP binding sites. Up to concentrations of 15 mu M, AZT was a mor e potent effector of fluorescence quenching than were ATP, ADP, AMP, a nd the AZT control, deoxythymidine. AZT strongly inhibited adenylate k inase in the direction of ATP synthesis (K-i, 8 mu M), the inhibition being of the partial competitive type, whereas deoxythymidine inhibiti on, also partially competitive, was much weaker (K-i 90 mu M). When me asured in the direction of ADP synthesis, AZT failed to demonstrate an y inhibition at concentrations up to 10 mu M. Experiments on isolated intact rat Liver mitochondria with the enzyme activity measured in bot h directions confirmed the isolated enzyme results. Respiratory contro l by these mitochondria was not affected by AZT. The finding of AZT af finity for ATP/ADP binding sites may open new avenues of approach to t he study of AZT toxicity.