Characterization of two electrophoretic lactate dehydrogenase-A mutants in Mus musculus

Two lactate dehydrogenase (LDH) mutations were recovered independently among offspring of ethylnitrosourea-treated male mice by screening for alterations of isoelectric focusing pattern in liver homogenates. Investigations of physicochemical and kinetic properties of the mutant enzymes indicated that the mutant traits resulted from point mutations at the Ldh-1 structural locus. Therefore, the new alleles were designated Ldh-1a-m5Neu and Ldh-1a-m6Neu, respectively. Both mutant alleles code for proteins which exhibit an altered stability to heat, in addition to changes in isoelectric focusing pattern and a reduction in anodal electrophoretic mobility. While LDH-Aa-m5Neu proteins are markedly less heat stable, LDH-Aa-m6Neu proteins are more heat stable than the wild-type enzyme. Furthermore, a small elevation of Km for pyruvate, a slightly reduced inhibition by high pyruvate concentrations, and a slight acidic shift of the pH activity profile distinguish LDH-Aa-m6Neu from both wild-type and LDH-Aa-m5Neu enzymes. Significant alterations of LDH activity were detected in some tissues from LDH-Aa-m5Neu individuals but not in those from LDH-Aa-m6Neu animals. Erythrocytes and blood of LDH-Aa-m5Neu mutants revealed activity levels which were reduced by approximately 6 and 13% compared with those of wild types in heterozygous and homozygous individuals, respectively. In addition, an elevation of approximately 6% in LDH activity was found in skeletal muscle in homozygous mutants. Consistent with the unaltered or only slightly altered LDH activity in tissues, the genetic as well as the physiological characterization yielded no easily detectable effects from either mutation on metabolism or fitness of the affected individuals.