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Abstract

During the last few years the information on the penetration characteristics of slowly permeating anion species such as sulphate and phosphate has considerably grown and, for the first time, reasonably reliable experimental data on the penetration characteristics of rapidly permeating anion species such as chloride and the other halides has become available. The expansion of factual knowledge was accompanied by a critical reconsideration of existing hypotheses on the molecular mechanism of anion penetration in the red blood cell. In addition new concepts emerged which differ basically from the previously accepted ones. However, in view of the high rate at which new data is collected, one can expect that today’s assessment of the relative merits of the various proposed models of anion transfer across the red blood cell membrane may need to be changed again tomorrow. Nevertheless, a brief discussion of certain theoretical models of the kinetics of anion permeability may help to clarify current issues and contribute to the formulation of better and more lasting concepts. The present paper is devoted to such a discussion. We shall first focus on slowly penetrating sulphate ions and consider the relative merits of a fixed charge model and a carrier model of anion transfer. In the carrier model the assumption is made that the combination between the anion to be transported and the carrier takes place much faster than the penetration of the loaded carrier across the membrane. Turning to halide anions which penetrate about 104–105 times faster than sulphate, we shall discuss the merits and problems of another carrier model which involves the assumption that the combination between the anion to be transported and the carrier, rather than the penetration of the loaded carrier across the membrane, is rate limiting. Finally, we shall formulate a series of questions which would have to be answered by any useful model of anion transfer across the red blood cell membrane.