© 1982 by The Society for Integrative and Comparative Biology
Kinetic and Thermodynamic Aspects of Sodium-Coupled Amino Acid Transport by Marine Invertebrates1
Department of Biology, Illinois State University Normal, Illinois 61761
Department of Physiology, School of Medicine, University of California, Los Angeles Los Angeles, California 90024
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.