Occurrence of two forms of H2M2 isoenzyme of lactate dehydrogenase

Serum from healthy human subjects and serum from animals demonstrated a splitting of the H2M2 isoenzyme fraction of lactate dehydrogenase (LDH) into two subbands on polyacrylamide gel electrophoresis. These subbands were designated H2M2-a and H2M2-b, based on their relative electrophoretic mobilities, with the H2M2-a band being more anodal. The doublet of the H2M2 band was also demonstrated in human erythrocytes and in the hybridization mixture from porcine LDH, indicating the production of two forms of H2M2 isoenzyme in tissues. Supportive evidence for the presence of two H2M2 subforms was obtained by ion-exchange chromatography of the hybridization mixture. The recombination experiments of LDH-binding IgA with both normal human serum and LDH M subunit variant serum indicated the difference of molecular structure between the two subforms. On the basis of the results obtained in this study, a theoretical model of the probable molecular structure of LDH isoenzymes is proposed which could account for both the occurrence of two subforms of H2M2 and their selective binding to LDH-linking IgA.