Active transport of L-phenylalanine by snail intestine, Helix (Cryptomphalus) aspersa

Snail intestine was found to actively accumulate L-phenylalanine. The uptake of L-phenylalanine was a saturable function of the amino acid concentration and sodium-dependent. The KT for L-phenylalanine uptake increased as the Na+ concentration was reduced, being 2.2 mM at 71.4 mM Na+, 5.9 mM at 10 mM Na+ and 9.9 mM at 0 mM Na+. Variations in Na+ concentration were without effect on the Vmax. At low concentrations of L-phenylalanine (0.5 and 1 mM) the tissue was able to concentrate the amino acid even in the absence of Na+. L-phenylalanine uptake was competitively inhibited by L-methionine (KI = 0.58 mM). This finding suggests that both amino acids might share a common transport system, being the affinity for L-methionine higher than for L-phenylalanine.