Compartmentation of free valine and its relation to protein turnover in perfused rat liver.

Livers from nonfasted rats were perfused in situ with concentrations of valine (plus [I-14C]valine) in the medium ranging from 0.3 to 15 m&r. The relationship of total free intracellular valine (y) to external valine (x) was of the form y = a + bx, where the slope was about 1.0 and the intercept nearly 0.4 mM. At low valine concentrations the ratio of intracellular to extracellular valine, after isotopic equilibrium was attained, was less than 0.5; as external valine was increased, the ratio increased and approached unity. These findings suggested that the intracellular valine pool comprised two components, one which readily equilibrated with extracellular valine and another which was independent of external valine. Rates of valine incorporation into liver protein, calculated from the average specific activity of total intracellular valine, were not constant over the concentration range of external valine, as expected, but were 2to 3-fold greater at the lowest concentrations than at the highest levels. This lack of constancy was explained by assuming that the two valine components represented two separate pools and that precursor sites of protein synthesis were in continuity with the expandable or equilibrating pool. Thus if label transported inward from the medium were restricted to the expandable pool, its specific activity and that of precursor amino acids would be higher than the determined specific activity of total intracellular valine. Recalculation of valine incorporation, based on the presumed specific activity of the expandable pool, gave uniform rates at all external valine concentrations. The second or nonexpandable pool, which apparently failed to equilibrate with label in the medium, was thought to arise from intracellular proteolysis since total intracellular and extracellular specific activities were the same when the only source of free label was the breakdown of protein in perfused livers that had been previously labeled with [l14C]valine in vivo.