Facilitated Diffusion of CO2 across Albumin Solutions

The steady-state CO(2) flux across thin layers of 30 g/100 ml albumin solutions was measured in two different CO(2) partial pressure ranges (boundary PCO(2) values 3 and 8 torr, and 160 and 650 torr, respectively). From the data the apparent diffusion coefficient for CO(2), DCO(2), was calculated. In the high PCO(2) range a value of DCO(2) was found which is to be expected on the basis of diffusion of dissolved CO(2) only. In the low PCO(2) range DCO(2) was about 100% higher than in the high PCO(2) range, when carbonic anhydrase was present and the pH was approximately 7.7. DCO(2) depended on the concentration of carbonic anhydrase. It increased with increasing pH. It is concluded that an additional diffusion of bound CO(2) (facilitated CO(2) diffusion) occurs in the low PCO(2) range and that this transport involves the hydration of CO(2). From the diffusion coefficients in the two PCO(2) 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 CO(2) diffusion: bicarbonate diffusion and simultaneous proton transport by albumin diffusion. The view that facilitated CO(2) diffusion is mediated by the diffusion of albumin is supported by the experimental finding of a considerable suppression of the facilitated CO(2) flux in the presence of gelatinized agar-agar.