Recovery of (13)CO(2) from infused [1-(13)C]leucine and [1,2-(13)C(2)]leucine in healthy humans.

Carbon (C) in the 1-position of leucine is released as CO(2) with the decarboxylation of alpha-ketoisocaproate (KIC). Carbon in the 2-position of leucine undergoes several additional metabolic steps before entering the tricarboxylic acid (TCA) cycle in the 1-position of acetyl-CoA, where it can be released as CO(2) or be incorporated into other compounds. This study examined the metabolic fate of C in the 2-position of leucine. We infused 11 healthy subjects with [1-(13)C]leucine and [1,2-(13)C(2)]leucine for 3.5--4 h to measure leucine kinetics and the oxidation of the tracers from enrichments of (13)C in blood and expired CO(2). The fraction of leucine infused that was oxidized (f(ox)) was used to define the degree of recovery of the (13)C label(s) for each tracer. As expected, leucine appearance (means +/- SE) did not differ between tracers ((13)C(1): 92.1 +/- 3.1 vs. (13)C(2): 89.2 +/- 3.2 micromol x kg(-1) x h(-1)) when calculated using plasma leucine enrichments as an index of intracellular enrichment. A small (3%) but significant (P = 0.048) difference between tracers was found when KIC was used to calculate leucine appearance ((13)C(1): 118.0 +/- 4.1 vs. (13)C(2): 114.4 +/- 4.5 micromol x kg(-1) x h(-1)). The value of f(ox) was 14 +/- 1% for [1,2-(13)C(2)]leucine and was lower than the f(ox) for [1-(13)C]leucine (19 +/- 1%). From the f(ox) data, we calculated that the recovery of the 2-(13)C label in breath CO(2) was 58 +/- 6% relative to the 1-(13)C label. These findings show that, although a majority of the 2-(13)C label of leucine is recovered in breath CO(2), a significant percentage (approximately 42%) is retained in the body, presumably by transfer to other compounds, via TCA exchange reactions.