Plasma protein binding of tetrodotoxin in the marine puffer fish Takifugu rubripes.

To elucidate the involvement of plasma protein binding in the disposition of tetrodotoxin (TTX) in puffer fish, we used equilibrium dialysis to measure protein binding of TTX in the plasma of the marine puffer fish Takifugu rubripes and the non-toxic greenling Hexagrammos otakii, and in solutions of bovine serum albumin (BSA) and bovine alpha-1-acid glycoprotein (AGP). TTX (100-1000 microg/mL) bound to protein in T. rubripes plasma with low affinity in a non-saturable manner. The amount of bound TTX increased linearly with the TTX concentration, reaching 3.92+/-0.42 microg TTX/mg protein at 1000 microg TTX/mL. Approximately 80% of the TTX in the plasma of T. rubripes was unbound in the concentration range of TTX examined, indicating that TTX exists predominantly in the unbound form in the circulating blood of T. rubripes at a wide range of TTX concentrations. TTX also bound non-specifically to H. otakii plasma proteins, BSA, and bovine AGP. The amount of the bound TTX in the plasma of H. otakii and BSA, respectively, was 1.86+/-0.36 and 4.65+/-0.70 microg TTX/mg protein at 1000 microg TTX/mL, and that in the bovine AGP was 8.78+/-0.25 microg TTX/mg protein at 200 microg TTX/mL.