Preferential utilization of exogenously supplied leucine for protein synthesis in estradiol-induced and uninduced cockerel liver explants.

A cockerel liver explant system has been used to study protein synthesis and ribosome transit times. After a 2-hr preincubation of explant tissue in the presence of a large concentration of nonradioactive leucine, a small quantity of [3H]leucine was added and the kinetics of uptake of [3H]leucine into the intracellular acid-soluble leucine pool was compared to the incorporation of [3H]leucine into protein. Incorporation of [3H]leucine into protein reaches a linear rate almost immediately after addition of label, whereas the acid-soluble pool does not reach constant specific activity until much later. The length of time needed to reach a linear rate of incorporation of [3H]leucine into protein is approximately equal to the length of time needed to equilibrate nascent polypeptide chains with labeled precursor--that is, one average ribosome transit time. Therefore, it seems that the immediate precursor pool for protein synthesis reaches constant specific activity almost instantly after addition of [3H]leucine. The results indicate that at least part of the supply of leucine for protein synthesis is derived directly from the exogenous incubation medium and not from the intracellular acid-soluble amino acid pool.