Glucose phosphate isomerase enzyme-activity mutants in Mus musculus: genetical and biochemical characterization

Two glucose-6-phosphate isomerase (GPI) mutants with approximately 60% residual activity in blood compared to wild type have been independently detected in offspring derived from 1-ethyl-1-nitrosourea-treated male mice. Homozygous mutants with about 20% residual activity were recovered in progeny of inter se matings of heterozygotes. However, in both mutant lines the number of homozygous mutants was less than expected suggesting an increased lethality of these animals. Results of linkage studies and of investigations of physiochemical properties of the mutant enzymes indicate point mutations at the Gpi-ls structural locus on chromosome 7. Based on these findings the two new alleles were designated Gpi-1sb-m1Neu and Gpi-1sb-m2Neu, respectively. The b-m1Neu allele codes for an erythrocyte enzyme which, in the homodimeric form, exhibits a decreased stability toward heat and urea, an altered isoelectric point, normal pH dependence, an increased Km for fructose-6-phosphate, and increased Ki's for 6-phosphogluconate and 2,3-diphosphoglycerate (2,3-DPG) compared to the wild-type enzyme. The GPI-1sb-m2Neu homodimer, in contrast, is characterized by an even stronger instability, slightly altered pH dependence, an increased Ki for 2,3-DPG, normal other kinetics, and normal isoelectric point. The different degree of stability of the mutant homodimers in vitro seems to be reflected in a different degree of stability in vivo, since GPI deficiency in general is more strongly expressed in the tissues of the homozygous Gpi-1sb-m2Neu mutant compared to the homozygous Gpi-1sb-m1Neu mutant. The similarity of the mutant enzymes to the allozymes found in human GPI deficiencies indicates the GPI deficient mouse mutants to be excellent models for the human disease.