Electrophoretic and kinetic properties of Rana pipiens lactate dehydrogenase isozymes

Lactate dehydrogenase of Rana pipiens exists in at least three isozymic forms. Extracts from heart tissue contain all three isozymes, and extracts from the gastrocnemius muscle contain only one isozyme. LDH extracted from testis is identical to that from the heart. The isozymes are tetramers of about 140,000 molecular weight. The five possible tetramers of LDH are not formed in frog preparations by the freeze‐thaw method applied so successfully to mammalian LDH. Kinetic data show that heart LDH exhibits substrate inhibition at lower substrate levels than does muscle LDH and that all the isozymes have a lower apparent Km value for pyruvate than for lactate. Thermal inactivation is a first order, unimolecular event with more heat energy required to inactivate the heart extract. Since inactivation of heart LDH occurred at a lower temperature and with considerably greater disruption of the molecule, the isozyme of muscle LDH has the more stable conformation. Synthesis of lactate dehydrogenase in this species appears to differ from the usual case of random association of subunits into tetramers. Homodimers may be formed first and then randomly associate to form homo‐ or hetero‐tetramers.