Mor2, Supernatant Malate Dehydrogenase, Is Linked to wa2 and Hba on Mouse Chromosome 11 in a Region of Homology with Human Chromosome 2p

The reversible oxidation of malate to oxaloacetate is catalyzed by malate dehydrogenase (EC 1.1.1.37). Two forms of the enzyme are known, one in the cytosol and one in the mitochondria, and each is integral to the malate-aspartate shuttle, a process vital for the complete oxidation of glucose in red skeletal muscle, heart muscle, and brain. In the mouse the cytosolic form is determined by the gene Mor2 and the mitochondrial form by Mor1. Mor1 has been positioned on Chromosome (Chr) 5 using an electrophoretic variant, but Mor2 has remained unmapped, for no variants were known until recently. We have induced a mutant allele Mor2 super(am1H) and used this to map Mor2 to Chr 11 in a region of homology with human chromosome 2. The original mutant arose in an experiment in which C3H/HeH males were given an intraperitoneal injection (250 mg/kg) of a point mutagen, ethylnitrosourea (ENU), and 2 months later mated to females of the Harwell mutation testing stock, PTP. Thus, any ENU-induced mutations would have arisen in spermatogonial stem cells. Offspring were screened for activity levels of 10 enzymes that occur in blood, and a female with 66% wildtype MOR2 activity was found. Breeding tests showed that this female was heterozygous, Mor2 super(a)/Mor2 super(am1H), for the wildtype and mutant alleles. The mean activity in heterozygotes (n = 40, range 44-65 U/g Hb) is 53% of wildtype (n = 65, range 83-124 U/g Hb). Backcrosses, Mor2 super(a)/Mor2 super(a) x Mor2 super(a)/Mor2 super(am1H), yielded 28 Mor2 super(a)/Mor2 super(a) and 36 Mor2 super(a)/Mor2 super(am1H). Intercrosses Mor2 super(a)/Mor2 super(am1H) x Mor2 super(a)/Mor2 super(am1H) gave 12 Mor2 super(a)/Mor2 super(a) and 29 Mor2 super(a)/Mor2 super(am1H). No Mor2 super(am1H)/Mor2 super(am1H) were recovered. For both backcrosses and intercrosses Mor2 super(a) was derived from C3H/HeH. These data suggest that Mor2 super(am1H) is a null or low activity allele and that homozygotes are lethal prenatally. Mor2 was assigned to Chr 11 following analysis of a linkage cross, designed to screen almost 50% of the genome, in which 11 Emv loci were segregating (8). Linkage with Emv14 on Chr 11 was found; in the cross Mor2 super(a)+/Mor2 super(a)+ female x (Mor2 super(a) Emv14/Mor2 super(am1H)+)F sub(1) male, there were 15 recombinants among 51 offspring, giving a recombination frequency of 29.4 plus or minus 6.4%. Two further linkage tests were then undertaken to position Mor2 more precisely.