Rate at which glutamine enters TCA cycle influences carbon atom fate in intestinal epithelial cells

Glutamine carbon entry into the tricarboxylic acid (TCA) cycle was assessed in small intestinal epithelial cells by measuring CO2 production from [1-C -14]glutamine, and these data together with [U-C-14]glutamine data were use d to calculate fractional oxidation rates for glutamine. CO2 production fro m either [1-C-14]glutamine or [U-C-14]glutamine showed saturation kinetics, and the concentration needed to achieve the half-maximal rate of CO2 produ ction was 0.7 and 0.4 mmol/l, respectively. Maximal rate for [1-C-14]glutam ine was twice that for [U-C-14]glutamine. Increasing glutamine concentratio n did not cause proportional increases in glutamine entry into the TCA cycl e and glutamine oxidation. Consequently, fractional oxidation of glutamine decreased with increasing glutamine concentration. Fractional oxidation cou ld be predicted from the rate at which glutamine carbon entered the TCA cyc le. (Aminooxy)-acetic acid, an aminotransferase inhibitor, reduced entry of glutamine into the TCA cycle and increased fractional oxidation of glutami ne. Glutamate carbon entered the TCA cycle at about one-half the rate of gl utamine-derived glutamate carbon and had a higher fractional oxidation rate when provided at equivalent concentrations to glutamine. These differences in the rate of entry predictably account for the differences in the metabo lic fate of glutamine vs. glutamate carbon.