Anionic Amino Acid Transport Across the Plasma Membrane of Cultured Rat Hepatocytes and Hepatoma Cell Lines.

Abstract

The charge on the amino acid molecule has emerged as the primary criterion determining which mediation will serve for its transport across the membrane of mammalian cells. Three systems serving for the uptake of anionic amino acids into rat hepatocytes, fetal rat hepatocytes and the hepatoma cell lines HTC, H4-II-E-C(,3) and McA-RH7777 have been characterized, two of which systems (x(,c)('-), x(,AG)('-)) are kinetically distinguishable from others serving for neutral (A, ASC, L and N) or for basic (y('+)) amino acids. The third system, first defined as x(,A)('-), and later apparently shown identical with System ASC, serves for neutral amino acids at pH 7.4 and for anionic ones at reduced pH. Interconvertible service for amino acids of unlike charge appears in this case consistent with protonation of the mediator rather than the substrate molecules. The chain length of an anionic amino acid determines which of the three systems will serve for its transport. The Na('+)-independent System x(,c)('-) observed in fetal hepatocytes and HTC cells, but barely perceptible in the other cells of hepatic origin tested (hepatocytes, H4-II-E-C(,3), McA-RH7777), serves for uptake of anionic amino acids as long or longer than glutamate but not longer than cystine. The formally neutral cystine becomes a substrate for x(,c)('-) at those pH values which allow it to be at least partially anionic, shown by the increase in glutamate-inhibitable cystine uptake on raising the pH from 5 to 7.5. Uptake of anionic amino acids by x(,c)('-) is both pH-independent and stereoselective. The two Na('+)-dependent systems x(,AG)('-) and x(,A)('-)/ASC were detected in all the cells of hepatic origin tested. System x(,AG)('-) serves with high affinity for aspartate, glutamate and for the short-chain analogs cysteate and cysteinesulfinate. Uptake by x(,AG)('-) is pH-independent and displays a stereoselective anomaly, readily distinguishing the D- and L-isomers of glutamate but not of aspartate. System x(,A)('-)/ASC serves for both short-chain neutral amino acids and the short-chain anionics transported by x(,AG)('-), but excludes L-glutamate. Systems x(,AG)('-) and x(,c)('-), when present, are the primary routes of anionic amino acid uptake under physiological conditions; uptake by x(,A)('-)/ASC is largely inhibited at physiological pH by the preferred neutral amino acid substrates.