Rapid activation of hepatic glutaminase in rats fed on a single high-protein meal.

We report that hepatic glutaminase is rapidly activated in rats fed on a single high-protein (60% casein) meal. Rats previously fed on a normal-protein (15% casein) diet for 3-4 days were given a high-protein meal for 2 h. The high-protein meal increased the rate of flux through glutaminase in intact liver mitochondria nearly 3-fold (20.6 +/- 1.7 nmol/min per mg of protein versus 7.5 +/- 2.9 nmol/min per mg of protein) at a P(i) concentration of 10 mM. The activation of flux through glutaminase by a high-protein meal involved an increased sensitivity of glutaminase to P(i), an activator of the enzyme. The Ka for P(i) was 1.0 mM and 24.1 mM in mitochondria from rats fed on the high-protein and normal-protein meals respectively. We measured the concentration of P(i) in the mitochondrial matrix and found that it did not differ in mitochondria from rats fed on the high-protein and normal-protein meals, suggesting that the effect of the high-protein meal on the P(i)-sensitivity of glutaminase was not due to a change in the distribution of P(i) across the mitochondrial inner membrane. Glutaminase activity was measured by using mitochondrial membranes from frozen-thawed mitochondria. Glutaminase activity and its dependence on P(i) were similar for preparations from rats fed on high-protein and normal-protein meals. These findings show that hepatic glutaminase is stimulated rapidly by a high-protein meal. This is part of the physiological hepatic response to increased protein intake which permits the liver to cope with the influx of glutamine occurring at this time.