Protein turnover in muscle and the whole body

Abstract

The first experiment conducted was designed to determine the effect of lysine (lys) and threonine (thr) supplementation of wheat gluten on the growing rat. The results show that when wheat gluten, a poor quality protein, is supplemented with lys and thr, good rates of growth (5.68 g/day) and feed efficiency (.202 g of grain/g of feed) are obtained in rats. Lysine supplementation of gluten resulted in good rates of growth (3.18 g/day) and feed efficiency (.137 g of gain/g of feed) but not approaching that seen with lys and thr supplementation. Gluten and gluten+thr resulted in poor rates of growth (0.56 and 0.64 g/day, respectively) and feed efficiency (.031 and .037 g of gain/g of feed, respectively). The next experiment conducted had the purpose of determining the effect of dietary protein quality on rates of whole body protein synthesis and degradation and lys and thr reutilization in the growing rat. "Apparent" and "true" rates of whole body protein synthesis, degradation and net protein synthesis were determined using ('14)C-labeled lys and thr (apparent) and 2-('3)H labeled amino acids (true) in growing rats fed a gluten, gluten+lys or gluten+lys+thr diet. "Apparent" rates of protein synthesis, degradation and net protein synthesis were slower in nearly all cases than the rates determined by the {2-('3)H}amino acid method. Improving the quality of dietary protein not only increased rates of growth and nitrogen retention, but also increased "true" rates of whole body protein synthesis, degradation and net protein synthesis, although not always significantly. Increased rates of protein synthesis were also associated with increased lys and thr reutilization. Lysine reutilization was 45.7% (gluten) and 71.4% (gluten+lys+thr). Threonine reutilization was 26.4% (gluten+lys) and 36.9% (gluten+lys+thr). The final experiment was designed to test the possible regulatory effects of individual and groupings of essential amino acids, and several nonessential amino acids, on protein degradation in L6 muscle cells. A pulse-chase procedure was used to label cell protein and {('3)H}tyrosine was used as a non-metabolizable label. Results from this experiment show that nearly all amino acids result in small, if not always significant, inhibition of protein degradation when compared to controls. Inhibition of proteolysis observed for several amino acids and groupings of amino acids which had large effects were leucine (15.1%); methionine (14.5%) and histidine (13.6%) amino acids plus glutamine (29.8%) and the branched chain amino acids (19.07%).