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It has been shown that amino acids have to be available simultaneously and in proper relative concentrations for optimal utilization in the tissues( 1). The first step in the utilization of dietary amino acids is the intestinal transfer to the body proper and therefore the speed and rate of this transfer may significantly influence the simultaneous availability of the amino acids(2). There is only little information available on the mechanism of amino acid absorption. Some data have been published in the literature which suggest that the absorption of amino acids is not a process of simple diffusion, but 4hat some other mechanisms probably some "accelerating factors" may be also involved(3). Further it has been shown by several authors that amino acids are absorbed selectively at varying rates from the gastrointestinal tract. It was the purpose of the present study to determine how the absorption of an amino acid, such as histidine or tryptophan, is influenced by the presence of other amino acids in the intestinal tract.Method. The Cori technic(4) was used to investigate the absorption of amino acids from the gastrointestinal tract of adult male Sprague-Dawley rats. The rats were fed the amino acid solutions via stomach tube. L-tryptophan and dl-phenylalanine were fed as such, and L-'histidine and L-arginine were fed as the monohydTochlorides. All of the amino acid solutions were adjusted to a pH of 7.0-8.0 before feeding. After a predetermined absorption period, the residual amounts of histidine and tryptophan which remained ia the gastrointestinal tract were colonmetrically determined( 5 , 6 ) . The absorption coefficients were computed on a basis of body surface area rather than of body weight in an attempt to obviate any variability due to differences in *This work was supported ,by a grant from the Williams-Waterman Fund of the Research Corporation.the weights of the animals used.For the determination of histidine Lang's modification of the Pauly-Weiss method was used( 5 ) , by which a red color is developed when histidine is treated in alkaline solution with freshly prepared diazotized sulfanilic acid. The only other amino acid giving this oobr under these conditions is tyrosine, which was completely removed by precipitation with HgC12. The test for tryptophan is based on the Voisenet-Rhode reaction ( 6 ) , by which tryptophan specifically produces a blue color in an acidic medium of pdimethylaminobenzaldehyde. In preliminary control experiments it was shown that the presence of tryptophan in the concentrations used did not interfere with the determination of histidine. Similarly, the color test for tryptophan was not modified by the presence of histidine.Results. Table I lists the mean values of the absorption coefficients for histidine and tryptophan t the standard deviations of the mean. Their absorption rates appear to depend to a certain limit on the concentration in which they were fed. The quantity absorbed increased with increasing concentration of the solution fed from 24 till 51) mg/7.5 ml. Greate...

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