Genetic control of amino acid transport in sheep erythrocytes

An inherited amino acid transport deficiency results in low concentrations of glutathione (GSH) in the erythrocytes of certain sheep. Earlier studies based on phenotyping according to GSH concentrations indicated that the gene Tr^H, which controls normal levels of GSH, behaves as if dominant or incompletely dominant to the allele Tr^h, which controls the GSH deficiency. The present paper shows that when sheep are classified according to amino acid transport activity, the Tr^H gene behaves as if codominant to Tr^h. Erythrocytes from sheep homozygous for the Tr^H gene exhibit rapid saturable l-alanine influx (apparent K_m ,21.6mm; V_max, 22.4 mmol/liter cells/hr). Cells from sheep homozygous for the Tr^h gene exhibit slow nonsaturable l-alanine uptake (0.55 mmol/liter cells/hr at 50mm extracellular l-alanine). Cells from heterozygous sheep show saturable l-alanine uptake with a diminished V_max (apparent K_m, 19.1mm; V_max, 12.7 mmol/liter cells/hr). These erythrocytes have a significantly lower GSH concentration than cells from Tr^H, Tr^H sheep but similar intracellular levels of dibasic amino acids.The authors are grateful to the M.R.C. for a Project Grant.