Oxygen-linked CO2 binding independent of pH in cephalopod blood.

Abstract

The oxygen affinity of the haemocyanin-containing blood of some cephalopods is very dependent on the pH of the blood1,2. The effect of pH on the oxygen affinity is expressed quantitatively as the slope of the function between log P50 and pH, where P50 denotes the oxygen tension at an oxygen saturation fraction of 0.5. This Bohr coefficient, Δ log P50/Δ pH, is about −1.8 in the case of Loligo pealei1 and about −1.5 for Sepia officinal is2. These pronounced Bohr effects are physiologically significant as they should allow the metabolically produced CO2 in the tissues to promote the dissociation of oxyhaemocyanin and thus facilitate the unloading of oxygen to the tissues. However, the Bohr effect is tantamount to an oxygen-linked hydrogen ion binding to the respiratory pigment and the quantitative parameter of this Haldane effect is the change in the concentration of haemocyanin-bound hydrogen ion (ΔcHcH) accompanying a change in the concentration of haemocyanin-bound oxygen (ΔcHcO2) at a constant pH. According to the linkage equation3, the Bohr coefficient and the Haldane coefficient are identical: Δ log p50/Δ pH = (ΔcHcH/ΔcHcO2)pH. This implies that in a homogeneous system, such as in cephalopod blood, the amount of protons bound to the haemocyanin upondeoxygenation from the fully oxygenated state, at constant pH,equals the negative value of the Bohr coefficient times the oxygen capacity. Thus, a Bohr coefficient of < −1 implies that more than1 mmol of protons will be bound to the pigment per mmol of O2 unloaded. Since the CO2 produced in the tissues can maximally yield Immol of protons per mmol of oxygen consumed, at a respiratory exchange ratio of unity, we rein-vestigated the reciprocal effects of O2 and CO2 on the respiratory properties of cephalopod blood to see whether blood pH can decrease during O2 unloading when CO2 is the only ‘proton source’ and the Bohr coefficient is < −1.