Adaptation of the oxygen transport system to hypoxia in the blue crab, Callinectes sapidus

Adaptation of the Oxygen Transport System to Hypoxia in the Blue Crab, Callinectes sapidus'

CHARLOTTE P MANGUM2

Department of Biology, College of William & Mary, Williamsburg, Virginia 23187-8795

SYNOPSIS. Among the oxygen carrying proteins, two groups are known to adapt to environmental challenges in the adult stage. Both vertebrate hemoglobins and arthropod hemocyanins adapt to chronic hypoxia by responding to the actions of allosteric co-factors. The design strategy, however, differs fundamentally in the two groups. Even within the arthropods, chelicerate and crustacean hemocyanins respond to co-factors very differently. Only in the crustaceans does the oxygen carrier adapt by shifts in intrinsic molecular properties. In hypoxic blue crabs, increases in the ratio of the primitive 1 x 6-meric oligomer bring about a higher oxygen affinity relative to that in normoxic animals, in which greater proportions of the derived 2 x 6-meric oligomer are responsible for a lower oxygen affinity.

INTRODUCTION

A priori, the functional properties of a protein can adapt by either or both of two mechanisms: 1) changes in the actions of extrinsic co-factors that influence the active site, and 2) changes in molecular structure that influence the active site. Although both mechanisms are responsible for adaptations of the vertebrate hemoglobins (Hbs) to hypoxia, intrinsic molecular adaptation is known only during embryonic development in some viviparous groups. Among obligatory air-breathers, adaptations of the vertebrate Hbs by either mechanism are infrequent in adults, which is not surprising (for an interesting exception of an adaptation unrelated to altitude, see Ingermann, 1992). And among vertebrate water-breathers, the design strategy entails the release of an allosteric action of organic POns, which normally lower oxygen affinity. ATP is one of the chief allosteric co-factors in fishes. Therefore, one might suppose that adaptation to hypoxia automatically results from lowering the adenylate energy charge in RBCs, as in non-circulating cells, in a simple regulatory device which is tightly coupled to the status of aerobic metabolism.

Quite the reverse, however. In a species that utilizes both ATP and GTP (guanosine triPO4), ATP levels are defended while the increased O2 affinity is due entirely to decreased GTP (Jensen and Weber, 1985). Although several investigators have inferred that the changes in organic PO4s are brought about by changes in oxidative phosphorylations, in fact the metabolic control mechanisms are not understood (reviewed by Weber and Jensen, 1988). Vertebrate Hbs also adapt to hypoxia by purely quantitative increases of the O2 carrying capacity of the blood. Since Hb synthesis is not tightly coupled to organic PO4 metabolism within the red blood cell, the control mechanism must be quite different. The endocrine control of mammalian red blood cell manufacture, and thus of Hb synthesis, by erythropoetin is discussed in this symposium by Hochachka (1997).

Arthropod hemocyanins (AHcs) also adapt to hypoxia in the adult stage of the life cycle, and the adaptations entail both extrinsic and intrinsic mechanisms. As in vertebrates, sublethal hypoxia induces higher quantities of AHcs in the blood, with an attendant increase in Oz carrying capacity. For details, I refer the reader to a discussion of this phenomenon written a few years ago (Mangum, 1990). In the adaptation of the AHcs to hypoxia, however, the induction is not always purely quantitative; it is believed to expedite adaptation at the intrinsic molecular level. The possibility of endocrine control of the induction has not been investigated.

HYPOXIA ADAPTATION BY COFACTORS IN ARTHROPODS

The simplest design strategy, which produces an extremely effective response to hypoxia, is allosteric modulation of many decapod AHcs by a co-factor, as in the lower vertebrate hemoglobins. The AHc effector first understood at a physiological level was the H+, which works by the well known Bohr shift. Despite extremely similar molecular structures, however, the AHcs in different groups of arthropods exhibit directly opposite responses to pH, and their physiological roles are also quite different.

The importance of the reversed Bohr shift as an adaptation to hypoxia was first pointed out by the late workers Kjell Johansen and Jorge Petersen (1975). In the chelicerate horseshoe crab Limulus polyphemus, hypoxia severe enough to require anaerobic metabolism results in accumulation in the blood of lactate, oddly enough the D- stereoisomer, which lowers blood pH and sharply raises O2 affinity.

PH can also be an effector of an increased O2 affinity in crustaceans, but the physiological aspects differ fundamentally because the Bohr shift is normal; alkalinization, not acidification, of the blood brings it about. In a number of crabs, hypoxia not severe enough to require anaerobic metabolism stimulates hyperventilation, which enhances CO2 excretion; blood pH and, therefore, AHc O2 affinity, rise (Truchot, 1975).