Facilitated Diffusion of CO2 across Albumin Solutions

Abstract

The steady-state CO2 flux across thin layers of 30 g/100 ml albumin solutions was measured in two different C02 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 CO2, 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 CO2 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 -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 CO2 diffusion) occurs in the low Pco 2 range and that this transport involves the hydration of CO2. 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 CO2 flux in the presence of gelatinized agar-agar.

Diffusion coefficients of CO2 molecule and bicarbonate ion in hemoglobin solution measured by fluorescence technique.

“…The Pco2 profile shows a linear decay and the gaps of 0.8 Torr at the boudaries. The diffusion rate per unit Pco2 difference was 0.35 x 10-5 ml/(cm2 sec Torr), and the effective diffusion coefficient was 2.4>< 10-5 cm2/sec, which is in good agreement with the value, 2.1 x 10-5 cm2/sec, estimated from the data obtained by GROS and MOLL (1974) in a P0, range of 4.2 to 7.3 Torr, by using the above temperature factors. In order to enhance the accuracy of the diffusivity, further studies are of course necessary, but it may be safe to say even at the present stage that the facilitated C02 diffusion described by GROS and MOLL (1974) is to a great extent due to the HC03-diffusion.…”

“…9. In other words, the integrated dif fusivity increases as the Pco2 decreases, as already demonstrated by LONGMUIR et al (1966) andMOLL (1974).…”

“…it becomes 1.6 x 10-5 cm2/sec, which is about 60 % higher than the value, 0.9>< 10-5 cm2/sec, estimated from the data obtained by GROS and MOLL(1971) in Hb solutions of 22°C in a PC02 difference of about 1 atm. The temperature factors of the solubility and the diffusion constant used in the estimation were 1.48 and 0.925, respectively, which were referred to the data of BARTELS et al (1959) and GROS and MOLL (1974). The lower curve (2) of Fig.…”

Interstitial PCO2 and pH, and their role as chemostimulants in the isolated respiratory network of neonatal rats.

“…Taking into account the effect of ionic strength on DHCo,-(see Appendix in Gros & Moll, 1974), the limiting equivalent conductivity of bicarbonate (44 5 S cm2 equiv-'; Robinson & Stokes, 1959) yields DHco3-= 0 88 x 10O5 cm2 S-1 in saline solutions used in the present study. The data therefore give A2/a = 4-75 + 2-3 for anoxic tissue.…”

The chemistry, physiology and pathology of pH in cancer

“…Although the spontaneous hydration reaction is very slow (time constant 5 s), it can be catalysed by exo-facial carbonic anhydrase (CA) enzymes [23][24][25], such as the tumour-associated isoforms CAIX and CAXII [26][27][28][29][30]. Catalysed conversion of CO 2 to HCO 3 À and H þ can facilitate overall CO 2 diffusion by means of a parallel flux of H þ þ HCO 3 À , a phenomenon first described in vitro by Gros & Moll [31]. Akin to the diffusion of other ionized weak acids, CA-facilitated CO 2 diffusion also requires adequate mobile buffering to carry H þ ions in parallel to CO 2 and HCO 3 À [32] (figure 2b).…”