Cell volume dependence of 1H spin-echo NMR signals in human erythrocyte suspensions. The influence of in situ field gradients

In-cell NMR: Why and how?

NMR spectroscopy has been applied to cells and tissues analysis since its beginnings, as early as 1950. We have attempted to gather here in a didactic fashion the broad diversity of data and ideas that emerged from NMR investigations on living cells. Covering a large proportion of the periodic table, NMR spectroscopy permits scrutiny of a great variety of atomic nuclei in all living organisms non-invasively. It has thus provided quantitative information on cellular atoms and their chemical environment, dynamics, or interactions. We will show that NMR studies have generated valuable knowledge on a vast array of cellular molecules and events, from water, salts, metabolites, cell walls, proteins, nucleic acids, drugs and drug targets, to pH, redox equilibria and chemical reactions. The characterization of such a multitude of objects at the atomic scale has thus shaped our mental representation of cellular life at multiple levels, together with major techniques like mass-spectrometry or microscopies. NMR studies on cells has accompanied the developments of MRI and metabolomics, and various subfields have flourished, coined with appealing names: fluxomics, foodomics, MRI and MRS (i.e. imaging and localized spectroscopy of living tissues, respectively), whole-cell NMR, on-cell ligand-based NMR, systems NMR, cellular structural biology, in-cell NMR… All these have not grown separately, but rather by reinforcing each other like a braided trunk. Hence, we try here to provide an analytical account of a large ensemble of intricately linked approaches, whose integration has been and will be key to their success. We present extensive overviews, firstly on the various types of information provided by NMR in a cellular environment (the "why", oriented towards a broad readership), and secondly on the employed NMR techniques and setups (the "how", where we discuss the past, current and future methods). Each subsection is constructed as a historical anthology, showing how the intrinsic properties of NMR spectroscopy and its developments structured the accessible knowledge on cellular phenomena. Using this systematic approach, we sought i) to make this review accessible to the broadest audience and ii) to highlight some early techniques that may find renewed interest. Finally, we present a brief discussion on what may be potential and desirable developments in the context of integrative studies in biology.

Sub-minute kinetics of human red cell fumarase: 1 H spin-echo NMR spectroscopy and 13 C rapid-dissolution dynamic nuclear polarization

Fumarate is an important probe of metabolism in hyperpolarized magnetic resonance imaging and spectroscopy. It is used to detect the release of fumarase in cancer tissues, which is associated with necrosis and drug treatment. Nevertheless, there are limited reports describing the detailed kinetic studies of this enzyme in various cells and tissues. Thus, we aimed to evaluate the sub-minute kinetics of human red blood cell fumarase using nuclear magnetic resonance (NMR) spectroscopy, and to provide a quantitative description of the enzyme that is relevant to the use of fumarate as a probe of cell rupture. The fumarase reaction was studied using time courses of 1 H spin-echo and 13 C-NMR spectra. 1 H-NMR experiments showed that the fumarase reaction in hemolysates is sufficiently rapid to make its kinetics amenable to study in a period of approximately 3 min, a timescale characteristic of hyperpolarized 13 C-NMR spectroscopy. The rapid-dissolution dynamic nuclear polarization (RD-DNP) technique was used to hyperpolarize [1,4-13 C]fumarate, which was injected into concentrated hemolysates. The kinetic data were analyzed using recently developed FmRα analysis and modeling of the enzymatic reaction using Michaelis-Menten equations. In RD-DNP experiments, the decline in the 13 C-NMR signal from fumarate, and the concurrent rise and fall of that from malate, were captured with high spectral resolution and signal-to-noise ratio, which allowed the robust quantification of fumarase kinetics. The kinetic parameters obtained indicate the potential contribution of hemolysis to the overall rate of the fumarase reaction when 13 C-NMR RD-DNP is used to detect necrosis in animal models of implanted tumors. The analytical procedures developed will be applicable to studies of other rapid enzymatic reactions using conventional and hyperpolarized substrate NMR spectroscopy.

