Epidermal taurine transport in marine mussels

We examined the structural specificity and kinetic characteristics of epidermal taurine transport in the marine mussels Mytilus edulis and M. californianus. The gill was the primary site of taurine uptake in both species, responsible for 70% of the accumulation of radioactively labeled substrate. Taurine transport was inhibited by short-chain beta-amino acids, including beta-alanine and beta-aminobutyric acid (beta-ABA), and by gamma-ABA; alpha-neutral, acidic, and basic amino acids had no effect on this transport. The uptake process was clearly a saturable phenomenon and was adequately described by Michaelis-Menten-type kinetics in both intact animals and isolated preparations of gill. The maximal rates of taurine uptake, expressed per gram of gill tissue, were 8.4 and 3.2 mumol X g-1 X h-1 for M. edulis and M. californianus, respectively. In intact mussels the half-saturation constant for both species ranged from 5 to 8 microM. A mathematical model was developed that describes the epidermal transport of taurine in the gill of the actively pumping mussel. The model takes into account the geometry and pattern of water flow in the gill. Calculations based on this model indicate that because of the convective movement of water through the gill, the actual Michaelis constant of the taurine transporter must be much lower (i.e., 0.5-2.5 microM) than the half-saturation constants determined with intact animals and isolated gills. The model also predicts that the presence of the transporter in the gill can result in a significant reaccumulation of taurine lost from the gill by passive diffusion. We suggest that epidermal taurine transport is involved with maintenance of the high concentrations of this compound commonly associated with gill tissue in mussels.