Haemolymph protein in xanthid crabs: Its selective binding of saxitoxin and possible role in toxin bioaccumulation

A saxitoxin (STX)-binding factor has been found in the haemolymph of the xanthid crabs Lophozozymus pictor, Liomera tristis, Chlorodiella nigra and Actaeodes tomentosus. Lophozozymus pictor A. tomentosus are both xanthids previously known to be toxic, with the toxins in some L. pictor individuals having been identified as STX analogues. All the individual crabs which had this STX-binding factor did not have any STX-like activity in their haemolymph or tissues. In contrast, no such factor could be found in two other reportedly toxic xanthids, Atergatis floridus and Platypodia granulosa, each individual of which did contain STX-Iike bioactivity. Analysis of saturable H-3-STX-binding, binding kinetics and competition curves in the haemolymph of both Lophozozymus pictor and Liomera tristis indicated that a single, pharmacologically homogeneous population of STX-binding sites was present in each species. The analysis also revealed that the haemolymph from the non-toxic xanthid Liomera tristis had an affinity for STX of 0.12 nM, which was 20-fold greater than that from Lophozozymus pictor (2 nM). This tighter toxin-binding by Liomera tristis haemolymph predominantly results from a much slower rate of toxin dissociation. The protease trypsin inactivated the toxin-binding factor in both Lophozozymus pictor and Liomera tristis haemolymph. Tetrodotoxin (TTX) could not displace H-3-STX from any of the haemolymph samples, distinguishing this factor from the voltage-gated sodium channel and a TTX-neutralizing haemolymph protein previously found in a grapsid crab and a horseshoe crab. Analogues of STX, which may occur in marine organisms, possess markedly reduced abilities to displace H-3-STX from haemolymph samples. Toxin-binding was inhibited by acidic pH, producing titrations with pKs of 5.5 and 5.1 for Lophozozymus pictor and Liomer a tristis haemolymph, respectively. Binding characteristics are reminiscent of a recently found amphibian plasma protein which belongs to the transferrin family and of proteins and is unrelated to the voltage-gated sodium channel, the only previously known STX-binding protein. This is the second putative STX-resistance mechanism to be found in this family of crabs.