A null mutation of cytoplasmic malic enzyme in mice

In the course of conducting a biochemical screening program for mutant enzymes in mice, individuals with an apparent nonfunctional allele at the locus (Mod-1) responsible for cytoplasmic malic enzyme were observed. The variant, later attributed to a germinal mutation, was identified by starch gel electrophoresis and by enzyme activity measurements. A series of matings were made, and mice homozygous for the nonfunctional, null, allele (Mod-1) were produced. In liver, kidney, and testis homogenates, the homozygous mutant exhibited less than 10% of the enzyme activity of the control mice. By an enzyme immuno-inactivation study, the residual enzyme activity was shown to be mitochondrial malic enzyme in all of the tissues examined. By double immuno-diffusion experiments, the kidney homogenate of the mutant formed no precipitin lines with the antiserum to cytoplasmic malic enzyme. Thus, the null mutants express no proteins that crossreact with the antiserum to cytoplasmic malic enzyme (CRM negative). Tissue enzyme assays revealed no significant differences between the normal and the mutant mice in activities of other enzymes in the related metabolic pathways. Because malic acid and malic enzyme together are reported to serve as a pump for NADPH generation in cytoplasm, total cellular NADP⁺ and NADPH concentrations in liver were determined for the control and the mutant mice. In liver from two individual mutant mice, lower NADPH/NADP⁺ ratio was detected in comparison to the level in liver from control mice. In spite of the lower levels of NADPH in the mutant mice, their body weight and lipid content were not significantly altered. Mice without cytoplasmic malic enzyme exhibited no striking deficiencies in metabolism or viability.