The contribution of muscle, kidney and splanchnic tissues to leucine transamination in humans

The first steps of leucine utilization are reversible deamination to α-ketoisocaproic acid (α-KIC) and irreversible oxidation. Recently the regulatory role of leucine deamination over oxidation was underlined in rodents. Our aim was to measure leucine deamination and reamination in the whole-body, in respect to previously determined rates across organs, in humans. By leucine and KIC isotope kinetics, we determined whole-body leucine deamination and reamination, and we compared these rates to those already reported across the sampled organs. As an in vivo counterpart of the "metabolon" concept, we analysed ratios between oxidation to either deamination or reamination. Leucine deamination to KIC was greater than KIC reamination to leucine in the whole-body (p=0.005), muscle (p=0.005) and the splanchnic area (p=0.025).These rates were not significantly different in the kidneys. Muscle accounted for ≈60% and ≈78%, the splanchnic bed for ≈15% and ≈15%, and the kidney for ≈12% and ≈18%, of whole-body leucine deamination and reamination rates, respectively. In the kidney, percent leucine oxidation over either deamination or reamination was >3-fold greater than muscle and the splanchnic bed. Skeletal muscle contributes by the largest fraction of leucine deamination, reamination and oxidation. However, in relative terms, the kidney plays a key role in leucine oxidation.