Incorporation of 3H-N-ethylmaleimide into sheep red cell membrane thiol groups following protection by diamide-induced oxidation

Abstract

The thiol oxidant diazene dicarboxylic acid bis [N,N-dimethylamide] (diamide) is known to reversibly activate K-Cl cotransport in sheep red blood cells. Although the detailed mechanism of activation is unknown, functional thiols at the membrane or at the cytoplasmic level are recognized as important. To search for membrane bound thiols involved in the regulation of K-Cl cotransport, sheep red cells were first exposed to diamide at concentrations activating K-Cl cotransport, and then to the alkylating agent N-ethylmaleimide (NEM) in order to block non-oxidized thiols. White ghosts, prepared by osmotic lysis from these cells, were again treated with NEM followed by reduction of the diamide-induced dithiols with dithiothreitol (DTT) concentrations known to reverse the diamide-induced K-Cl flux. Maximum 3H-NEM incorporation into the DTT-reduced thiols occurred at 50 microM DTT. Saturation labelling by 3H-NEM of about 2 x 10(4) diamide-protected thiols/cell occurred at 25 microM NEM. Diamide protected about 0.1% of all membrane thiols chemically determined earlier. Membranes from high K (HK) and low K (LK) sheep red cells did not differ significantly in the number of diamide-protected thiols, and polyacrylamide gels revealed a similar protein distribution of 3H-NEM-labelled thiols. Since diamide is known to stimulate K-Cl flux in LK cells ten times more than in HK cells this finding is consistent with the hypothesis of a cytoplasmic control effecting different K-Cl flux activities in the membranes of the two cation genotypic red blood cells.