ORGAN SPECIFICITY AND LACTATE-DEHYDROGENASE ACTIVITY. DIFFERENTIAL INHIBITION BY UREA AND RELATED COMPOUNDS.

1. The effect of urea on the lactate-dehydrogenase activities of human-heart and -liver tissue extracts and on crystalline ox-heart and rabbit-muscle enzyme have been determined. Similar studies on electrophoretically separated isoenzyme fractions have shown an inverse relationship between sensitivity to urea inhibition and electrophoretic mobility. 2. With pyruvate as substrate a sharp change in the nature of the inhibition of tissue lactate dehydrogenase with increasing concentrations of urea occurs at 1 m or 4 m with the electrophoretically slow and fast isoenzymes respectively. 3. At concentrations of urea less than 1 m, inhibition of the purified enzymes is competitive with respect to pyruvate and 2-oxobutyrate. 4. Similar studies have been carried out with methylurea and hydantoic acid, both of which are more potent inhibitors than urea.