The action of isoniazid on the transaminases of Mycobacterium tuberculosis (B.C.G.)

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Abstract
No qualitative differences in the range of transaminase activities of isoniazid-sensitive and resistant strains of M. tuberculosis (BCG) were found. The enzymes in extracts of sensitive and resistant organisms were equally sensitive to isoniazid. Below a concentration of 0.05 [image], the amount of a-oxoglutarate employed in estimating the aspartic-glutamic transaminase activity affected the rate of the reaction and the degree of inhibition by a given concentration of isoniazid. The presence of equimolar concentrations of pyridoxal phosphate prevented the inhibition by isoniazid. Stable hydrozones of isoniazid did not inhibit transaminase activity. These compounds, which were stable in the presence of heavy suspensions of BCG organisms, had tuberculostatic activity comparable to that of isoniazid. Transaminase activity was found in extracts of cells which had been exposed to isoniazid under bactericidal conditions. It appears improbable that the bactericidal activity of isoniazid can be explained by its action on a transaminase or other enzyme requiring pyridoxal phosphate.