Comparative studies of water permeability of red blood cells from humans and over 30 animal species: an overview of 20 years of collaboration with Philip Kuchel

NMR measurements of the diffusional permeability of the human adult red blood cell (RBC) membrane to water (P(d)) and of the activation energy (E(a,d)) of the process furnished values of P(d) ~ 4 × 10(-3) cm/s at 25 °C and ~6.1 × 10(-3) cm/s at 37 °C, and E(a,d) ~ 26 kJ/mol. Comparative NMR measurements for other species showed: (1) monotremes (echidna and platypus), chicken, little penguin, and saltwater crocodile have the lowest P(d) values; (2) sheep, cow, and elephant have P(d) values lower than human P(d) values; (3) cat, horse, alpaca, and camel have P(d) values close to those of humans; (4) guinea pig, dog, dingo, agile wallaby, red-necked wallaby, Eastern grey kangaroo, and red kangaroo have P(d) values higher than those of humans; (5) mouse, rat, rabbit, and "small and medium size" marsupials have the highest values of P(d) (>8.0 × 10(-3) cm/s at 25 °C and >10.0 × 10(-3) cm/s at 37 °C). There are peculiarities of E(a,d) values for the RBCs from different species. The maximum inhibition of diffusional permeability of RBCs induced by incubation with p-chloromercuribenzene sulfonate varied between 0% (for the chicken and little penguin) to ~50% (for human, mouse, cat, sheep, horse, camel, and Indian elephant), and ~60-75% (for rat, guinea pig, rabbit, dog, alpaca, and all marsupials). These results indicate that no water channel proteins (WCPs) or aquaporins are present in the membrane of RBCs from monotremes (echidna, platypus), chicken, little penguin and saltwater crocodile whereas WCPs from the membranes of RBCs from marsupials have peculiarities.

Magnetic field anisotropy based MR tractography

Non-invasive measurements of structural orientation provide unique information regarding the connectivity and functionality of fiber materials. In the present study, we use a capillary model to demonstrate that the direction of fiber structure can be obtained from susceptibility-induced magnetic field anisotropy. The interference pattern between internal and external magnetic field gradients carries the signature of the underlying anisotropic structure and can be measured by MRI-based water diffusion measurements. Through both numerical simulation and experiments, we found that this technique can determine the capillary orientation within 3°. Therefore, susceptibility-induced magnetic field anisotropy may be useful for an alternative tractography method when diffusion anisotropy is small at higher magnetic field strength without the need to rotate the subject inside the scanner.

Simulations of molecular diffusion in lattices of cells: insights for NMR of red blood cells

The pulsed field-gradient spin-echo (PGSE) nuclear magnetic resonance (NMR) experiment, conducted on a suspension of red blood cells (RBC) in a strong magnetic field yields a q-space plot consisting of a series of maxima and minima. This is mathematically analogous to a classical optical diffraction pattern. The method provides a noninvasive and novel means of characterizing cell suspensions that is sensitive to changes in cell shape and packing density. The positions of the features in a q-space plot characterize the rate of exchange across the membrane, cell dimensions, and packing density. A diffusion tensor, containing information regarding the diffusion anisotropy of the system, can also be derived from the PGSE NMR data. In this study, we carried out Monte Carlo simulations of diffusion in suspensions of "virtual" cells that had either biconcave disc (as in RBC) or oblate spheroid geometry. The simulations were performed in a PGSE NMR context thus enabling predictions of q-space and diffusion tensor data. The simulated data were compared with those from real PGSE NMR diffusion experiments on RBC suspensions that had a range of hematocrit values. Methods that facilitate the processing of q-space data were also developed.

Dysprosium-bearing red cells as potential transverse relaxation agents for MRI

The cytosol of intact human red blood cells was loaded with 28.1 +/- 3.4 mM of dysprosium DTPA-BMA using a hypoosmotic technique. When loaded cells were diluted with saline and control cells to give an average dysprosium concentration of 3.3 +/- 0.5 mM, the transverse relaxation rate constants R(*)(2) and R(2) increased. R(*)(2) increased from 7.5 +/- 0.9 sec(-1) to 356 +/- 50 sec(-1), and R(2) increased from 7.4 +/- 0.7 sec(-1) to 148 +/- 40 sec(-1). After lysing, R(*)(2) was 6.0 +/- 0.6 sec(-1) in the control and 13.4 +/- 1.5 sec(-1) in the mixture; R(2) was 6.4 +/- 1.1 sec(-1) and 9.8 +/- 2.4 sec(-1), respectively. Thus, the relaxivity effects were enhanced by sequestration of the dysprosium within intact red cells, and this effect was lost after lysis. At a circulating whole-blood concentration of 0.81 +/- 0.15 mM in rats, the liver signal intensity dropped 29.9% +/- 3.7% and kidney signal intensity dropped 19.4% +/- 8.7%. Dysprosium-loaded cells might be useful in the study of perfusion and tissue blood volume.

