Interamino Acid Inhibition of Transport in Higher Plants

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Abstract
Data from published experiments were analyzed to determine the number and specificities of amino acid transport channels in cells of higher plants [tobacco, corn, wheat, barley, mustard, melon, soybean, asparagus, carrot]. Each experiment measured the uptake of a labeled amino acid in the presence of unlabeled amino acids, used one at a time, in the incubating medium. The observed interamino acid inhibitions can be accounted for by 2 transport channels, each with characteristic affinities that were computed from the observed interamino acid inhibitions. The 1st channel is a general transport system with the following relative affinities for the amino acids: methionine 75, alanine 75, phenylalanine 64, tyrosine 64, leucine 63, cysteine 58, serine 57, glycine 56, tryptophan 54, glutamine 51, threonine 49, valine 44, isoleucine 44, glutamic acid 44, proline 43, histidine 33, lysine 32, asparagine 22, arginine 22, aspartic acid 18. The 2nd channel is a basic amino acid transport system with relative affinities for arginine, lysine and histidine of 66, 39 and 21, respectively. The affinities for the other acids in the 2nd channel are lower. Despite considerable diversity in the species, tissues and solute concentrations employed in the experiments, multiple regression equations (Y = .alpha. + .beta.1X1 + .beta.X2, in which Y is the observed transport inhibition and X1 and X2 are the relative transport affinities of the 2 channels) account for 50-99% of the variance in all but 6 experiments, 5 of which employed usually high solute concentrations.