Free carboxylate groups required for transport of neutral amino acids by the Ehrlich ascites-tumor cell.

Although uncharged structures analogous to the carboxyl group of the amino acid molecule have served in place of that group for transport into some cells, we find that substitution either by the carboxamide group or by the chloromethyl ketone group eliminates inhibition of transport Systems A or L of the Ehrlich ascites tumor cell. Comparison of the loss of System L transport on acidification shows that the pH at which this loss occurs is correlated with pK'1 of the amino acid substrate, suggesting that transport is terminated by protonation of the site-bound carboxylate group.