Evidence of red cell alignment in the magnetic field of an NMR spectrometer based on the diffusion tensor of water

The alignment of human erythrocytes in aqueous suspensions in the magnetic field B(0) (called the z-direction) of an NMR spectrometer was shown by calculating the diffusion tensor for water in the sample. The diffusion was measured using a pulsed-field-gradient spin-echo NMR method. The extent of diffusion anisotropy for water was exemplified by the values of the apparent diffusion coefficients with erythrocytes of normal shape and volume: for a typical experiment the values for the x-, y-, and z-directions were (6.88 +/- 0.17) x 10(-10), (7.07 +/- 0.17) x 10(-10), and (10.20 +/- 0.17) x 10(-10) m(2) s(-1), respectively. Cells in hypo- and hyperosmotic media were also studied and they too showed the anisotropy of the apparent diffusion coefficients but the extents were different. A new method of data analysis was developed using the Standard Add-On Packages in a Mathematica program. The experimental findings support evidence of erythrocyte alignment that was previously obtained with a high-field-gradient q-space method.

Relaxation rates of blood with osmotically modified red cell volume: application of the two-compartment fast exchange model

Blood has been considered as a simplified tissue model, both physiologically and physically consisting in two compartments, extra-cellular and intra-cellular. In the physiologic condition (300 mOsm), the relaxation rates of red cell suspensions in saline increased linearly with the hematocrit in the range 0-0.80 according to Fullerton's model of fast proton exchanges between the two compartments (Fullerton GD, Potter JL, Dornbluth NC. NMR relaxation of protons in tissues and other macromolecular water solutions. Magn Reson Imaging 1982; 1:209-228). In experiments of osmotic variations, between 200 and 900 mOsm at three constant red cell numbers in the samples, non-linear variations of relaxation rates with red cell volume were observed. In the hyperosmotic domain, the particularly high increase in blood transverse relaxation rate with the decreasing cell volume has been attributed to the progressive water-protein organization in the cellular compartment. A generalised form of the fast exchange model has been applied to extended experimental conditions of red cells, by introducing the red cell volume ratio of modified to iso-osmotic values, and the volume fraction of iso-osmotic red cells.

A model for diffusive transport through a spherical interface probed by pulsed-field gradient NMR

In biological systems, because of higher intracellular viscosity and/or the restriction of the diffusion space inside cells, the (apparent) diffusion coefficient of an intracellular species (e.g., water) is generally smaller than when it is in the extracellular medium. This difference affects the spin-echo signal attenuation in the pulsed field gradient NMR experiment and thus affords a means of separating the intracellular from the extracellular species, thereby providing a basis for studying transmembrane transport. Such experiments have commonly been analyzed using the macroscopic model of Kärger (see Adv. Magn. Reson. 21:1-89 (1988)). In our previous study, we considered a microscopic model of diffusive transport through a spherical interface using the short gradient pulse approximation (J. Magn. Reson. A114:39-46 (1995)). The spins in the external medium were modeled with the "partially absorbing wall" condition or as having a small but finite lifetime. In the present paper, we extend our treatment to the case in which there is no limitation upon the lifetime in either medium. We also consider a simple modification of Kärger's model that more properly accounts for the restricted intracellular diffusion. Importantly, it was found that the exact solution within the short gradient pulse approximation developed here and the modified Kärger model are in close agreement in the (experimentally relevant) long-time limit. The results of this study show that when there is no limitation upon the lifetime of the transported species in either phase, the spin-echo attenuation curve is very sensitive to transport.

