Molecular heterogeneity of the enzyme was determined on the basis of kinetic data and electrophoretic isolation on a starch block. The uninfected red blood cell showed a major anodal and a minor cathodal peak of lactic dehydrogenase activity, and was further characterized by a kinetic constant representing a high pH optimum with low concentrations of substrate. Isolated P. lophurae had a single, cathodal peak of activity dissimilar from that of the uninfected erythrocyte, and a kinetic constant describing a low pH optimum with a high concentration of substrate. Infected erythrocytes showed a combination of these electrophoretic entities and an intermediate range of kinetic constants.

The data indicate that the avian malaria parasite P. lophurae contains a lactic dehydrogenase qualitatively dissimilar from that of its host cell, and the increased enzymatic activity of infected erythrocytes is a result of the enzyme content of the growing parasite added to that of the red blood cell. It is suggested that the LDH of the parasite has a physiological advantage under those conditions which prevail inside the red blood cell.