Testing the Hypothesis that System y + L Accounts for High- and Low-Transport Phenotypes in Chicken Erythrocytes Using L -Leucine as Substrate

Experiments were carried out to test the hypothesis that system y + L accounts for the high (HT) and low (LT) amino-acid transport phenotypes in chicken erythrocytes and to explain the different effect of selective breeding on lysine and leucine fluxes. L -Leucine transport was characterized in individuals which had been separated into two groups (HT and LT) according to their capacity to transport L -lysine across the erythrocyte membrane. Whereas lysine influx (1 μ M ) in the two groups differed by 32-fold (HT/LT), leucine influx was not significantly different. Average rates (nmol/ L cells/ min) were: 227 (HT) and 7.0 (LT) for L -lysine, and 8.9 (HT) and 7.1 (LT) for L -leucine. The differential ability of L -lysine and L -leucine fluxes to discriminate between the HT and LT phenotypes was shown to be consistent with the interactions of these substrates with system y + L and to vary depending on the conditions of the assay. It is shown that the two phenotypes can be clearly discriminated by measuring L -leucine influx in the presence of Li + . These results support the hypothesis that the HT and LT phenotypes reflect alterations in the function of system y + L and illustrate that the choice of the appropriate substrate and medium composition must be carefully considered when investigating the consequences of either experimental or natural alterations of broad-scope transporters.