Role of glucose carrier in human erythrocyte water permeability

Abstract

Although the transport properties of human erythrocyte water channels have been well characterized, the identity of the protein(s) mediating water flow remains unclear. Recent evidence that glucose carriers can conduct water raised the possibility that the glucose carrier, which is abundant in human erythrocytes, is the water channel. To test this possibility, water permeabilities and glucose fluxes were measured in large unilamellar vesicles (LUV) containing human erythrocyte lipid alone (lipid LUV), reconstituted purified human erythrocyte glucose carrier (Glut1 LUV), or reconstituted glucose carrier in the presence of other human erythrocyte ghost proteins (ghost LUV). In glucose and ghost LUV, glucose carriers were present at 25% of the density of native erythrocytes, were oriented randomly in the bilayer, and exhibited characteristic inhibition of glucose flux when exposed to cytochalasin B. Osmotic water permeability (Pf, in centimeters per second; n = 4) averaged 0.0012 +/- 0.00033 in lipid LUV, 0.0032 +/- 0.0015 in Glut1 LUV, and 0.006 +/- 0.0014 in ghost LUV. Activation energies of water flow for the three preparations ranged between 10 and 13 kcal/mol; p-(chloromercuri)benzenesulfonate (pCMBS), an organic mercurial inhibitor of erythrocyte water channels, and cytochalasin B did not alter Pf. These results indicate that reconstitution of glucose carriers at high density increases water permeability but does not result in water channel activity. However, because the turnover number of reconstituted carriers is reduced from that of native carriers, experiments were also performed on erythrocyte ghosts with intact water channel function. In ghosts, Pf averaged 0.038 +/- 0.013 (n = 9), while the activation energy for water flow averaged 3.0 +/- 0.3 kcal/mol.(ABSTRACT TRUNCATED AT 250 WORDS)

Maturational Changes in Rabbit Renal Basolateral Membrane Vesicle Osmotic Water Permeability

“…Thus, the water channels may not have been totally blocked and allowed some water transport to occur. Second, there is accumulating data that other transporters (e.g., the glucose transporter) may transport water and would therefore allow for water transport in the presence of mercury (Fischbarg et al, 1990;Zeidel et al, 1992;Fischbarg & Vera, 1995). Third, the lipid content of the membrane is known to be different from the brush border membrane and may alter its interaction with the water channel and water transport (Le Grimellec et al, 1982).…”

Functional Analysis of Aquaporin-1 Deficient Red Cells

“…Low molecular weight solutes do not permeate through AQP1 (Zeidel et al 1992a(Zeidel et al , 1992b(Zeidel et al , 1994, and urea is now known to be transported across red cell membranes by a protein unrelated to the aquaporins (Olives et al, 1995). Therefore, it is reasonable to conclude that water goes through AQP1 by single file diffusion (reviewed by Finkelstein (1987)).…”

“…4), which appears to be comprised of a small PCMBS-inhibitable pathway in addition to the PCMBS-resistant pathway, and their composite actions exhibit a high activation energy indicating that they are not aqueous pores. While the identities of these two minor water permeation pathways are unknown, it may be speculated that the PCMBS-inhibitable pathway is a protein, and the glucose transporter has been shown to transport water at a very low rate (Fischbarg et al, 1990;Zeidel et al, 1992a). The PCMBS-resistant pathway may represent simple diffusion through the lipid bilayer.…”

Inactivation by Hg2+ and methylmercury of the glutamine/amino acid transporter (ASCT2) reconstituted in liposomes: Prediction of the involvement of a CXXC motif by homology modelling

“…Indeed, it was reported that some transport systems catalyze the transport of mercuric compounds through biological membranes [2,[7][8][9]. Furthermore, interaction of mercuric compounds with transport systems of plasma membrane [1,[10][11][12][13][14][15] and also of mitochondrial membrane [16][17] have been reported to cause alterations of function. In several cases the effects and the sites of interaction of mercurials have been characterized [14-23; 24 and refs.…”