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Journal of Experimental Biology 136,363-381 (1988)
Published by Company of Biologists 1988

Electroneutral Na⁺-2 Cl^--Leucine Cotransport by Lobster Hepatopancreatic Brush-Border Membrane Vesicles

GREGORY A. AHEARN ¹ and LAUREL P. CLAY ¹

¹ Department of Zoology, 2538 The Mall, University of Hawaii at Manoa, Honolulu, HI 96822, USA

Uptake of L-[³H]leucine by lobster hepatopancreatic brush-border membrane vesicles was stimulated by a transmembrane NaCl gradient (o>i), but not by identical gradients of NaSCN or other Cl^- salts (e.g. K⁺, Li⁺, NH₄⁺, Cs⁺ or choline), suggesting that amino acid transfer depended upon both Na⁺ and Cl^-. In NaCl medium at acidic pH, leucine uptake was largely electroneutral and unresponsive to a transmembrane potential generated by permeable anions; however, in Na⁺-free medium, amino acid transport was strongly electrogenic under the same conditions. Leucine influx occurred by a combination of two carrier processes at physiologically acidic pH. One exhibited an influx K_t of 0.59 mmol l^-1, a J_M of 390 pmol mg protein^-1 S^-1 and a cotransport stoichiometry of 1 Na⁺: 2 Cl⁺: 1 leucine. This process was most strongly cis-inhibited by the non-polar amino acids phenylalanine, methionine and isoleucine, and most weakly inhibited by the more polar species methylaminoisobutyric acid (MeAIB), hydroxyproline, glutamate and arginine. The second leucine carrier system showed a very low binding affinity and could not be distinguished from diffusion, was Na⁺- and Cl^--independent, and was cis-inhibited by more polar amino acids such as lysine, hydroxyproline, MeAIB, alanine and glutamate. These results suggest that brush-border leucine transport in lobster hepatopancreas at acidic pH may occur by a combination of a modified L-system, that includes ion cosubstrates, and either by a second undefined Na⁺-independent process with a broad structural specificity or by multiple Na⁺-independent processes.

Key words: cotransport, leucine transport, brush-border membrane vesicles, Na⁺-dependent, hepatopancreas, Homarus americanus, ion gradients

Accepted on November 25, 1987

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