Effect of the level of dietary protein on the utilization of alpha-ketoisocaproate for protein synthesis.

The efficiency of alpha-ketoisocaproate (KIC) as a dietary substitute for leucine in rats on varying protein intake was estimated by an isotopic method, previously shown to yield the same results as comparative growth experiments. 14C-KIC and 3H-leucine are injected orally. Six hours later the ratio, R, of 14C/3H in isolated proteins, divided by the same ratio in the injectate is measured. This ratio has been shown to be approximately equal to nutritional efficiency of KIC relative to leucine. As dietary protein increased from 6.3% to 48.3%, whole body protein R decreased from 0.515 +/- 0.045 to 0.299 +/- 0.016. Variations with protein intake were noted in R of protein isolated from individual organs. The magnitude of R in these organs varied two-fold, in the following sequence: brain greater than heart greater than or equal to skeletal muscle greater than or equal to salivary gland greater than or equal to kidney greater than liver. Whole body protein R could be confidently predicted (r2 = 0.992) from R in the protein of kidney and muscle. Thus the nutritional efficiency of KIC as a dietary substitute for leucine in individual organs as well as in the whole animal is strongly dependent on the level of protein intake.