Survey of normal appearing mouse strain which lacks malic enzyme and NAD(+)-linked glycerol phosphate dehydrogenase: Normal pancreatic beta cell function, but abnormal metabolite pattern in skeletal muscle

We studied a mouse doubly homozygous for mutations in the genes encoding ma lic enzyme (EC1.1.1.40) and cytosolic glycerol phosphate dehydrogenase (EC 1.1.1.8) (cGPD). This mouse, which we call the mmgg mouse and which is the product of intercrosses between the Mod-1 mouse and the BALB/cHeA mouse, la cks activity of both enzymes. Like both parental strains the mmgg mouse is completely normal in appearance. cGPD is one of the two enzymes that cataly ze the reactions of the glycerol phosphate shuttle. The activity of the oth er enzyme of the glycerol phosphate shuttle, mitochondrial glycerol phospha te dehydrogenase (EC 1.1.99.5) (mGPD), is abundant in tissues, such as brai n, skeletal muscle and the pancreatic islet, suggesting that the glycerol p hosphate shuttle is important in these tissues which rapidly metabolize glu cose. Cytosolic malic enzyme activity is important for shuttles which trans port NADPH equivalents from mitochondria to the cytosol. The major finding of the study was a highly abnormal metabolite pattern in tissues of the mmg g mouse suggesting a block in the glycerol phosphate shuttle due to cGPD de ficiency. The metabolite pattern did not suggest that malic enzyme deficien cy caused an abnormality. Tissue levels of glycerol phosphate (low) and dih ydroxyacetone phosphate (high) were only abnormal in skeletal muscle. Glyco lytic intermediates, situated at or before the triose phosphates in the pat hway, such as fructose bisphosphate and glyceraldehyde phosphate were incre ased depending on the tissue. Taken together with previous extensive data o n the mouse deficient only in cGPD this suggests a block in glycolysis at t he step catalyzed by glyceraldehyde phosphate dehydrogenase caused by an ab normally low NAD/NADH ratio resulting from a nonfunctional glycerol phospha te shuttle. Consistent with this idea the lactate/pyruvate ratio was high i n skeletal muscle signifying a low cytosolic NAD/NADH ratio. The mmgg mouse was normal in all other factors studied including blood glucose and serum insulin levels, pancreatic islet mass, insulin release from isolated pancre atic islets, as well as the activities of five metabolic enzymes, including mGPD, in liver, kidney, skeletal muscle and pancreatic islets. cGPD enzyme activity was undetectable in pancreatic islets, 0.5% of normal in liver, a nd 2.1% of normal in kidney and skeletal muscle. Malic enzyme activity was undetectable in these same tissues.