PHOSPHATE-DEPENDENT GLUTAMINASE OF RAT SKELETAL-MUSCLE - SOME PROPERTIES AND POSSIBLE ROLE IN GLUTAMINE-METABOLISM

A relatively high activity (26.7 nmol/min per mg mitochondrial protein ) of phosphate-dependent glutaminase (EC 3.5.1.2; L-glutamine amidohyd rolase) was found in rat skeletal muscle (mixed type from hindlegs) mi tochondria incubated in 200 mM potassium phosphate (pH 8.2); the activ ity was lower in rat heart and diaphragm mitochondria. Phosphate-depen dent glutaminase was also found in human skeletal muscle mitochondria, but the activity was about 3-5 times lower than in rat skeletal muscl e. Multiplying the specific activity of mitochondrial glutaminase by t he amount of mitochondrial protein present in 1 g of rat skeletal musc le the maximum glutaminase activity was found to be 0.352 mumol/min pe r g wet tissue. The rat skeletal muscle enzyme appears to be similar i n many respects to phosphate-dependent glutaminase of the kidney (e.g. , S0.5 for glutamine, K0.5 for phosphate, the pH activity profile, inh ibition by glutamate). These properties make the skeletal muscle enzym e very similar to the 'kidney type' glutaminase isoenzyme of rat tissu es. A significant difference between rat kidney and skeletal muscle en zymes is their adaptive response during acidosis. While the kidney enz yme increases during acidosis, the skeletal muscle glutaminase activit y does not. A possible role of glutaminase in the glutamine metabolism in rat skeletal muscle is discussed.