The functioning of the haemocyanin of the terrestrial Christmas Island redcrab Gecarcoidea natalis and roles for organic modulators

Gecarcoidea natalis is a land crab that migrates annually several kilometre s to breed. The O-2-binding properties of haemocyanin in G. natalis were in vestigated in vitro to test the idea that the O-2-binding properties of the haemocyanin of land crabs are not dependent on circulating modulators and to provide a model of haemocyanin functioning during exercise. The affinity of the haemocyanin for O-2 decreased with increasing temperature (change i n the heat of oxygenation; Delta H=-59kJ mol(-1)). The haemocyanin of G, na talis apparently differs from that of other terrestrial crabs in showing ha emocyanin O-2 modulation by both organic and inorganic molecules. Haemocyan in O-2-affinity,vas not affected by Mg2+ but was sensitive to changes in Ca 2+ concentration (Delta logP(50)/Delta log[Ca]=-0.61, where P-50 is the par tial pressure of Oz required for half-maximal Oz binding), The Bohr factor was modest (phi=-0.26+/-0.03, N=4, in whole haemolymph at 25 degrees C) and there was no specific effect of CO2 on the O-2-binding properties of the h aemocyanin, An increase in urate concentration increased haemocyanin O-2-af finity, but the effect was linear (Delta logP(50)/Delta[urate]=-0.06) and n ot logarithmic as is the case in other species, The effect of L-lactate on the haemocyanin O-2-affinity in G. natalis was unique among the crustaceans , because an increase in L-lactate concentration decreased the haemocyanin O-2-affinity. The effect of L-lactate on haemocyanin O-2-affinity (Delta lo gP(50)/Delta log[lactate]) was time-dependent and decreased from a maximum of 0.044 on day 1 to 0.001 after 4 days of storage at 4 degrees C, The pres ence of an unknown dialysable and unstable factor in the haemolymph is post ulated to explain the time-dependent effect of L-lactate on haemocyanin O-2 -binding properties. Model oxygen equilibrium curves constructed for in viv o conditions showed that the reverse effect of L-lactate was advantageous b y decreasing the O-2-affinity of the haemocyanin beyond that predicted by t he Bohr shift alone and assisted in O-2 off-loading at the tissues. This ef fect of lactate can only provide an advantage if the gas-exchange organs ma intain arterial O-2 loading and thus is dependent on lung function in land crabs and must have occurred coincident with the evolution of these other f eatures.