Low-molecular weight lanthanide contrast agents: evaluation of susceptibility and dipolar effects in red blood cell suspensions

Red blood cell (RBC) suspensions, containing low-molecular weight (LMW) dysprosium (Dy) and gadolinium (Gd) chelates, were selected as a two-compartment system for the evaluation of the magnetic dipolar and susceptibility contributions to the transverse (T2) relaxation of solvent water protons. The influence of RBC geometry and degree of metal chelate compartmentalization on T2 was investigated by variation of the osmolality and hematocrit (HC), respectively. The T2-relaxation ability of Dy-chelates was markedly improved in RBC suspensions, in comparison to aqueous solutions, due to the presence of susceptibility effects that more than compensated for the low dipolar relaxation efficacy. Despite a smaller susceptibility effect, the Gd-chelates were still the most efficacious in shortening T2 due to their comparatively larger dipolar relaxation contribution. The results obtained with the Dy-chelates allowed the evaluation of the relative contributions of susceptibility and dipolar mediated relaxation for the Gd-chelates. The RBC geometry and degree of compartmentalization influenced strongly the T2 relaxation efficacy of Dy-chelates, as opposed to the Gd-chelates. Hemolysis eliminated the susceptibility effect, essentially removing the T2 relaxation ability of Dy-chelates. The T2 relaxation efficacy of Gd-chelates was improved by hemolysis due to enhancement of the dipolar relaxation. As a conclusion, RBC suspensions have clearly been shown to be a suitable ex vivo model with which to distinguish the different contrast mechanisms of LMW Dy- and Gd-based MRI contrast agents.

Molecular diffusion, tissue microdynamics and microstructure

Diffusion NMR is the only method available today that noninvasively provides information on molecular displacements over distances comparable to cell dimensions. This information can be used to infer tissue microstructure and microdynamics. However, data may be fairly difficult to interpret in biological tissues which differ markedly from the theoretical "infinite isotrope medium", as many factors may affect the NMR signal. The object of this paper is to analyze the expected effects of temperature, restriction, hindrance, membrane permeability, anisotropy and tissue inhomogeneity on the diffusion measurements. Powerful methods, such as q-space imaging, diffusion tensor imaging and diffusion spectroscopy of metabolites further enhance the specificity of the information obtained from diffusion NMR experiments.

Time-dependent diffusion of water in a biological model system

Packed erythrocytes are ideally suited as a model system for the study of water diffusion in biological tissue, because cell size, membrane permeability, and extracellular volume fraction can be varied independently. We used a pulsed-field-gradient spin echo NMR technique to measure the time-dependent diffusion coefficient D(t) in packed erythrocytes. The long-time diffusion constant, D(eff), depends sensitively on the extracellular volume fraction. This may explain the drop in D(eff) during the early stages of brain ischemia, where just minutes after an ischemic insult the extra-cellular volume in the affected region of the brain is significantly reduced. Using an effective medium formula, we estimate the erythrocyte membrane permeability, in good agreement with measurements on isolated cells. From the short-time behavior of D(t), we determine the surface-to-volume ratio of the cells, approximately (0.72 micron)-1.

NMR visibility studies of N-delta proton of proximal histidine in deoxyhemoglobin in lysed and intact red cells

Recent in vivo 1H MRS studies of muscle oxygen levels by means of myoglobin (Mb) oxygenation have necessitated an investigation of the possible contamination from deoxy-hemoglobin (Hb). To determine the contribution of Hb in these NMR measurements, we studied the NMR visibility of histidyl N-delta proton of Hb in lysed and intact RBC (red blood cell) suspensions. We found that the visibility of deoxy-Hb was significantly lower in intact RBCs (ca. 16%) than the solution form at an equivalent concentration of beta heme-iron, even though the visibility of deoxy-Hb from lysed RBC and deoxy-Mb was almost identical. Our study also shows that visibility increases with RBC swelling. Based on the difference in concentration and compartmentation, we conclude that less than 17% contribution from Hb is expected in Mb measurements in vivo. The possible mechanisms which cause the low visibility of Hb in RBCs are discussed.

