Kinetics of glutamine-efflux from liver mitochondria loaded with the 14C-Labeled substrate.

Glutamine transport across the inner membrane of rat liver mitochondria was studied by the method of loading the organelles with [14C]glutamine and by measuring efflux of the metabolite at 0 degree C. The release of [14C]glutamine from loaded mitochondria was prevented by mersalyl, whereas the efflux was started by the addition of glutathione. The rate of glutamine efflux from the mitochondria was measured by the inhibitor stop technique with mersalyl plus N-ethylmaleimide. It was found that up to 10 mM glutamine there is no significant activity of glutaminase, whereas at about 20 mM of the substrate the enzyme is activated. The rate of the efflux measured after the addition of the optimal amount of glutathione was 10 nmol glutamine/min per mg protein. This is 5-times faster than the rate of glutaminase activity at 0 degree C. The pH optimum of glutamine carrier is between 6.5 and 7.0. Low concentration of succinate inhibits the efflux due to formation of pH gradient in coupled mitochondria, whereas a higher concentration of succinate inhibits the carrier directly. 2-Oxoglutarate and glutamate strongly inhibit the rate of glutamine efflux, the inhibition by glutamate being very pronounced at its physiological concentration. D-Glutamine does not inhibit the rate of the efflux, indicating that the transport of L-glutamine is stereospecific.