Histidine transport into isolated animal cells.

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.