Stability and nonreactivity of ergothioneine in human erythrocytes studied by 1H NMR

The N(CH3)3 resonance of ergothioneine in 1H spin-echo Fourier transform (SEFT) NMR spectra of red blood cells is usually a large singlet and it has been common practice to use this apparently unchanging resonance as an intensity reference. Recently, Reglinski et al. (Magn. Reson. Med. 6, 217-223 (1988)) have questioned this practice, reporting changes seen in the resonance in response to oxidative stress induced by arsenicals. We propose that the changes in the ergothioneine resonance that were reported are artifacts due to alterations in osmolality and magnetic susceptibility induced by the addition of nonisotonic solutions to red blood cell suspensions. These factors change the specific intensity of the intracellular resonances of all compounds. Ergothioneine was observed not to take part in any chemical reactions with arsenicals in free solution or in intact erythrocytes, and we conclude that ergothioneine may still be used as an internal intensity reference in 1H SEFT NMR spectra, bearing in mind the above physical factors.

NMR studies of diffusional water permeability of red blood cells from macropodid marsupials (kangaroos and wallabies)

1. The water permeabilities of the red blood cell (RBC) membranes of five species of macropodid marsupials were monitored by using a Mn(2+)-doping 1H nuclear magnetic resonance (NMR) technique. 2. Since this appears to be the first time that this type of measurement at 400 MHz for 1H has been reported, an analysis of instrumental parameters influencing the estimated value of the water exchange time (Te), and of the diffusional water permeability (Pd), was performed on samples of human RBC. 3. It was found that a short interpulse delay in the Carr-Purcell-Meiboom-Gill pulse sequence had to be used (around 100 microseconds) to avoid an underestimation of the relaxation times, that occurred as the result of molecular diffusion through non-uniform local magnetic fields in and around the cells. 4. There were no significant differences, in the water permeabilities of the RBC membranes, between the five species (Macropus rufogriseus, M. parma, M. eugenii, M. parryi and Wallabia bicolor). 5. There were also no significant differences between various colonies of M. eugenii living in different habitats. 6. The average values of Pd were 9.1 x 10(-3) cm/sec at 24.6 degrees C, 10.4 x 10(-3) cm/sec at 30 degrees C, 12.6 x 10(-3) cm/sec at 37 degrees C, and 14.7 x 10(-3) cm/sec at 42.1 degrees C; these were more than twice as high as those for human RBC. 7. In agreement with the high water permeability the RBC of macropodids displayed a relatively low activation energy of water diffusion across their membranes, approximately 21 kJ/mol, compared with 25 kJ/mol for human RBC.

Aqueous shift reagents for high-resolution cation NMR. VI. Titration curves for in vivo 23Na and 1H2O MRS obtained from rat blood

Frequency shift/concentration calibration curves applicable to the use of shift reagents (SRs) for in vivo 23Na MRS studies can be obtained from experiments with whole blood. Here, they are reported for titrations of rat blood with the SRs DyTTHA3- and TmDOTP5-. There are a number of considerations that must be made in order to derive accurate calibration curves from the experimental data. These include the effects of bulk magnetic susceptibility (BMS, since the SRs are paramagnetic), the effects of water flux (since addition of the SR stock solution to blood renders the plasma hyperosmotic), and the consequences of restricted distribution of the SR anion in the erythrocyte suspension. We give in some detail the BMS shift theory that obtains in this case and show also how it applies to excised perfused organ as well as in vivo studies. Also, we report significant effects of adjuvant Ca2+ additions in the TmDOTP5- titrations. These are very important to the successful use of this SR in vivo. Finally, our considerations of BMS lead naturally to an understanding of its manifestations in the shifts of the 1H2O resonance frequencies of cell suspensions and tissues induced by SRs. Since these are being increasingly reported, and often misinterpreted, we devote an experiment and some discussion to this subject. We show that this phenomenon cannot be used to quantitatively discriminate intra- and extracellular 1H2O signals.

Application of missing pulse steady state free precession to the study of renal microcirculation

Missing pulse steady state free precession (MP-SSFP), an extension of steady state free precession (SSFP), was evaluated for its ability to measure slow fluid flows. In experiments using flow phantoms, the MP-SSFP signal was sensitive to fluid velocities in the millimeters per second range. Isolated perfused rabbit kidneys were then used to determine if MP-SSFP could measure perfusion in a biological tissue. The signal intensities in the different anatomical regions of the kidney were observed to be related to the total flow to the organ. Furthermore, increasing the flow sensitivity of the pulse sequence by increasing the gradient strength resulted in decreases in the image signal intensity. The MP-SSFP signal was more sensitive to flow in the medulla than in the cortex. This can be related to slow flow sensitivity of MP-SSFP and the known differences in velocity profiles between these two regions. These results suggest that MP-SSFP may be a powerful tool for the noninvasive measurement of slow fluid flows in different regions of the kidney.

