Heterogeneity of histidine transport in the Ehrlich cell.
We have reexamined the heterogeneity shown by histidine in its uptake by the Ehrlich ascites tumor cell, in the face of a contradiction of our earlier interpretation. We again find the fraction of histidine uptake at neutral pH inhibitable by the model substrate for System A, 2-(methylamino)-isobutyric acid, to be fully dependent on the presence of Na+ or Li+. The small Na+ -independent component not attributable to System L can be identified with System Ly+ through its inhibitability by homoarginine. This component increases as the pH is lowered with an apparent pK' a of 6.1. The simultaneous decrease in the uptake by the neutral systems could be identified, for System L, with the same titration of histidine to its cationic form, but for System A the sharp decrease is identified with the protonation of a structure on the membrane rather than one on the substrate. The action of H+ in the latter case proved approximately non-competitive with Na+ when tested with ordinary substrates.
D000621:Aminoisobutyric Acids / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D001692:Biological Transport; D001693:Biological Transport, Active; D002264:Carboxylic Acids / Q000494:pharmacology; D002286:Carcinoma, Ehrlich Tumor / Q000378:metabolism*; D006639:Histidine / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008094:Lithium / Q000494:pharmacology; D012964:Sodium / Q000494:pharmacology
D000621:Aminoisobutyric Acids; D002264:Carboxylic Acids; D006639:Histidine; D008094:Lithium; D012964:Sodium