title: L-leucine, L-methionine, and L-phenylalanine share a Na(+)/K (+)-dependent amino acid transporter in shrimp hepatopancreas.
abstract: Hepatopancreatic brush border membrane vesicles (BBMV), made from Atlantic White shrimp (Litopenaeus setiferus), were used to characterize the transport properties of (3)H-L-leucine influx by these membrane systems and how other essential amino acids and the cations, sodium and potassium, interact with this transport system. (3)H-L-leucine uptake by BBMV was pH-sensitive and occurred against transient transmembrane concentration gradients in both Na(+)- and K(+)-containing incubation media, suggesting that either cation was capable of providing a driving force for amino acid accumulation. (3)H-L-leucine uptake in NaCl or KCl media were each three times greater in acidic pH (pH 5.5) than in alkaline pH (pH 8.5). The essential amino acid, L-methionine, at 20 mM significantly (p < 0.0001) inhibited the 2-min uptakes of 1 mM (3)H-L-leucine in both Na(+)- and K(+)-containing incubation media. The residual (3)H-L-leucine uptake in the two media were significantly greater than zero (p < 0.001), but not significantly different from each other (p > 0.05) and may represent an L-methionine- and cation-independent transport system. (3)H-L-leucine influxes in both NaCl and KCl incubation media were hyperbolic functions of [L-leucine], following the carrier-mediated Michaelis-Menten equation. In NaCl, (3)H-L-leucine influx displayed a low apparent K M (high affinity) and low apparent J max, while in KCl the transport exhibited a high apparent K M (low affinity) and high apparent J max. L-methionine or L-phenylalanine (7 and 20 mM) were competitive inhibitors of (3)H-L-leucine influxes in both NaCl and KCl media, producing a significant (p < 0.01) increase in (3)H-L-leucine influx K M, but no significant response in (3)H-L-leucine influx J max. Potassium was a competitive inhibitor of sodium co-transport with (3)H-L-leucine, significantly (p < 0.01) increasing (3)H-L-leucine influx K M in the presence of sodium, but having negligible effect on (3)H-L-leucine influx J max in the same medium. These results suggest that shrimp BBMV transport (3)H-L-leucine by a single L-methionine- and L-phenylalanine-shared carrier system that is enhanced by acidic pH and can be stimulated by either Na(+) or K(+) acting as co-transport drivers binding to shared activator sites.
authors.affiliation: Department of Biology, 1 UNF Drive, University of North Florida, Jacksonville, FL 32224, USA.
authors.forename: Ada
authors.initials: A
authors.lastname: Duka
authors.forename: Gregory A
authors.initials: GA
authors.lastname: Ahearn
mesh_terms: D026905:Amino Acid Transport Systems / Q000378:metabolism; D000818:Animals; D043143:Hepatopancreas / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007930:Leucine / Q000378:metabolism*; D008715:Methionine / Q000378:metabolism*; D008871:Microvilli / Q000378:metabolism; D033561:Penaeidae / Q000378:metabolism*; D010649:Phenylalanine / Q000378:metabolism*; D011188:Potassium / Q000378:metabolism; D012964:Sodium / Q000378:metabolism
chemical_list: D026905:Amino Acid Transport Systems; D010649:Phenylalanine; D012964:Sodium; D008715:Methionine; D007930:Leucine; D011188:Potassium