pH-dependent heterogeneity of acidic amino acid transport in rabbit jejunal brush border membrane vesicles.
Initial rates of Na(+)-dependent L-glutamic and D-aspartic acid uptake were determined at various substrate concentrations using a fast sampling, rapid filtration apparatus, and the resulting data were analyzed by nonlinear computer fitting to various transport models. At pH 6.0, L-glutamic acid transport was best accounted for by the presence of both high (Km = 61 microM) and low (Km = 7.0 mM) affinity pathways, whereas D-aspartic acid transport was restricted to a single high affinity route (Km = 80 microM). Excess D-aspartic acid and L-phenylalanine served to isolate L-glutamic acid flux through the remaining low and high affinity systems, respectively. Inhibition studies of other amino acids and analogs allowed us to identify the high affinity pathway as the X-AG system and the low affinity one as the intestinal NBB system. The pH dependences of the high and low affinity pathways of L-glutamic acid transport also allowed us to establish some relationship between the NBB and the more classical ASC system. Finally, these studies also revealed a heterotropic activation of the intestinal X-AG transport system by all neutral amino acids but glycine through an apparent activation of Vmax.
Department of Physiology, Faculty of Medicine, Université de Montréal, Québec, Canada.
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Maenz
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Chenu
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Breton
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Berteloot
D000596:Amino Acids / Q000494:pharmacology; D000818:Animals; D001224:Aspartic Acid / Q000378:metabolism* / Q000494:pharmacology; D001692:Biological Transport / Q000187:drug effects; D005971:Glutamates / Q000378:metabolism*; D018698:Glutamic Acid; D006863:Hydrogen-Ion Concentration; D007413:Intestinal Mucosa / Q000378:metabolism*; D007583:Jejunum / Q000378:metabolism*; D007700:Kinetics; D008871:Microvilli / Q000187:drug effects / Q000378:metabolism*; D011817:Rabbits
D000596:Amino Acids; D005971:Glutamates; D001224:Aspartic Acid; D018698:Glutamic Acid