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
Mj. Macdonald et Lk. Marshall, 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, MOL C BIOCH, 220(1-2), 2001, pp. 117-125
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.