Respective contribution of plasma branched-chain amino acids and 2-keto acids to the hepatic metabolism of the carbon moiety of branched-chain amino acids in fed rats.

The splanchnic balances of branched-chain amino acids (BCAA) and ketoacids (BCKA) concentrations were compared in rats fed a 15% (basal) or a 70% (HP) casein diet. Arterial BCAA concentrations were four- to fivefold and BCKA concentrations were twofold higher in rats fed HP diet than in rats fed basal diet. In rats fed basal diet, a higher proportion (45-58%) of BCKA was removed by the liver than of BCAA (7-12%). Thus, plasma BCKA were probably the major source of substrates for hepatic BCKA dehydrogenase. In rats fed HP diet, hepatic uptake of BCAA was 10 times greater than in rats fed basal diet, owing to their higher extraction; BCKA uptake was only twice as great; thus BCKA were a minor source of substrates for hepatic metabolism. In both diet groups, uptake of ketomethylvalerate (KMV) was less efficient than ketoisovalerate (KIV) or ketoisocaproate (KIC) uptake. Studies of metabolism of intraportal loads of BCKA showed that capacities for KIV and KIC uptake were high, KMV uptake plateaued at lower portal concentrations. With HP diets, the liver may be the major site of BCAA utilization; the interorgan transfer of the carbon moiety of BCAA to the liver as ketoacids does not keep pace with an increased availability of BCAA.