Pools and protein synthesis: studies with the D- and L-isomers of leucine.

D-leucine and L-leucine produce pools of an identical nature in HeLa cells. Both isomers noncompetitively inhibit to the same extent pool formation and incorporation of valine. In the presence of D-leucine, [3H]-L-leucine at high specific activity is avidly incorporated into protein while forming a highly radioactive pool. The development of this pool was suppressed to normal levels by the presence of cycloheximide. It therefore represented a largely catabolic pool derived from proteins which had already become labelled. Discharge of pools of D-leucine followed first order kinetics and was significantly retarded when medium contained 10(-2) M of either isomer. Discharge of catabolic pools was equally as fast but the continual flow of labelled amino acid from protein sustained its intracellular level. The presence of 10(-2) to 10(-4) M leucine in the chase medium did not apparently alter the rate of discharge of this catabolic pool. The results are discussed in terms of the specificity of amino acids for different stages of the pathway leading to and from protein synthesis, and support the intracellular perfusion mechanism described elsewhere (Wheatley and Inglis, 1979).