BOHR AND ROOT EFFECTS IN CEPHALOPOD HEMOCYANINS - PARADOX OR PRESSUREIN SEPIA-OFFICINALIS

The oxygen affinity, expressed as the P-50, of the haemocyanin of most cephalopods so far studied is relatively low (P-50 > 20 Torr in 75% o f the studies). At the same time the Bohr effect, the dependence of ox ygen affinity on pH, is large (Bohr coefficient < -1.0 in 80% of the s tudies). Spectrophotometric measurements of oxygen dissociation curves of the haemocyanin from Sepia officinalis at high oxygen tensions (> 500 Torr) indicate that 100% saturation is not attained at low pH (6.5 ) compared to high pH (7.5). Cooperativity, expressed as n(50) also de creases with pH and these two characteristics suggest the presence of a Root effect, i.e. a dependence of maximum oxygen carrying capacity ( O-2 cap) on pH. Oxygen dissociation curves at low pH however, were not asymptotic, indicating that full saturation may be reached at higher oxygen tensions. Gasometric measurements of oxygen carrying capacity u sing low pH and high PO2 values (< 650 Torr) also indicated the presen ce of a Root effect. However, high oxygen pressure measurements (> 1 a tmosphere) in a specially constructed chamber show that 100% saturatio n was reached at higher oxygen tensions, indicating that this apparent Root effect is not due to an absolute conformational change, as in so me fish haemo_globins, but to an exaggerated Bohr effect. Physiologica lly however, neither such high oxygen pressures nor such low pH values have been measured to date in the blood of cephalopods in vivo making the functional use of a Root effect doubtful.