ABSTRACT
In freshly prepared uninjected folliculated oocytes, Na(+)-independent leucine uptake is mediated predominantly by a system L-like transport system. Removal of follicular cells, however, results in an irreversible loss of this transport activity. When total poly(A)+ mRNA derived from Chinese hamster ovary (CHO) cells was injected into prophase-arrested stage V or VI Xenopus laevis oocytes, enhanced expression of Na(+)-independent leucine transport was observed. The injected mRNAs associated with increased levels of leucine uptake were between 2 and 3 kb in length. The newly expressed leucine transport activity exhibited important differences from the known characteristics of system L, which is the dominant Na(+)-independent leucine transporter in CHO cells as well as in freshly isolated folliculated oocytes. The CHO mRNA-dependent leucine uptake in oocytes was highly sensitive to the cationic amino acids lysine, arginine, and and ornithine (> 95% inhibition). As with the leucine uptake, an enhanced lysine uptake was also observed in size-fractionated CHO mRNA-injected oocytes. The uptakes of leucine and lysine were mutually inhibitable, suggesting that the newly expressed transporter was responsible for uptakes of both leucine and lysine. The inhibition of uptake of lysine by leucine was Na+ independent, thus clearly distinguishing it from the previously reported endogenous system y+ activity. Furthermore, the high sensitivity to tryptophan of the CHO mRNA-dependent leucine transport was in sharp contrast to the properties of the recently cloned leucine transport-associated gene from rat kidney tissue, although leucine transport from both sources was sensitive to cationic amino acids. Our results suggest that there may be a family of leucine transporters operative in different tissues and possibly under different conditions.
