The absorption of lysine, arginine, phenylalanine and methionine by Taenia crassiceps larvae is linear with respect to time for at least 2 min. Arginine uptake occurs by a mediated system and diffusion, and arginine, lysine and ornithine (in order of decreasing affinity) are completely competitive inhibitors of arginine uptake. The basic amino acid transport system has a higher affinity for l-amino acids than d-amino acids, and blocking the α-amino group of an amino acid destroys its inhibitory action. Phenylalanine uptake by T. crassiceps larvae is inhibited in a completely competitive fashion by serine, leucine, alanine, methionine, histidine, phenylalanine, tyrosine and tryptophan (in order of increasing affinity). Methionine apparently binds non-productively to the phenylalanine (aromatic amino acid-preferring) transport system. l-methionine uptake by larvae is inhibited more by d-alanine and d-valine than by their respective l-isomers, while d- and l-methionine inhibit l-methionine uptake equally well. The presence of an unsubstituted α-amino group is essential for an inhibitor to have a high affinity for the methionine transport system. Uptake of arginine, phenylalanine and methionine is Na +-insensitive, and both phenylalanine and methionine are accumulated by larvae against a concentration difference in the presence or absence of Na +. Arginine accumulation is precluded by its rapid metabolism to proline, ornithine and an unidentified compound.
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