MOLECULAR, GENETIC AND BIOCHEMICAL-CHARACTERIZATION OF LACTATE DEHYDROGENASE-A ENZYME-ACTIVITY MUTATIONS IN MUS MUSCULUS

Four independent heterozygous lactate dehydrogenase (LDH) mutations wi th approximately 60% of wild-type enzyme activity in whole blood have been recovered. The mutant line Ldhl(a2Neu) proved to be homozygous le thal, whereas for the three lines Ldhl(a7Neu), Ldhl(a11Neu), and Ldhl( a12Neu) homozygous mutants with about 20% residual activity occurred i n the progeny of heterozygous inter se matings. However, the number of homozygous mutants was less than expected, suggesting an increased le thality of these animals. Various physicochemical and kinetic properti es of LDI-I are altered. Exons of the Ldhl gene were PCR amplified and sequenced to determine the molecular Lesion in the mutant alleles. Ld hl(a2Neu) carried nrl AIT --> GIC transition in codon If:! (in exon 3) , resulting in an Asn --> Arp substitution; Asn112 is part of the heli x alpha D, which is involved in the coenzyme-binding domain. Ldhl(a7Ne u) contained an AIT --> C/G transversion within the codon for residue 194 in exon rf, causing an Asp --> Ala substitution. which may affect the arrangement of the substrate-binding sire. Three base substitution s were discovered for the mutation Ldhl(a11Neu) in exon 7: the transit ion C/G --> T/A, a silent mutation, and two transversions C/G --> AIT and C/G --> GIG, both missense mutations which led to the amino acid r eplacements Ala319 --> Glu and Thr321 --> Ser, respectively, located i n the alpha H helix structure of the COOH tail uf LDHA We suggest that thr mutation is the result of a gene conversion event between Ldhl(a) wild-type gene and the pseudogene Ldhl(a12Neu). The alteration Ile -- > Thr of codon 241 in exon 6 caused by the base pair change T/A --> C/ G was identified in the mutation Ldhl(a12Neu): Ile24 is included in th e heir alpha 2G, a structure that is indirectly involved in coenzyme b inding. Each of the sequence alterations has a potential impact on the structure of the LDHA protein, which is consistent with the decreased LDH activity and biochemical and physiological alterations.