Facilitated Diffusion of CO₂ across Albumin Solutions

The steady-state CO₂ flux across thin layers of 30 g/100 ml albumin solutions was measured in two different CO₂ partial pressure ranges (boundary PCO₂ values 3 and 8 torr, and 160 and 650 torr, respectively). From the data the apparent diffusion coefficient for CO₂, DCO₂, was calculated. In the high PCO₂ range a value of DCO₂ was found which is to be expected on the basis of diffusion of dissolved CO₂ only. In the low PCO₂ range DCO₂ was about 100% higher than in the high PCO₂ range, when carbonic anhydrase was present and the pH was ∼7.7. DCO₂ depended on the concentration of carbonic anhydrase. It increased with increasing pH. It is concluded that an additional diffusion of bound CO₂ (facilitated CO₂ diffusion) occurs in the low PCO₂ range and that this transport involves the hydration of CO₂. From the diffusion coefficients in the two PCO₂ ranges the rate of facilitated diffusion was determined. Approximate calculations show that this rate (at pH ≤ 7.7) can be explained on the basis of the proposed mechanism of facilitated CO₂ diffusion: bicarbonate diffusion and simultaneous proton transport by albumin diffusion. The view that facilitated CO₂ diffusion is mediated by the diffusion of albumin is supported by the experimental finding of a considerable suppression of the facilitated CO₂ flux in the presence of gelatinized agar-agar.