l-leucine, l-methionine, and l-phenylalanine share a Na+/K+-dependent amino acid transporter in shrimp hepatopancreas
Hepatopancreatic brush border membrane vesicles (BBMV), made from Atlantic White shrimp (Litopenaeus setiferus), were used to characterize the transport properties of 3H-l-leucine influx by these membrane systems and how other essential amino acids and the cations, sodium and potassium, interact with this transport system. 3H-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. 3H-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 3H-l-leucine in both Na+- and K+-containing incubation media. The residual 3H-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. 3H-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, 3H-l-leucine influx displayed a low apparent KM (high affinity) and low apparent Jmax, while in KCl the transport exhibited a high apparent KM (low affinity) and high apparent Jmax. l-methionine or l-phenylalanine (7 and 20 mM) were competitive inhibitors of 3H-l-leucine influxes in both NaCl and KCl media, producing a significant (p < 0.01) increase in 3H-l-leucine influx KM, but no significant response in 3H-l-leucine influx Jmax. Potassium was a competitive inhibitor of sodium co-transport with 3H-l-leucine, significantly (p < 0.01) increasing 3H-l-leucine influx KM in the presence of sodium, but having negligible effect on 3H-l-leucine influx Jmax in the same medium. These results suggest that shrimp BBMV transport 3H-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.