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 ce llular tricarboxylic acid (TCA) cycle activity using C-13-NMR spectros copy. Glutamate is linked to the TCA cycle by the amino-transferase re actions, however the consequences of alterations in amino-transferase activity on glutamate labelling kinetics, at a constant total tricarbo xylic acid cycle activity, have not been investigated. Aspartate amino -transferase activity in [2-C-13]acetate-perfused beating rat hearts w as found to be similar to total TCA cycle flux in the presence of norm al perfusion conditions and was reduced by more than 50% with the subs equent administration of amino-oxyacetic acid (AOA). AOA did not reduc e contractile or kinetic measures of total TCA cycle flux, but did slo w the C-13 labelling of glutamate, in accord with current mathematical predictions. The impact of similar reductions in amino-transferase ac tivity on estimates of total TCA cycle flux derived from several previ ously reported methods was also evaluated. Because total TCA cycle and the amino-transferase activities both affect the kinetics of C-13-glu tamate labelling and because the amino-transferase activities are ofte n unknown under physiologic conditions and can be reduced under pathol ogic conditions, the calculation of total TCA cycle flux from C-13-NMR data in the future is probably best accomplished either with a suffic iently sophisticated mathematical model that assesses amino-transferas e activity or with an empiric model that is relatively insensitive to variations in amino-transferase activity.