Characterization of neutral and cationic amino acid transport in Xenopus oocytes

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Abstract
Amino acid transport was characterized in stage 6 Xenopus laevis oocytes. Most amino acids were taken up by the oocytes by way of both Na+‐dependent and saturable Na+‐independent processes. Na+‐dependent transport of 2‐aminoisobutyric acid (AIB) was insensitive to cis‐ or trans‐inhibition by the System A‐defining substrate 2‐(methylamino)‐isobutyric acid (MeAIB), although threonine, leucine, and histidine were found to be effective inhibitors, eliminating greater than 80% of Na+‐dependent AIB uptake. Lack of inhibition by arginine eliminates possible mediation by System Bo,+ and suggests uptake by System ASC. The Na+‐dependent transport of characteristic System ASC substrates such as alanine, serine, cysteine, and threonine was also insensitive to excess MeAIB. Evidence to support the presence of System Bo, + was obtained through inhibition analysis of Na+‐dependent arginine transport as well arginine inhibition of Na+‐dependent threonine uptake. The Na+‐independent transport of leucine was subject to trans‐stimulation and was inhibited by the presence of excess phenylalanine, histidine, and, to a lesser extent, 2‐amino‐(2,2,1)‐bicycloheptane‐2‐carboxylic acid (BCH). These observations are consistent with mediation by System L. The characteristics of Na+‐independent uptake of threonine are not consistent with assignment to System L, and appear to be reflective of Systems asc and bo,+. In its charged state, histidine appears to be transported by a carrier similar in its specificity to System y+, but is taken up by System L when present as a zwitterion.