Determination of iron in biological material by spectrophotometry of ferric perchlorate.

3. The presence of equimolar concentrations of pyridoxal phosphate prevented the inhibition by isoniazid. Stable hydrazones of isoniazid did not inhibit transaminase activity. These compounds, which were stable in the presence of heavy suspensions of B.C.G. organisms, had tuberculostatic activity comparable with isoniazid. 4. Transaminase activity was found in extracts of cells which had been exposed to isoniazid under bactericidal conditions. 5. It appears improbable that the bactericidal activity of isoniazid can be explained by its action on a transaminase or other enzyme requiring pyridoxal phosphate.