Kinetic and Thermodynamic Aspects of Sodium-Coupled Amino Acid Transport by Marine Invertebrates

SYNOPSIS. Marine invertebrates absorb amino acids directly across their external body surfaces. This absorption process occurs via carrier-mediated transport systems which, recent evidence suggests, may be sodium-dependent. Steady-state amino acid gradients are maintained at levels exceeding 103–106 times that of the external environment. Examination of the standing gradients of total free amino acids, Na, and K in seven invertebrates suggests that an Na/amino acid cotransport model, or an Na/K/amino acid cotransport model can account for amino acid gradients of this magnitude. However, the Na : amino acid coupling coefficients must be 2 or 3 depending on factors such as membrane potential and the intracellular Na activities. Evidence from studies of L-alanine transport in the integument of the polychaete Glycera dibranchiata is presented showing that, for this case, the Na : alanine coupling coefficient is 3. It is concluded that the models presented are plausible and readily testable explanations for the observed amino acid gradients in marine invertebrates.