Phosphate-dependent glutaminase of rat skeletal muscle. Some properties and possible role in glutamine metabolism

Abstract

A relatively high activity (26.7 nmol/min per mg mitochondrial protein) of phosphate-dependent glutaminase (EC 3.5.1.2; L-glutamine amidohydrolase) was found in rat skeletal muscle (mixed type from hindlegs) mitochondria incubated in 200 mM potsssium phosphate (pH 8.2); the activity was lower in rat heart and diaphragm mitochondria. Phosphate-dependent glutaminase was also found in human skeletal muscle mitochondria, but the activity was about 3–5 times lower than in rat skeletal muscle. Multiplying the specific activity of mitochondrial glutaminase by the amount of mitochondrial protein present in 1 g of rat skeletal muscle the maximum glutaminase activity was found to be 0.352 μmol/min per g wet tissue. The rat skeletal muscle enzyme appears to be similar in many respects to phosphate-dependent glutaminase of the kidney (e.g., S 0.5 for glutamine, K 0.5 for phosphate, the pH activity profile, inhibition by glutamate). These properties make the skeletal muscle enzyme very similar to the ‘kidney type’ glutaminase isoenzyme of rat tissues. A significant difference between rat kidney and skeletal muscle enzymes is their adaptive response during acidosis. While the kidney enzyme increases during acidosis, the skeletal muscle glutaminase activity does not. A possible role of glutaminase in the glutamine metabolism in rat skeletal muscle is discussed.