Rapid Exchange of Fluoroethylamine via the Rhesus Complex in Human Erythrocytes: 19F NMR Magnetization Transfer Analysis Showing Competition by Ammonia and Ammonia Analogues

The NMR 'split peak effect' in cell suspensions: Historical perspective, explanation and applications

The physicochemical environment inside cells is distinctly different from that immediately outside. The selective exchange of ions, water and other molecules across the cell membrane, mediated by integral, membrane-embedded proteins is a hallmark of living systems. There are various methodologies available to measure the selectivity and rates (kinetics) of such exchange processes, including several that take advantage of the non-invasive nature of NMR spectroscopy. A number of solutes, including particular inorganic ions, show distinctive NMR behaviour, in which separate resonances arise from the intra- and extracellular solute populations, without the addition of shift reagents, differences in pH, or selective binding partners. This 'split peak effect/phenomenon', discovered in 1984, has become a valuable tool, used in many NMR studies of cellular behaviour and function. The explanation for the phenomenon, based on the differential hydrogen bonding of the reporter solutes to water, and the various ways in which this phenomenon has been used to investigate aspects of cellular biochemistry and physiology, are the topics of this review.

Determination of glucose exchange rates and permeability of erythrocyte membrane in preeclampsia and subsequent oxidative stress-related protein damage using dynamic-19F-NMR

The cause of the pregnancy condition preeclampsia (PE) is thought to be endothelial dysfunction caused by oxidative stress. As abnormal glucose tolerance has also been associated with PE, we use a fluorinated-mimic of this metabolite to establish whether any oxidative damage to lipids and proteins in the erythrocyte membrane has increased cell membrane permeability. Data were acquired using ¹⁹F Dynamic-NMR (DNMR) to measure exchange of 3-fluoro-3-deoxyglucose (3-FDG) across the membrane of erythrocytes from 10 pregnant women (5 healthy control women, and 5 from women suffering from PE). Magnetisation transfer was measured using the 1D selective inversion and 2D EXSY pulse sequences, over a range of time delays. Integrated intensities from these experiments were used in matrix diagonalisation to estimate the values of the rate constants of exchange and membrane permeability. No significant differences were observed for the rate of exchange of 3-FDG and membrane permeability between healthy pregnant women and those suffering from PE, leading us to conclude that no oxidative damage had occurred at this carrier-protein site in the membrane.

NMR magnetization-transfer analysis of rapid membrane transport in human erythrocytes

Nuclear magnetic resonance (NMR) magnetization-transfer (MT) experiments provide a convenient tool for studying rapid sub-second membrane-transport processes in situ in metabolically active cells. These experiments are used with membrane-permeable substances when separate (resolved) NMR signals are observed from their populations inside and outside the cells. Here, we provide a description of the theory and practice of the most common NMR MT experiments that have been used to study membrane-transport processes in human erythrocytes (red blood cells; RBCs). The procedures, involved in the analysis of the experimental data for defining mechanisms of transport, and for estimating values of kinetic parameters in the corresponding mathematical models, are given special attention.

Mathematical models of naturally "morphed" human erythrocytes: stomatocytes and echinocytes

We present two mathematical models that describe human red blood cells (RBCs) with morphologies that are attained naturally under certain patho-physiological conditions, namely stomatocytes and echinocytes. Muñoz San Martín et al. (Bioelectromagnetics 27:521-527, 2006) recently presented models of these shapes based on our previous set of parametric equations (Kuchel and Fackerell, Bull. Math. Biol. 61:209-220, 1999) that involve Jacobi elliptic functions and integrals. Thus, both discocytes and stomatocytes are described. Here, we derived the Cartesian forms of these new equations; and, in addition, present a realistic model of a Type III echinocyte, using prolate spheroids 'decorating' a central sphere at the vertices of an internal dodecahedron. The RBC models based on Cartesian equations have been used for representing the shape changes (morphological transformations or "morphing") that occur in RBCs under various experimental conditions; specifically, when the shape changes have been monitored by nuclear magnetic resonance (NMR) micro-imaging.

The monovalent cation leak in overhydrated stomatocytic red blood cells results from amino acid substitutions in the Rh-associated glycoprotein

Overhydrated hereditary stomatocytosis (OHSt) is a rare dominantly inherited hemolytic anemia characterized by a profuse membrane leak to monovalent cations. Here, we show that OHSt red cell membranes contain slightly reduced amounts of Rh-associated glycoprotein (RhAG), a putative gas channel protein. DNA analysis revealed that the OHSt patients have 1 of 2 heterozygous mutations (t182g, t194c) in RHAG that lead to substitutions of 2 highly conserved amino acids (Ile61Arg, Phe65Ser). Unexpectedly, expression of wild-type RhAG in Xenopus laevis oocytes induced a monovalent cation leak; expression of the mutant RhAG proteins induced a leak about 6 times greater than that in wild type. RhAG belongs to the ammonium transporter family of proteins that form pore-like structures. We have modeled RhAG on the homologous Nitrosomonas europaea Rh50 protein and shown that these mutations are likely to lead to an opening of the pore. Although the function of RhAG remains controversial, this first report of functional RhAG mutations supports a role for RhAG as a cation pore.