The sulphide-binding protein that occurs in high concentrations in the vascular blood and coelomic fluid of the hydrothermal vent tube-worm Riftia pachyptila Jones is the haemoglobin. Sulphide binding does not occur at the oxygen-binding sites of the haem, but may occur via thiol-disulphide exchange at the interchain disulphide bridges on the macromolecule. We have confirmed the report that vascular blood is heterogeneous for two haemoglobins (FI and FII) that are different in Mr, but we conclude that the coelomic fluid is homogeneous for the lower Mr haemoglobin FII, in the intact, living animal. These two haemoglobins occur naturally in the living animals, and FII is not a dissociation product of the higher Mr FI. The sulphide-binding capacities of the two haemoglobin species differ by about a factor of two. Consequently, the vascular blood and the coelomic fluid also have different sulphide-binding capacities. These differences in sulphide-binding capacity may have important ramifications for the physiology of this unusual animal.

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