1., Intravenous infusions of l-valine (600 μmol/min), l-isoleucine (150 μmol/min), l-leucine (300 μmol/min) and a mixture of the three branched-chain amino acids (70% l-leucine, 20% l-valine, 10% l-isoleucine; 270 μmol/min) were given to four groups of healthy volunteer subjects. Whole-blood concentrations of amino acids and glucose and serum insulin were measured before and during the infusions.
2. Valine and isoleucine infusions resulted in twelve- and six-fold increases in the respective amino acid. During valine infusion, tyrosine was the only amino acid for which a decrease in concentration was seen (25%, P < 0.05). With isoleucine administration, no significant changes were found. In contrast, leucine infusion (during which the leucine concentration rose about sixfold) was accompanied by significant decreases in tyrosine (35%), phenylalanine (35%), methionine (50%), valine (40%) and isoleucine (55%). The arterial glucose concentration fell slightly (5%) and the insulin concentration increased 20% during leucine infusion.
3. Infusion of the mixture of the three branched-chain amino acids resulted in marked decreases in tyrosine (50%), phenylalanine (50%) and methionine (35%). The decreased amino acid levels remained low for 2 h after the end of the infusion.
4. The present findings demonstrate that intravenous infusion of leucine (not infusion of valine or isoleucine) results in marked reductions in the concentrations of the aromatic amino acids and methionine. Infusion of a mixture of the three branched-chain amino acids gives results similar to those obtained with leucine infusion alone. Thus a mixed branched-chain amino acid solution with leucine as its main constituent seems to be the best alternative in the treatment of patients with hepatic cirrhosis and encephalopathy.