The specific influence of carbon dioxide and carbamate compounds on the buffer power and Bohr effects in human haemoglobin solutions

Abstract

1. The difference of pH (DeltapH) between human deoxygenated haemoglobin (Hb) and oxygenated haemoglobin (O(2)Hb) solutions when equilibrated with physiological pressures of carbon dioxide is (experimentally) much less than previously supposed.2. This smaller DeltapH is in contradiction to Wyman's (1948) theoretical calculations, wherein no allowance was made for the specific effect of carbamino-compounds on the amount of base neutralized by haemoglobin. Other previous authorities have also neglected this factor, which when properly allowed for restores the role of carbamino-compounds in CO(2) transport practically to that previously estimated by Ferguson & Roughton (1934a, b).3. At a given pH and P(CO(2) ), more CO(2) is bound by Hb solutions than by O(2)Hb, the difference increasing with pH. This result provides further, and seemingly decisive, evidence that the bound-CO(2) in the blood other than HCO(3)- (i.e. x-bound CO(2)) is oxygen-linked.4. According to a modified form of the Henderson-Hasselbalch equation for haemoglobin solutions [Formula: see text]. The value of pK(1)' is the same in Hb as in O(2)Hb solution and from the data in 3 is found to have the value 6.15 at 37 degrees C.5. The difference between the titration curves of O(2)Hb and Hb (DeltaX), at a given pH had been hitherto supposed to be the same in presence of CO(2) as in its absence. Our experiments show, however, that DeltaX is less in presence of CO(2) and at pH > 7.5 may even change sign. This paradoxical effect is also explicable, at any rate semi-quantitatively, by the effect of carbamino compounds on the buffer power, according to the theory put forward in the paper.6. The results show that the buffer power (dB/dpH) of haemoglobin solution under physiological conditions is 20-30% greater than previously estimated, and this also is in line with the new theory.7. In graphs of total CO(2) versus P(CO(2) ) in haemoglobin solutions (or blood) it has been customary to suppose that points on straight lines radiating from the origin are points of equal pH. Our data, however, show that the iso-pH lines drawn through the experimental points in the pH range, 7.2-7.4 do not, when produced as straight lines, pass through the origin, but intercept the P(CO(2) ) axis significantly to the right thereof.8. Calculations indicate that most of the x-bound CO(2) in haemoglobin solutions at pH 7.2-7.4 and at 37 degrees C can be accounted for by carbaminobound CO(2) without the need of postulating the existence of appreciable amounts of yet other forms, i.e. y-bound CO(2), in this range.