Possibility for the transfer of reducing equivalents from the cytosol to the mitochondrial compartment in Ehrlich ascites tumor cells by the malate-aspartate shuttle.

Abstract

Intensive oxidation of glutamine in Ehrlich ascites tumor cells results in a high intracellular accumulation of aspartate. Since Km of aspartate aminotransferase for aspartate is high it was assumed that the level of aspartate may be the rate‐limiting step in the operation of the malate‐aspartate shuttle. In that case the oxidation of glutamine would stimulate or activate the shuttle. However, it was found that if aerobic glycolysis takes place in the presence of glutamine the production of lactate was the same whereas the accumulation of aspartate by glutamine oxidation was strongly inhibited. Preincubation of the cells with glutamine and the addition of glucose at the moment when the level of aspartate was high and the level of glutamate was low brought about significant decrease in lactate production suggesting activation of the shuttle. This was supported by the experiments with an oxygen electrode. Namely, if the cells were preincubated with glutamine the addition of lactate stimulated oxygen consumption in the presence of arsenite. This effect of lactate was lacking or abolished in the presence of pyruvate, aminooxyacetate or rotenone.These and other experimental findings suggest that low level of aspartate is the limiting step in the operation of the malate‐aspartate shuttle in the strain of Ehrlich ascites cells studied. Analysis of all experimental data indicates that the shuttle probably does not normally operate in these cells since in the presence of glucose the accumulation of aspartate was strongly inhibited. This together with high level of glutamate strongly suppresses the activity of aspartate aminotransferase in the direction of oxaloacetate production in the cytosol.