Effects of Pyrazinamide on Fatty Acid Synthesis by Whole Mycobacterial Cells and Purified Fatty Acid Synthase I

The effects of low extracellular pH and intracellular accumulation of weak organic acids were compared with respect to fatty acid synthesis by whole cells of Mycobacterium tuberculosis and Mycobacterium smegmatis. The profile of fatty acids synthesized during exposure to benzoic, nicotinic, or pyrazinoic acids, as well as that observed during intracellular hydrolysis of the corresponding amides, was not a direct consequence of modulation of fatty acid synthesis by these compounds but reflected the response to inorganic acid stress. Analysis of fatty acid synthesis in crude mycobacterial cell extracts demonstrated that pyrazinoic acid failed to directly modulate the fatty acid synthase activity catalyzed by fatty acid synthase I (FAS-I). However, fatty acid synthesis was irreversibly inhibited by 5-chloro-pyrazinamide in a time-dependent fashion. Moreover, we demonstrate that pyrazinoic acid does not inhibit purified mycobacterial FAS-I, suggesting that this enzyme is not the immediate target of pyrazinamide.