EFFECTS OF FLOODING AMINO-ACIDS ON INCORPORATION OF LABELED AMINO-ACIDS INTO HUMAN MUSCLE PROTEIN

We investigated the effects of the nature of the flooding amino acid o n the rate of incorporation of tracer leucine into human skeletal musc le sampled by biopsy. Twenty-three healthy young men (24.5 +/- 5.0 yr, 76.2 +/- 8.3 kg) were studied in groups of four or five. First, the e ffects of flooding with phenylalanine, threonine, or arginine (all at 0.05 g/kg body wt) on the incorporation of tracer [C-13]leucine were s tudied. Then the effects of flooding with labeled [C-13]glycine [0.1 g /kg body wt, 20 atoms percent excess (APE)] and [C-13]serine (0.05 g/k g body wt, 15 APE) on the incorporation of simultaneously infused [C-1 3]leucine were investigated. When a large dose of phenylalanine or thr eonine was administered, incorporation of the tracer leucine was signi ficantly increased (from 0.036 to 0.067 %/h and 0.037 to 0.070 %/h, re spectively; each P < 0.01). However, when arginine, glycine, or serine was administered as a flooding dose, no stimulation of tracer leucine incorporation could be observed. These results, together with those p reviously obtained, suggest that large doses of individual essential, but not nonessential, amino acids are able to stimulate incorporation of constantly infused tracer amino acids into human muscle protein.