On the origin of paramagnetic inhomogeneity effects in whole blood

Asymmetric 7 T proton NMR signals of water in RBC suspensions containing intracellular deoxyhemoglobin are composites of chemically shifted extracellular and intracellular resonances broadened by gradient diffusion and modulated by transmembrane water exchange. This allows assessment of field dependences of acute hematoma intensities in proton MRIs at lower field strengths (less than or equal to 1.5 T).

Determination of extracellular/intracellular fluid ratios from magnetic resonance images: accuracy, feasibility, and implementation

This study determines the accuracy and feasibility of using localized spin-lattice (T1) relaxation time measurements from magnetic resonance (MR) images to follow changes in extracellular/intracellular fluid ratios in defined subvolumes of living tissue. A red blood cell suspension was used as a test system and a simple two-compartment model incorporating fast exchange was found to suffice for the conversion of T1 values to volume ratios. The technique requires the addition of gadolinium-DTPA to the model system to selectively enhance relaxation in the extracellular fluid space. No detectable amount of gadolinium-DTPA was found to enter the intracellular fluid space, and all magnetization decay plots obtained from both intracellular constituents and complete RBC suspensions consisted of a single exponential. Both of these results are compatible with assumptions underlying our physical model. The NMR-determined fluid ratio values were compared to those measured via the microhematocrit technique. Partial saturation image-mode determinations are strongly correlated to microhematocrit data (R2 = 0.945) and indicate that localized cell volume changes may be followed with a sensitivity of +/- 2.2%. These values compare favorably with those produced when nonimaging inversion-recovery techniques are used to determine the MR hematocrit (R2 = 0.962, sensitivity = +/- 1.1%). This technique, with modification, should be applicable to the comparison of ratios of extracellular/intracellular fluid volumes in structurally complex tissues where small subvolumes of homogeneous cell structure could be examined.

On the origin of paramagnetic inhomogeneity effects in blood

Hydrogen, sodium, and fluorine (added F-) NMR spectra of venous and oxygenated blood were measured. The fluorine resonance was seen as a single peak in both samples, and all three resonances exhibited the same deoxy-oxy shift. Because F- exchanges slowly across the red cell membrane, and because sodium is 95% extracellular, these results suggest that the intra-extracellular field difference delta B is less than 0.1 ppm. A small value of delta B tends to rule out transmembrane exchange as an important contributor to relaxation in MRI of blood and hematomas. However, the broadening of the resonances with deoxygenation, by 0.3-0.4 ppm, indicates that both intra- and extracellular gradients are of comparable and sufficient magnitude to produce the T2-weighted hypointensity seen in clinical magnetic resonance images of hematomas at high fields.

Magnetic potential and field gradients of a model cell

The magnetic potential within and outside a nucleated cell placed in a uniform magnetic field is described for a model consisting of two concentric diamagnetic spheres. The analytical description of the magnetic potential in and around a system consisting of a finite number of concentric diamagnetic spheres in a uniform magnetic field was derived. The solution was employed to calculate the field gradients outside and inside a model chicken red blood cell. The form and magnitude of the gradients provide a theoretical basis on which to discuss experimental results relating to the attenuation of signals obtained using proton nuclear magnetic resonance spectroscopy with chicken erythrocyte suspensions. The form of the magnetic field, field difference and field gradients in the external medium of a cell suspension was simulated for a distribution of spheres in a uniform magnetic field, such as might occur in an idealised dilute cell suspension.

Two-site exchange revisited: a new method for extracting exchange parameters in biological systems

A new analysis is presented which links real volume fractions, relaxation rates, and intracompartmental exchange rates directly with apparent volume fractions and relaxation rates obtained from biexponential fits of transverse magnetization decay curves. The analysis differs from previous methods in that measurements from two paramagnetic doping levels are used to close the two-site exchange equations. Both the new method and one previously described by Herbst and Goldstein (HG) have been applied to paramagnetically doped whole-blood data sets. Significant differences in the calculated exchange parameters are found between the two methods. A small dependence of the intracellular relaxation rate on extracellular paramagnetic agent concentration, assumed nonexistent with the HG method, is inferred from the new analysis. The analysis was also applied to published data on perfused rat hearts, and we obtained a limited assessment of two-site exchange in this system.

