Malic dehydrogenese (MDH) activities and isozymes of various human organs were measured. The assays of isozyme were carried out by means of agargel electrophoresis. The highest MDH activity was demonstrated in heart muscle. Liver, skeletal muscle, kidney and brain showed considerably high MDH activities. Six fractions were distinguished in many MDH isozymograms. In order of movility towards the cathod, each MDH isozyme was named MD(1), MD(2)…MD(6). Intracellular distribution of each MDH isozyme was investigated by means of cell fractionation, and MD(1)～MD(5) were confirmed mitochondrial, MD(6) cytoplasmic, in origin. MDH isozyme-patterns of human organs were divided into 3 groups according to the ratio of mitochondrial MDH (m-MDH) and cytoplasmic MDH (c-MDH).
Group Ⅰ (m-MDH>c-MDH): heart muscle, kidney, skeletal muscle, white blood cell, red blood cell and lung-tissues.
Group Ⅱ (m-MDH≒c-MDH): liver, pancreas. Group Ⅲ (This group situates between Group Ⅰ and Group Ⅱ): brain, gastric mucosa, and spleen. Studies on inhibitory effect of p-chlor-mercuri-benzoate (PCMB) on each MDH isozyme and heat stability of each MDH isozyme demonstrated that m-MDH was not inhibited by PCMB and stable to a heat test at 50℃ for 30,' when 1-malate was used as substrate. It was also observed that m-MDH was activated by a high concentration of 1-malate. According to the standard deviation of MD(6) and MD(3) activites, c-MDH considered regulatory enzyme and m-MDH constitutive enzyme, respectively.