Heterogeneity of amino acid transport in horse erythrocytes: a detailed kinetic analysis of inherited transport variation.

Abstract

SUMMARY1. Thoroughbred horses were divisible into five distinct amino acid transport subgroups on the basis of their erythrocyte permeability to L-alanine, measured uptake rates ranging from 5 to 625 pmol 1 cells-' h-1 (0-2 mM-extracellular L-alanine, 37 0C).2. Erythrocytes from animals belonging to the lowest L-alanine permeability subgroup (5-15 /tmol 1 cells-' h-1) (transport-deficient type) exhibited slow nonsaturable transport of this amino acid. In contrast, cells from horses of the four transport-positive subgroups possessed additional high-affinity (apparent L-alanine Km (Michaelis constant) -03 mM) and/or low-affinity (apparent L-alanine Km -13 mM) Na+-independent transport routes selective for L-neutral amino acids of intermediate size. The two transporters, designated systems asc, and asc2, respectively, also possessed a significant affinity for dibasic amino acids.3. Amino acid transport activity in horse erythrocytes behaved as if controlled by three co-dominant alleles (s, h and 1), where s is a silent allele, and h and 1 code for the functional presence of systems ascl and asc2, respectively. 4. At physiological temperature, system asc, operated preferentially in an exchange mode. In contrast, system asc2 did not participate in exchange reactions at 37 0C, but did exhibit significant trans-acceleration at 25 'C.5. Reduction of the incubation temperature also resulted in dramatic decreases in apparent Km and Vmax for L-alanine uptake by system asc2, whereas the effects of temperature on system asc1 were much less marked. At 5 'C the two transporters exhibited equivalent kinetic constants for L-alanine influx. L-Alanine uptake by transport-deficient cells was relatively insensitive to temperature. Influx by this route may represent the ground-state permeability of the lipid bilayer.6. The effects of low temperature on system asc2 suggest a preferential impairment of the mobility of the unloaded carrier relative to that of the loaded transporter. Similarly, the different kinetic properties of systems asc1 and asc2 at physiological temperature are attributed to a difference in the mobilities of the empty carriers, this difference being minimized at 5 'C.

Expression of amino acid transport systems in cultured human umbilical vein endothelial cells.

“…Lack of uptake for the system A-selective analogue 2-MeAlB and the negligible uptake for L-proline ( Table 1), suggest that system A is not expressed in umbilical vein endothelial cells. L-Serine transport was markedly dependent upon extracellular sodium, as expected if uptake occurred primarily via a Na+-dependent system ASC (alanine-serine-cysteine) and not a Na+-independent system as recently described in erythrocytes (Fincham, Mason, Paterson & Young, 1987) and the exocrine pancreas (Mann & Peran, 1986).…”

Amino acid efflux in the isolated perfused rat pancreas: trans‐stimulation by extracellular amino acids.

“…The exchange mechanism for radiolabelled L-serine was stereospecific and did not accept anionic, cationic or f-amino acids. The substrates which most readily stimulated L-serine efflux matched those most effective in accelerating L-alanine efflux via system asc in horse erythrocytes (Fincham et al 1985(Fincham et al , 1987. Unlike system asc in erythrocytes, leucine and methionine were as effective as valine in accelerating serine efflux in the pancreas (Fig.…”

“…7). Although the Na+-independent system asc in horse erythrocytes interacts with dibasic amino acids (Fincham et al 1987), system asc in pigeon erythrocytes (Vadgama & Christensen, 1985) and the rat pancreas reveals no such interaction (Fig. 7).…”

“…Borghetti & Gazzola, 1978;Shotwell, Kilberg & Oxender, 1983). Pancreatic transport of L.-serine is mediated bv more than one system , with -82 % of influx occurring through a Na+-independent carrier resembling system ase, recently characterized in horse erythrocytes (Fincham et al 1987). and the remnant Na+-dependent fraction being mediated systems A and ASC.…”

“…Recent transport studies in the perfused rat pancreas have established that the high rates of amino acid influx are mediated by at least four basolateral transporters: a small neutral Na'-dependent system A , a Na+-independent system asc selective for neutral amino acids of intermediate size (Fincham, Mason & Young, 1985;Vadgama & Christensen, 1985;Fincham, Mason, Paterson & Young, 1987), a large neutral Na+-independent system L (Alann. Habara & Peran, 1986;) and a cationic system v (Mann.…”