Pyrazinamide is a
prodrug that stops the growth of
M. tuberculosis. Pyrazinamide diffuses into the granuloma of
M. tuberculosis, where the tuberculosis enzyme
pyrazinamidase converts pyrazinamide to the active form
pyrazinoic acid. Under acidic conditions of pH 5 to 6, the pyrazinoic acid that slowly leaks out converts to the protonated conjugate acid, which is thought to diffuse easily back into the bacilli and accumulate. The net effect is that more pyrazinoic acid accumulates inside the bacillus at acid pH than at neutral pH. Pyrazinoic acid was thought to inhibit the enzyme
fatty acid synthase (FAS) I, which is required by the bacterium to synthesize
fatty acids although this has been discounted. The accumulation of pyrazinoic acid was also suggested to disrupt membrane potential and interfere with energy production, necessary for survival of
M. tuberculosis at an acidic site of infection. However, since an acidic environment is not essential for pyrazinamide susceptibility and pyrazinamide treatment does not lead to intrabacterial acidification nor rapid disruption of membrane potential, this model has also been discounted. Pyrazinoic acid was proposed to bind to the ribosomal protein S1 (RpsA) and inhibit
trans-translation, but more detailed experiments have shown that it does not have this activity. The current hypothesis is that pyrazinoic acid blocks synthesis of
coenzyme A. Pyrazinoic acid binds weakly to the
aspartate decarboxylase PanD, triggering its degradation. This is an unusual mechanism of action in that pyrazinamide does not directly block the action of its target, but indirectly triggers its destruction.
Resistance Mutations in the
pncA gene of
M. tuberculosis, which encodes a pyrazinamidase and converts pyrazinamide to its active form pyrazinoic acid, are responsible for the majority of pyrazinamide resistance in
M. tuberculosis strains. A few pyrazinamide-resistant strains with mutations in the
rpsA gene have also been identified. Further, this strain was found to be susceptible to pyrazinamide in a mouse model of tuberculosis. Thus, current data indicate that
rpsA mutations are not likely to be associated with pyrazinamide resistance. Currently, three main methods of testing are used for pyrazinamide resistance: 1) phenotypic tests where a tuberculosis strain is grown in the presence of increasing concentrations of pyrazinamide, 2) measuring levels of pyrazinamidase enzyme produced by the tuberculosis strain, or 3) looking for mutations in the
pncA gene of tuberculosis. Global resistance of tuberculosis to pyrazinamide has been estimated to be in 16% of all cases, and 60% of people with
multidrug-resistant tuberculosis. == Abbreviations ==