Isoenzymic forms of NAD-linked glycerol-3-phosphate dehydrogenase from rabbit brain

Two major enzyme forms of cytosolic NAD-linked glycerol-3-phosphate dehydrogenase in rabbit brain have been purified to apparent homogeneity. One major enzyme form designated I_6.5 exhibits an iso-electric point at pH 6.5, and is indistinguishable from the major form I_6.5 found in other tissues. The other major form, designated I_5.9, has an isoelectric point at pH 5.9, and by amino acid analysis is shown to be a true isoenzyme distinct from form I_6.5. Form I_5.9 appears to be closely related to or identical with the major enzyme characteristic of heart. Neither the brain enzyme form I_5.9 nor the major heart isoenzyme are inhibited by antiserum to the muscle enzyme. Because of the high apparent K_m for NADH, it is postulated that the brain isoenzyme I_5.9 serves to maintain glycolysis when NADH levels rise under relatively anaerobic conditions especially during fetal and neonatal development.