LIMITATIONS IN THE USE OF [2-C-14]ACETATE FOR MEASURING GLUCONEOGENESIS INVIVO

This study was undertaken to test two assumptions critical for use of [2-C-14]acetate to measure gluconeogenesis in vivo. For the assumption that incorporation into glucose of products of [C-14]acetate metaboli sm does not affect the distribution of label within the glucose molecu le, we infused [2-C-14]acetate in 17 healthy subjects and [3-C-14]lact ate in 10 healthy subjects and compared the ratio of the resultant spe cific activities of plasma glucose carbons 1, 2, 5, 6, and 3, 4 obtain ed with each tracer. The ratio obtained with [2-C-14]acetate (2.99 +/- 0.07) was significantly different from the ratio obtained with [3-C-1 4]lactate, (3.82 +/- 0.2, P < 0.01). Because the model predicts that t hese ratios should be identical, these results indicate that either th e model is incorrect or that metabolism of [C-14]acetate to other comp ounds affects the distribution of the label within the glucose molecul e. To test the assumption that plasma 3-OH-butyrate specific activity approximates the specific activity of hepatic intramitochondrial acety l CoA, we compared the ratio of specific activities of plasma glucose and 3-OH-butyrate obtained in 7 healthy subjects infused with [2-C-14] acetate and [2-C-14] octanoate. The ratio obtained with [2-C-14]acetat e (0.18 +/- 0.03) was significantly different from that obtained with [2-C-14]octanoate, (0.10 +/- 0.02), P < 0.001. These results suggest c ompartmentalization of acetyl CoA within liver mitochondria and indica te that plasma 3-OH-butyrate specific activity may not necessarily app roximate intramitochondrial acetyl CoA specific activity during [2-C-1 4]acetate infusion. We conclude that assumptions critical for use of [ 2-C-14]acetate to measure gluconeogenesis in vivo are not valid.