The enzyme-inhibitor approach to cell-selective labelling--I. Sulphonamide inhibitors of carbonic anhydrase as carriers for red cell labelling: in vitro uptake of pIBS by human red blood cells.

Red cell carbonic anhydrase is identified as an ideal target in an enzyme-inhibitor approach to radiolabel localisation. Current problems in blood pool labelling could be overcome by using selective sulphonamide inhibitors as carriers. p-Iodobenzenesulphonamide (pIBS) was selected as the choice reagent for red blood cell labelling. Rapid uptake of [125I]-pIBS was found in vitro, consistent with passive diffusion across the cell membrane. The intracellular binding could be attributed to interaction with two specific acceptor sites, with dissociation constants of 4.9 +/- 1.0 and 0.10 +/- 0.05 mumol dm-3, and maximum binding capacities of 166 +/- 5 and 19.9 +/- 1.0 mumol dm-3, respectively under the experimental conditions. These data correlate with the two major carbonic anhydrase isozymes; acceptor assignments were confirmed by gel chromatography of the red cell lysate.