Rate of whole-body protein synthesis in the rat as calculated from fractional oxidation of leucine, valine, or methionine.

In the steady state, the ratio of the rate of utilization for whole-body protein synthesis of any essential amino acid to its rate of oxidation should be the same as the ratio of the peak fraction of a dose of tracer incorporated into protein (F) to the fraction oxidized (1-F) provided that negligible tracer remains in the free amino acid pool or remains unabsorbed in the gut. The total rate of amino acid catabolism (C) can be estimated from the rate of urinary excretion of urea nitrogen (N) plus ammonia N. Hence the rate of whole-body protein synthesis (S) can be estimated as CF/(1-F). This method, which is not new, was explored as follows: (1) Radioactivity in the leucine of whole-body protein of rats after intravenous (IV) injection of labeled leucine was shown to plateau from three to nine hours. (2) The fractions of labeled leucine, valine, and methionine remaining in the gut six hours after enteral injection were 1.2 +/- 0.4% (SD), 1.2 +/- 0.4%, and 7.1 +/- 2.9%, respectively; thus, enterally administered methionine cannot be used for this purpose. (3) Oxidation of [1-14C]-labeled leucine or valine, measured from 14CO2 excretion, was found to be the same whether these isotopes were given IV or enterally. (4) Negligible radioactivity remained in the free leucine of plasma within one hour after injection of labeled leucine.(ABSTRACT TRUNCATED AT 250 WORDS)