Factors controlling the in vitro and in vivo oxygen affinity of the hemocyanin in the crab Carcinus maenas (L.)

Abstract

The oxygen affinity of the hemocyanin of the crab Carcinus maenas was studied in vitro by measuring the O 2 partial pressure at 50% saturation, P 50 at various conditions of pH, temperature and salt concentration. At alkaline pH, the hemocyanin exhibits a normal Bohr effect, inverted below pH = pH 0 = 6.9 at 15 °C. A rise of temperature provokes an increase of both P 50 and pH 0. Within the physiological range, only Ca + + and Mg + + ions influence the oxygen affinity. Both ions decrease P 50, and Mg + + increases the Bohr factor, ΔlogP 50/ΔpH. The variations of P 50 due to changes of pH, temperature (°C) and Ca + + and Mg + + concentrations (rnEq.L −1) are satisfactorily described by the empirical quantitative expressions: log P 50 = k′ + 0.03 t − 0.0425[ Ca ++] + 1 2 − (0.029[ Mg ++] 1 2 + 0.242)( pH−pH 0) 2 pH 0 = 7.35−0.03 t Long-term temperature adaptation of the animals results in changes of the parameter k′ which appears to be dependent on the concentration of an unidentified dialysable substance in blood. As temperature rises, this phenomenon limits the in vivo increase of P 50 induced by both the intrinsic temperature effect and the decrease of in vivo blood pH. When external salinity decreases, lower blood Ca + + and Mg + + concentrations entail an increase of P 50 but without changes of k′, an effect compensated for in vivo by an increase of the blood pH. The observations suggest that the functional oxygen affinity of the pigment is physiologically regulated to ensure an adequate oxygen transport by the blood in quite varied conditions of temperature and salinity.