Respiratory Function of the Hemocyanins

Abstract

Recent studies clearly demonstrate the respiratory importance of the hemocyanins in each of the three animal phyla in which they occur. Despite their generally low oxygen affinity, hemocyanins can be highly oxygenated at the site of gas exchange with the medium as well as deoxygenated at the tissues. The functional range of a hemocyanin oxygen transport system is severely limited however by environmental change. These systems function under incipient hypoxia due largely to responses of blood pH which are not fully understood a normal Bohr shift is accompanied by a rise in blood pH and a reverse Bohr shift by a decrease in blood pH. In both instances blood oxygen affinity increases and its oxygenation state at the gill remains high in spite of its lower Po2. Dilution of the blood at low salinity generally alters its oxygenation properties both oxygen affinity and cooperativity. These properties may or may not be restored by concomitant changes in blood pH, which depend on the various mechanisms of osmotic adaptation. Within a homogeneous taxon the oxygenation properties of a hemocyanin appear to be highly conservative showing little interspecific adaptation except to extreme changes in the mode of gas exchange. Unlike that in vertebrates air-breathing in crustaceans is accompanied by an increase in blood oxygen affinity. Similar oxygen affinities in latitudinally separated species result in optimal functioning of the system at the same temperature, corresponding to different seasons. In eurythermal species a temperature acclimation of oxygen affinity extends the operating range of the crustacean hemocyanins but they cannot deoxygenate at very low temperatures. Unsolved problems of hemocyanin function include specific effects of pH and CO2 the basis of which is not entirely clear, and the postulated occurrence in native blood of both dialyzable and non-dialyzable substances that modify oxygen affinity the identity of which is unknown. With the exception of the crustacean oxygen carrier the hemocyanins confer a respiratory advantage over their predecessors. But the oxygen carrying capacity of crustacean blood never reaches the levels found in the annelids and molluscs due to the colloid osmotic pressure of the relatively low molecular weight hemocyanin and to the drop in blood hydrostatic pressure accompanying the loss of a fluid skeleton. The selection of a blood oxygen carrier with an apparently limiting combination of respiratory and osmotic properties is obscured by the uncertain phylogenetic position of the phylum.