Amino acid transport System L in muscle cells: biochemical properties and its relation to protein synthesis

Objectives were to characterize mechanisms and biochemical properties of transport systems responsible for the uptake of branched-chain amino acids (BCAAs) in muscle cells. Rat omega myoblasts (RMo) were grown to confluency and allowed to differentiate prior to conduct of transport assays. Myotubes concentrated cycloleucine (cLeu) in a sodium (Na)-free medium. The Na gradient-independent transporter possessed high affinity (Km = 0.12 mM) and high capacity (Vmax = 6.4 nmol cLeu/mg protein per min). Cycloleucine transport was strongly inhibited by nonpolar neutral amino acids but not by alpha-aminoisobutyric acid or lysine. Myotubes possessed a Na gradient-independent trans-exchange mechanism. Hence, myotubes possess a System L-like transporter. In the second part of the study we determined that various inhibitors (KCN, oligomycin, iodoacetamide and cycloheximide) increased leucine transport. Their actions were not mediated by reductions in ATP concentration but were instead associated with changes in protein synthesis. Hence, regulation of muscle protein synthesis may also influence transporter activity.