Hepatic transport system interconverted by protonation from service for neutral to service for anionic amino acids

What we had seen previously as two Na+-dependent agencies of amino acid transport in the hepatoma cell line HTC, now also in the cultured rat hepatocyte, fetal and mature, in primary culture, turns out, on the basis of 1) their pH profiles for transport, 2) mutual competitive interaction between their substrates, 3) similarity in selectivity for amino acid chain length, and 4) stimulation of threonine exodus by the anionic cysteine sulfinate, to represent a single mediating system. We conclude that protonation of the mediating ASC system, possibly substrate-assisted, rather than od the free anionic amino acid, converts the system from function for various short chained amino acids to function for aspartate, cysteine sulfinate, and cysteate. Other systems for anionic amino acid transport have shown no such relation to ASC.