Differential effects of amino acid and ketoacid on protein metabolism in humans

We examined the effects of insulin, amino acid (AA), and branched-chain ketoacid (KA) availability on leucine kinetics in eight healthy volunteers (age = 22 +/- 2 y, body mass index = 24 +/- 1 kg) by using the euglycemic insulin clamp and [1-C-14] leucine turnover techniques. Four experimental conditions were studied: study I, hyperinsulinemia; study IT, hyperinsulinemia with maintenance of basal plasma AA and branched-chain KA concentrations; study III, hyperinsulinemia with hyperaminoacidemia and basal plasma branched-chain KA concentrations; and study IV, hyperinsulinemia plus basal plasma AA concentrations and elevated branched-chain KA levels. Basal endogenous leucine flux (ELF) averaged 1.20 +/- 0.05 (mu mol . kg(-1) . min(-1), mean +/- SE); basal leucine oxidation (LOX) was 0.25 +/- 0.01; and basal non-oxidative leucine disposal (NOLD) was 0.95 +/- 0.04. UF significantly decreased in study I (0.77 +/- 0.06 mu mol . kg(-1) . min(-1), P < 0.01 versus basal). When plasma AA and branched-chain KA were either maintained at their basal levels (study II) or increased above baseline values (studies III and IV), ELF declined further (0.64 +/- 0.05, 0.66 +/- 0.02, and 0.66 +/- 0.03 mu mol . kg(-1) . min(-1), respectively; all Ps < 0.01 versus basal and P < 0.01 versus study I). LOX declined in study I (0.12 +/- 0.02 mu mol . kg(-1) . min(-1), P < 0.01 versus basal) but increased significantly in studies II, III, and IV (0.31 +/- 0.04, 0.37 +/- 0.03, and 0.40 +/- 0.03 mu mol . kg(-1) . min(-1), respectively, all Ps < 0.01 versus basal, P < 0.05 study IV versus study II, and P < 0.05 study III versus study Il). NOLD declined in study I (0.65 +/- 0.05 mu mol/kg . min, P < 0.01 versus basal), whereas neither the maintenance of basal plasma AA/branched-chain KA levels (study II; 0.89 +/- 0.2 mu mol . kg(-1) . min(-1)) nor the elevation of plasma branched-chain KA concentration (study IV; 0.96 +/- 0.1 mu mol . kg(-1) . min(-1)) increased NOLD above baseline level. A stimulation of NOLD was observed only when plasma AA levels were increased (study III; 1.23 +/- 0.03 mu mol/kg . min, P < 0.01 versus basal). In conclusion, the present data do not support the concept of a direct anabolic action of ketoanalogs but do provide additional evidence for the pivotal role of AA availability in the stimulation of whole-body protein synthesis. (C) Elsevier Science Inc. 2000.