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  • Title: Heterogeneity of histidine transport in the Ehrlich cell.
    Author: Im WB, Christensen HN.
    Journal: Biochim Biophys Acta; 1976 Nov 11; 455(1):144-51. PubMed ID: 11000.
    Abstract:
    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.
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