No evidence for bradykinin hydrolysis in human erythrocyte suspensions: 1H NMR studies

In view of their permeability to small peptides, it has been postulated that human erythrocytes may play a role in terminating the action of some circulating peptide hormones. Work using classical paper chromatographic techniques for detecting free amino acids indicated that the octapeptide, des-(Arg9)-bradykinin, enters these cells and its amino-terminal arginine residue is released by cytosolic aminopeptidase-P. We have used 1H NMR to monitor directly the release of arginine from bradykinin. The hydrolytic reaction rate in hemolysates, with an initial peptide concentration of 13.0 mmol l-1 was 6.5 mmol (1 packed red cell)-1 h-1. But no reaction was evident after a 4.5-h incubation with intact cells, thus contradicting the earlier suggestion that erythrocytes are involved in the primary inactivation of this hormone. This is consistent with our previous findings that the pentapeptide leu-enkephalin fails to enter human erythrocytes but that its lower-order degradation products may do so.

Viscosity of concentrated solutions and of human erythrocyte cytoplasm determined from NMR measurement of molecular correlation times. The dependence of viscosity on cell volume

Metabolically active human erythrocytes were incubated with [alpha-13C]glycine which led to the specific enrichment of intracellular glutathione. The cells were then studied using 13C-NMR in which the longitudinal relaxation times (T1) and nuclear Overhauser enhancements of the free glycine and glutathione were measured. The T1 values of labelled glycine were also determined in various-concentration solutions of bovine serum albumin and glycerol and also of the natural abundance 13C of glycerol in glycerol solutions. From the T1 estimates the rotational correlation time (tau r) was calculated using a formula based on a model of an isotropic spherical rotor or that of a symmetrical ellipsoidal rotor; for glycine the differences in estimates of tau r obtained using the two models were not significant. From the correlation times and by use of the Stokes-Einstein equations viscosity and translational diffusion coefficients were calculated; thus comment can be made on the likelihood of diffusion control of certain enzyme-catalysed reactions in the erythrocyte. Bulk viscosities of the erythrocyte cytoplasm and the above-mentioned solutions were measured using Ostwald capillary viscometry. Large differences existed between the latter viscosity estimates and those based upon NMR-T1 measurements. We derived an equation from the theory of the viscosity of concentrated solutions which contains two phenomenological interaction parameters, a 'shape' factor and a 'volume' factor; it was fitted to data relating to the concentration dependence of viscosity measured by both methods. We showed, by using the equation and interaction-parameter estimates for a particular probe molecule in a particular solution, that it was possible to correlate NMR viscosity and bulk viscosity; in other words, given an estimate of the bulk viscosity, it was possible to calculate the NMR 'micro' viscosity or vice versa. However, the values of the interaction parameters depend upon the relative sizes of the probe and solute molecules and must be separately determined for each probe-solute-solvent system. Under various conditions of extracellular osmotic pressure, erythrocytes change volume and thus the viscosity of the intracellular milieu is altered. The volume changes resulted in changes in the T1 of [alpha-13C]glycine. Conversely, we showed that alterations in T1, when appropriately calibrated, could be used for monitoring changes in volume of metabolically active cells.

Enkephalin degradation by human erythrocytes and hemolysates studied using 1H NMR spectroscopy

High resolution (400 MHz) 1H spin-echo NMR spectroscopy was used to monitor the degradation of leucine-enkephalin, and peptide fragments of it, by human erythrocytes and hemolysates. We showed that leucine-enkephalin is rapidly degraded by the cytosolic peptidases of the human erythrocyte, and we have elucidated the most probable pathway of degradation. Computer simulations of the proposed pathway, using a model incorporating the experimentally derived steady-state kinetic parameters obtained for the individual enzyme steps, showed close agreement with the experimental results. From a methodological perspective, the work demonstrates the value of 1H spin-echo NMR spectroscopy for rapidly elucidating, both qualitatively and quantitatively, an entire peptide-degradation pathway as it operates in situ.

