The anion transport system of the red blood cell. The role of membrane protein evaluated by the use of 'probes'.

Transport of L-lactate across the plasma membrane is of considerable importance to almost all mammalian cells. In most cells a specific H(+)-monocarboxylate cotransporter is largely responsible for this process; the capacity of this carrier is usually very high, to support the high rates of production or utilization of L-lactate. The best characterized H(+)-monocarboxylate transporter is that of the erythrocyte membrane, which transports L-lactate and a wide range of other aliphatic monocarboxylates, including pyruvate and the ketone bodies acetoacetate and beta-hydroxybutyrate. This carrier is inhibited by alpha-cyanocinnamate derivatives and some stilbene disulfonates and has been identified as a protein of 35-50 kDa on the basis of purification and specific labeling experiments. Other cells possess similar alpha-cyanocinnamate-sensitive H(+)-linked monocarboxylate transporters, but in some cases there are significant differences in the properties of these systems, sufficient to suggest the existence of a family of such carriers. In particular, cardiac muscle and tumor cells have transporters that differ in their Km values for certain substrates (including stereoselectivity for L- over D-lactate) and in their sensitivity to inhibitors. Mitochondria, bacteria, and yeast also possess H(+)-monocarboxylate transporters that share some properties in common with those in the mammalian plasma membrane but are adapted to their specific roles. However, there are distinct Na(+)-monocarboxylate cotransporters on the luminal surface of intestinal and kidney epithelia, which enable active uptake of lactate, pyruvate, and ketone bodies in these tissues. This article reviews the properties of these transport systems and their role in mammalian metabolism.

Transport of lactate and other monocarboxylates across mammalian plasma membranes

Mammalian cell membranes harbor several types of chloride channels, chloride-cation symporters/cotransporters, and several classes of anion exchangers/antiporters. These transport systems subserve different cellular or organismic functions, depending on the nature of the cell, the spatial organization of transporters, and their functional interplay. Chemical probing has played a central role in the structural and functional delineation of the various anion transport systems. The design of specific probes or their selection from existing sources coupled with their judicious application to the most appropriate biological system had led to the identification of specific anion transporters and to the elucidation of the underlying molecular transport mechanism. In many instances, chemical probing has remained the major or exclusive analytical tool for the functional definition or identification of a given transport system, particularly for discerning among the various anion transporters which operate in highly heterogeneous cell membrane systems. This work critically reviews the present state of the chemical armamentarium available for the most common anion transporters found in mammalian cell membranes. It encompasses the description of the most useful or commonly used probes in terms of their chemical, biochemical, physiological, and pharmacological properties. The review deals primarily with what chemical probes tell about anion transporters and, most importantly, with the limitations inherent in the use of probes in transport studies.

Chemical probes for anion transporters of mammalian cell membranes

CO2produced within skeletal muscle has to leave the body finally via ventilation by the lung. To get there, CO2 diffuses from the intracellular space into the convective transport medium blood with...

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Carbon dioxide transport and carbonic anhydrase in blood and muscle

Publisher Summary This chapter reviews that the transport of anions is such a prominent feature of the red cell membrane that failure to observe this feature in the membrane would be tantamount to overlooking the presence of hemoglobin in the erythrocyte cytoplasm. It explores that the membrane specialization, together with the relative ease with which transport experiments can be performed and membranes is prepared from red cells, has made this system one of the best-studied mammalian transport systems. It is one of the few such systems where sophisticated and detailed studies of both the transport kinetics and the biochemical nature of the proteins thought to mediate the transport have been performed. The development of a large number of chemical probes that are used to inhibit the system and/or to label sites for biochemical analysis, has provided an important link between these two fields of investigation. These techniques and approaches have proved useful for analyzing other transport systems, have produced a very large body of information concerning the red cell anion transport system in a relatively short time. The chapter focuses on assembling the evidence from both kinetic and biochemical sources concerning particular aspects of the mechanism of anion transfer, concentrating on recent findings, and speculates on the answers to some central questions about the nature of the transport system.

Erythrocyte Anion Exchange and the Band 3 Protein: Transport Kinetics and Molecular Structure

1. The penetration of organic anions into bovine red blood cells has been studied under experimental conditions where it could be distinguished from the penetration of undissociated acids which proceeds by diffusion through lipid zones of the membrane. 2. Several lines of evidence suggest that the entry of organic anions cannot be ascribed to simple diffusion across aqueous channels limited by positive charges but needs a specific interaction of the penetrating anion with a component of the membrane. 3. The structural requirements allowing for ionic transfer is a strong polar head for the smallest molecules and in addition an amphiphilic structure for acids with chain length greater than C4. Interaction between substrate and receptor requires at least a three point attachment involving three oxygen atoms in the substrate which react with complementary loci on the receptor to form ionic and hydrogen bonds. Such a three point attachment can be made by a sulphonic group or with carboxylic acid by alpha ketosubstitution, alpha hydroxysubstitution, addition of an amidegroup or addition of a second carboxyl group spatially close to the first. 4. As suggested by the behaviour of the formate anion, in such a transport system any carboxylic acid could interact transiently with the receptor and therefore interfere with the transport of an organic anion even though such ionic interaction with the receptor were insufficient to produce transport of the acid itself.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.1975.sp010884

R Motais

Papers

Molecular features of organic anion permeablity in ox red blood cell.