The extent to which pyridoxal-5'-phosphate stimulates the activities of aspartate aminotransferase and its cytoplasmic and mitochondrial isoenzymes was measured in six human left ventricular biopsies obtained freshly during open-heart surgery, and in 13 human left ventricular autopsies. A concentration of pyridoxal-5'-phosphate of 15 mumol/l for 1 h is sufficient to convert any apoenzyme to holo-enzyme. Pyridoxal-5'-phosphate-induced stimulation of aspartate aminotransferase is 26 +/- 6% (+/- SD) in myocardial biopsies (range 22-34%) and 112 +/- 54% in myocardial autopsies (range 35-200%). The extent of stimulation of the cytoplasmic and mitochondrial isoenzymes is 18 +/- 9% and 32 +/- 6%, respectively, in myocardial biopsies, and 150 +/- 57% and 100 +/- 62%, respectively, in myocardial autopsies. The greater extent and variation of the pyridoxal-5'-phosphate-induced stimulation of aspartate aminotransferase and its isoenzymes in myocardial autopsies compared to that in myocardial biopsies is caused by autolysis and its duration. Autolysis depresses myocardial aspartate aminotransferase activity measured in the absence of pyridoxal-5'-phosphate which effect is more prominent for the cytoplasmic than for the mitochondrial isoenzyme.