Transport of amino acids and glucose in brush border membrane vesicles from the gills of the marine mussel, Mytilus edulis.

Abstract

Marine mussels accumulate amino acids and glucose from seawater against considerable concentration gradients. The principal site for this uptake is the gill. Previous studies using intact, isolated gills from marine mussels have suggested that the transport mechanism involves coupling to Na⁺, similar to the mechanism of secondary active transport of amino acids and glucose in vertebrate epithelia, but until this dissertation there had been no rigorous test of this hypothesis. Brush border membrane vesicles (BBMV) were prepared from the gills of the marine mussel, Mytilus edulis, by differential and sucrose density centrifugation. The preparation procedure isolated a population of membranes enriched in brush border membrane markers. The transport of amino acids by two pathways, the alanine-lysine pathway (AK) and the alanine-proline pathway (AP), and the uptake of glucose was studied in the BBMV. The mechanism of transport through the three transport pathways is BBMV involved coupling to Na⁺. Concentrative uptake through the AK pathway, which transported alanine and lysine, also occurred in the presence of Li⁺ and K⁺ gradients. This pathway was the major route for alanine transport in BBMV. The AP pathway transported alanine and proline, and was strictly dependent on Na⁺. Glucose transport in gill BBMV resembled quite closely the Na⁺-coupled transport of glucose in vertebrate epithelia in such characteristics as Na⁺ and substrate specifically, and electrogenicity. Transport through the two amino acid uptake pathways (AK and AP) and through the glucose uptake pathway could be described by Michaelis-Menten kinetics, with high substrate affinities (K(t)'s below 10 μM). Furthermore, it is likely that multiple Na⁺ ions are involved in the transport of these amino acids and glucose in mussel gill BBMV. It appears that these transporters are adapted for function at low substrate concentrations and against large concentration gradients.