Histidine transport into isolated animal cells

Abstract

At pH values where it is predominantly without net charge, histidine in dilute solution divided its uptake by the Ehrlich cell between the Na +-independent L system and the Na +-requiring A system. As in every other case studied the L component was partially inhibitable by lysine, partially not. At pH 5, where it is chiefly a cation, an additional component became perceptible, one which could be inhibited by lysine but not by neutral amino acids, which is assigned to the Ly + system. These observations extend generalizations suggesting that the L and Ly + systems are associated so that partially competitive inhibition occurs between their respective substrates. An apparent heterogeneity in the interaction between histidine and phenylalanine in the constant-ratio test is probably explained by the role of the Ly + system in histidine uptake expected for the cationic form of histidine even at pH 7.4. In various erythrocytes histidine uptake was simpler because of the absence of an A system and the minimal reactivity of histidine with any other Na +-requiring system. Although it reacted here also both as a cation and a neutral amino acid, interaction between the corresponding transport systems was inconspicuous.