On the nature of the "non-saturable" migration of amino acids into Ehrlich cells and into rat jejunum.

Abstract The so-called non-saturable uptake of α-amino acids by the Ehrlich cell, even though it occurs at a characteristically slow rate for various neutral amino acids (whether they are in the d - or the l -form) is nevertheless structurally specific, since the uptake of β-alanine, taurine and betaine occurs only about one-third as rapidly as that of the α-amino acids. Furthermore the uptake shows a considerable sensitivity to pH, and a temperature sensitivity so high as to exclude simple diffusion as the rate-limiting step. The structural specificity is compatible with a reaction of the amino acid with a membrane site, either an abundant one or a relatively unreactive one, the reaction of the amino acid with which presumably need involve at most only its amino and carboxyl groups. Uptake of amino acids at high levels by rat-intestinal segments also showed high temperature sensitivities.