The effect of 4,4 '-diisothiocyanato-stilbene-2,2 '-disulfonate on CO2 permeability of the red blood cell membrane

It has long been assumed that the red cell membrane is highly permeable to gases because the molecules of gases are small, uncharged, and soluble in l ipids, such as those of a bilayer. The disappearance of (COO)-C-12-O-18-O-1 6 from a red cell suspension as the O-18 exchanges between labeled CO2 + HC O3- and unlabeled HOH provides a measure of the carbonic anhydrase (CA) act ivity (acceleration, or A) inside the cell and of the membrane self-exchang e permeability to HCO3- (P-m,P-HCO3-) TO test this technique, we added suff icient 4,4'-diisothiocyanato-stilbene-2,2'-disulfona (DIDS) to inhibit all the HCO3-/Cl- transport protein (Band III or capnophorin) in a red cell sus pension. We found that BIDS reduced P-m,P-HCO3- as expected, but also appea red to reduce intracellular A, although separate experiments showed it has no effect on CA activity in homogenous solution. A decrease in P-m,P-CO2 mo uld explain this finding. With a more advanced computational model, which s olves for CA activity and membrane permeabilities to both CO2 and HCO3-, we found that DIDS inhibited both P-m,P-HCO3- and P-m,P-CO2, whereas intracel lular CA activity remained unchanged. The mechanism by which DIDS reduces C O2 permeability may not be through an action on the lipid bilayer itself, b ut rather on a membrane transport protein, implying that this is a normal r oute for at least part of red cell CO2 exchange.