Enkephalins are transported by a novel eukaryotic peptide uptake system

We have identified an oligopeptide transporter in the yeast Saccharomyces c erevisiae which mediates the uptake of tetra- and pentapeptides, including the endogenous opioids leucine enkephalin (Tyr-Gly-Gly-Phe-Leu) and methion ine enkephalin (Tyr-Gly-Gly-Phe-Met). The transporter is encoded by the gen e OPT1. Yeast expressing OPT1 can utilize enkephalins to satisfy amino acid auxotrophic requirements for growth. The transport of radiolabeled leucine enkephalin exhibits saturable kinetics, with a K-m of 310 mu M. Transport activity is optimum at acidic pH and sensitive to reagents which uncouple o xidative phosphorylation, suggesting an energy dependence on the proton gra dient. Growth, transport, and chromatographic data indicate that leucine en kephalin is not hydrolyzed in the extracellular medium and as such is trans located intact across the cell membrane. The system is specific for tetra- and pentapeptides and can be inhibited by the opioid receptor antagonists n aloxone and naltrexone. To date, this is the first example of a eukaryotic transport system which can use enkephalins as a substrate, opening the poss ibility that a homologue exists in higher eukaryotes.