The transport of neutral amino acids and their interactions for uptake have been studied in skin-derived-diploid human fibroblasts cultured at confluency. Properly timed preincubations in a medium defined in composition and requirements have been adopted to control size of internal amino acid pools and regulatory interferences affecting amino acid transport. L-Proline, L-alanine, and L-leucine were used as natural amino acid substrates. Amino acid uptake and exchange with preaccumulated molecules have been measured under conditions approaching initial entry rates in the absence and in the presence of Na+ and of transport-specific model substrates. Cultured human fibroblasts were found to contain three neutral amino acid transport systems: A, ASC, and L. L-Proline was taken up preferentially by System A. System ASC appeared to mediate the largest fraction of L-Leucine entered the cells mainly by the Na+-independent System L. Preaccumulation of the cells with Site A-reactive amino acids caused trans-inhibition of the activity of System A. The activity of System L was strongly trans-stimulated in cells preloaded with Site L-reactive amino acids. The inward transport of L-alanine was trans-inhibited by internal Site A-reactive amino acids and trans-stimulated by preaccumulated L-alanine exchanging with the external amino acid through the operation of System ASC.
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