Oxygen and carbon dioxide transporting qualities of hemocyanin in the hemolymph of a natant decapodPalaemon adspersus

Abstract

1. The O2 and CO2 combining properties ofPalaemon adspersus hemolymph is studied, aiming to assess respiratory function and the environmental and metabolic adaptations of hemocyanin of natant decapods where, in contrast to the intensively-studied, larger and predominantly less active reptant decapods, virtually no information is available. 2. The hemolymph shows a high O2 carrying capacity (mean=2.8 vol%), a low O2 affinity (at 15 °C half-saturation tension,P50=16 and 37 mm Hg at pH 7.85 and 7.65, respectively), pronounced cooperativity in O2 binding (Hill's coefficient,n≃2.8) and a large, pH dependent Bohr factor (ϕ=ΔlogP50/ΔpH=−2.0 and −0.9 at pH 7.85 and 7.4, respectively) (Figs. 1 and 2). These qualities are distinct from those typifying reptant hemocyanins and appear illsuited for O2 transport at low ambient tensions, but well-adapted for O2 delivery in tissues at highPO2, supporting high levels of metabolism and activity. 3. CO2 has a specific, augmenting effect on O2 affinity at high pH (Fig. 3), indicating carbamate formation with an opposite oxygenation-linkage as in vertebrate hemoglobins. Astrup titrations indicate the presence of a small but distinct Haldane effect at physiological pH, and buffering capacity varies greatly (ΔHCO3/ΔpH≃−4.4 to −9.3 mmol·1−1·(pH unit)−1 depending on hemocyanin concentration) (Fig. 4). Equilibrium curves of total, non-protein-bound CO2 show large capacitance for transport at low, in vivo CO2 tensions (Fig. 4). 4. The data are discussed comparatively, particularly as regards hemocyanin function in reptant decapods, and the O2, CO2 and proton exchanges involved.