CONSEQUENCES OF ALTERED ASPARTATE-AMINOTRANSFERASE ACTIVITY ON C-13-GLUTAMATE LABELING BY THE TRICARBOXYLIC-ACID CYCLE IN INTACT RAT HEARTS

The appearance of C-13 label in glutamate has been used to quantify cellular tricarboxylic acid (TCA) cycle activity using C-13-NMR spectroscopy. Glutamate is linked to the TCA cycle by the amino-transferase reactions, however the consequences of alterations in amino-transferase activity on glutamate labelling kinetics, at a constant total tricarboxylic acid cycle activity, have not been investigated. Aspartate amino-transferase activity in [2-C-13]acetate-perfused beating rat hearts was found to be similar to total TCA cycle flux in the presence of normal perfusion conditions and was reduced by more than 50% with the subsequent administration of amino-oxyacetic acid (AOA). AOA did not reduce contractile or kinetic measures of total TCA cycle flux, but did slow the C-13 labelling of glutamate, in accord with current mathematical predictions. The impact of similar reductions in amino-transferase activity on estimates of total TCA cycle flux derived from several previously reported methods was also evaluated. Because total TCA cycle and the amino-transferase activities both affect the kinetics of C-13-glutamate labelling and because the amino-transferase activities are often unknown under physiologic conditions and can be reduced under pathologic conditions, the calculation of total TCA cycle flux from C-13-NMR data in the future is probably best accomplished either with a sufficiently sophisticated mathematical model that assesses amino-transferase activity or with an empiric model that is relatively insensitive to variations in amino-transferase activity.