The preferential interaction of L-threonine with transport system ASC in cultured human fibroblasts.
The transport of L-threonine was studied in cultured human fibroblasts. A kinetic analysis of L-threonine transport in a range of extracellular concentrations from 0.01 to 20 mM indicated that this amino acid enters cells through both Na(+)-independent and Na(+)-dependent routes. These routes are: (1) a non-saturable, Na(+)-independent route formally indistinguishable from diffusion; (2) a saturable, Na(+)-independent route inhibitable by the analog BCH and identifiable with system L; (3) a low-affinity, Na(+)-dependent component (Km = 3 mM) which can be attributed to the activity of system A since it is adaptively enhanced by amino acid starvation and suppressed by the characterizing analog MeAIB and (4) a high-affinity, Na(+)-dependent route (Km = 0.05 mM). This latter route is identifiable with system ASC since it is insensitive to adaptive regulation, uninhibited by MeAIB, trans-stimulated by intracellular substrates of system ASC, markedly stereoselective, and relatively insensitive to changes in external pH. At an external concentration of 0.05 mM more than 90% of L-threonine transport is referrable to the activity of system ASC; in these conditions, the transport of the amino acid exhibits typical ASC-features even in the absence of inhibitors of other transport agencies, and, therefore, it can be employed as a reliable indicator of the activity of transport system ASC in cultured human fibroblasts.
Istituto di Patologia Generale, Università di Parma, Italy.
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D000596:Amino Acids / Q000494:pharmacology; D001120:Arginine / Q000494:pharmacology; D001692:Biological Transport / Q000187:drug effects; D002478:Cells, Cultured; D005347:Fibroblasts / Q000187:drug effects / Q000378:metabolism; D005971:Glutamates / Q000494:pharmacology; D018698:Glutamic Acid; D006801:Humans; D007700:Kinetics; D008297:Male; D008954:Models, Biological; D012867:Skin / Q000378:metabolism*; D012964:Sodium / Q000494:pharmacology*; D013912:Threonine / Q000378:metabolism*
D000596:Amino Acids; D005971:Glutamates; D013912:Threonine; D018698:Glutamic Acid; D001120:Arginine; D012964:Sodium