Die Wirkung von N-�thylmaleinimid auf die Kationenpermeabilit�t von Menschenerythrocyten

Ion selectivity of aqueous leaks induced in the erythrocyte membrane by crosslinking of membrane proteins

The aqueous leak induced in the human erythrocyte membrane by crosslinking of spectrin via disulfide bridges formed in the presence of diamide (Deuticke, B., Poser, B., Lütkemeier, P. and Haest, C.W.M. (1983) Biochim. Biophys. Acta 731, 196-210) was further characterized with respect to its ion selectivity by means of (a) measurements of cell volume changes or hemolysis, (b) determination of membrane potentials and (c) analysis of potential-driven ion fluxes. The leak turned out to be slightly cation-selective (PK:PCl approximately equal to 4:1). It discriminates mono- from divalent ions (PNa:PMg greater than 100:1, PCl:PSO4 greater than 10:1) and to a much lesser extent monovalent ions among each other. The selectivities for monovalent ions follow the sequence of free solution mobilities, increasing in the order Li+ less than or equal to Na+ less than K+ less than or equal to Rb+ less than Cs+ and F- less than Cl- less than Br- less than I-. Polyatomic anions also fit into that order. Quantitatively, the ratios of permeabilities of the leak are larger than those of the ion mobilities in free solution. The ion permeability of the leak is concentration-independent up to at least 150 mM. The ion milieu, however, has marked effects on leak permeability, most pronounced for chaotropic ions (guanidinium, nitrate, thiocyanate), which increase leak fluxes of charged and uncharged solutes. The results support the view that, besides geometric constraints, weak coulombic or dipolar interactions between penetrating ions and structural elements of the leak determine permselectivity.

Studies on lithium transport across the red cell membrane. VI. Properties of a sulfhydryl group involved in ouabain-resistant Na+-Li+ (and Na+-Na+) exchange in human and bovine erythrocytes

The reactivity of the SH-group essential for ouabain-resistant Na+-Li+ (and Na+-Na+) exchange and its location within the membrane are studied on human and beef erythrocytes and beef red cell ghosts. N-ethylmaleimide (NEM), 1,6-hexane dimaleimide, and iodoacetamide can induce an irreversible, partial inhibition of Na+-Li+ exchange in erythrocytes of the two species. The development of the inhibition due to the alkylating agents is greatly accelerated by external Na+ and Li+. The inhibition takes 3 min (NEM) and 60 min (iodoacetamide) to come to completion in isotonic Na+ media, but is hardly detectable in choline+, K+ or Mg2+ media. The transport site of the exchange system and the site promoting NEM binding exhibit similar affinities for external Na+. The impermeable, monofunctional glutathione derivative of 1,6-hexane dimaleimide does not inhibit Na+-Li+ exchange. The mercurials PCMBS, PCMB, and Hg2+ inhibit Na+-Li+ exchange in beef, but not in human erythrocytes. The inhibitory action of PCMBS, being slightly accelerated by external Na+, is fully reversed by penetrating thiols such as 2-mercaptoethanol, whilst glutathione, an impermeable thiol, is ineffective. Pretreatment with PCMBS affords partial protection from the irreversible inhibition caused by NEM. Oxidation with copper orthophenanthroline inhibits Na+-Li+ exchange only when performed in the presence of penetrating thiols such as 2-mercaptoethanol. It is concluded that the SH-reagents studied inhibit Na+-Li+ exchange by modifying an essential SH-group of a membrane protein in such a way that the turnover number of the exchange system is reduced. This SH-group is separated from both the red cell exterior and interior by a penetration barrier and seems to be distinct from the cation binding site. The action of external Na+ and Li+ in promoting the reaction of alkylating inhibitors is interpreted to result from a conformational change of the transport protein induced by the binding of external Na+ or Li+.