Haemocyanin and the binding of cadmium and zinc in the haemolymph of the shore crab Carcinus maenas (L.)

The trace metals cadmium and zinc are taken up from solution into haemolymph of the shore crab, Carcinus maenas, where they bind to the respiratory pigment haemocyanin. Investigations using FPLC and PAGE to separate the constituent dodecamer and hexamer aggregations of haemocyanin showed a difference between the distribution of cadmium and zinc between the two aggregate states. In the haemolymph, labelled cadmium binds not only to haemocyanin but, at least initially, it is also associated with low molecular size material or exists as free cadmium ions. The binding of cadmium to haemocyanin is not an immediate process. Initially the binding of cadmium favours the hexamer; only later is a more even distribution between the two aggregate states apparent. Newly taken up labelled zinc is rapidly bound to haemocyanin. The majority of both total and labelled zinc in the haemolymph is bound to haemocyanin, with the hexamer having a greater binding affinity for zinc than the dodecamer. This suggests that formation of the dodecamer leads to a blocking of zinc-binding sites and/or that the hexamer consists of subunits which possess a relatively high affinity for zinc.