The assimilation of tri- and tetrapeptides by human erythrocytes

Evidence is presented that tripeptides enter human erythrocytes via saturable transport system(s) at rates similar to those previously described for dipeptides (King, G.F. and Kuchel, P.W. (1985) Biochem. J. 227, 833-842) but that the transmembrane flux rates for tetrapeptides are considerably less. 1H spin-echo NMR spectroscopy was used to monitor the coupled uptake and hydrolysis of peptides by red cells, since it enabled the simultaneous measurement of the levels of substrates and products of peptidase-catalysed reactions in suspensions with haematocrits similar to those found in vivo. Weighted non-linear least-squares regression of the integrated Michaelis-Menten equation onto progress curves obtained from the hydrolysis of Tyr-Gly-Gly and Gly-Gly-Gly in RBC lysates gave Km = 2.11 +/- 0.08 and 23.4 +/- 0.9 mmol/l and Vmax = 307 +/- 3 and 905 +/- 22 mmol/h per 1 packed cells, respectively. In whole cell suspensions, the rate of hydrolysis was considerably less and was dominated by the transmembrane flux of tripeptide. Progress curve analysis thus yielded the steady-state kinetic parameters for peptide transport; the values were Km = 11.6 +/- 1.1 and 56 +/- 18 mmol/l and Vmax = 12.9 +/- 3.0 and 36.4 +/- 3.2 mmol/h per 1 packed cells, respectively, for the previously mentioned peptides. The rate of transport of the tetrapeptide Gly-Gly-Gly-Gly was considerably less than either of the tripeptides. The above mentioned steady-state kinetic parameters were used in computer simulations of the coupled uptake and hydrolysis of tripeptides by human erythrocytes under physiological conditions; these simulations revealed certain similarities between the rates of peptide uptake by erythrocytes and the intestine in vivo.

Regulation of the human-erythrocyte hexose-monophosphate shunt under conditions of oxidative stress. A study using NMR spectroscopy, a kinetic isotope effect, a reconstituted system and computer simulation

The regulation of the hexose monophosphate shunt of human erythrocytes under conditions of oxidative stress has been investigated by monitoring the reduction of oxidised glutathione (GSSG) to reduced glutathione (GSH) in erythrocytes containing high levels of GSSG; 1H NMR and a biochemical assay were used to measure the changes. A reconstituted metabolic system prepared with the purified erythrocyte enzymes was used in conjunction with studies of intact cells and haemolysates to determine the dependence of the rate of GSH production on the activities of hexokinase and glucose-6-phosphate dehydrogenase. Both of these enzymes have previously been claimed to be the rate-limiting step of oxidatively stimulated flux through the hexose monophosphate shunt. The absence of a kinetic isotope effect on the rate of GSH production in these systems, when [1-2H]glucose replaced glucose as the source of reducing equivalents, showed that glucose-6-phosphate dehydrogenase activity was not a strong determinant of the flux. The dependence of the rate of GSH production on the concentration of the hexokinase inhibitors glucose 1,6-bisphosphate and glycerate 2,3-bisphosphate showed that, under conditions of oxidative stress, hexokinase was the principal determinant of flux through the shunt. Glucose 1,6-bisphosphate at the concentration present in vivo appears to be more important in limiting hexokinase activity, and thus the rate of glucose utilisation, than was previously assumed. A detailed computer model of the system was developed based on the reported kinetic parameters of the enzymes involved. A sensitivity analysis of this model predicted that the hexokinase reaction would have a sensitivity coefficient of 0.995 with respect to the maximal rate of GSH production.

Proton NMR spectroscopy of rabbit renal cortex

High resolution 1H-NMR spectra were obtained from renal cortical slices, homogenates, isolated cells in suspension, and lysates. Experimental conditions for acquisition of high resolution spin-echo spectra are described and assignments of prominent peaks to various metabolites are given. The in situ activity of the highly active cortical enzyme prolidase was determined (1.8 nmoles/min/mg wet kidney wt) by exploiting the marked difference in the spectral features of glycyl-L-proline and that of the free amino acids. The results suggest that this NMR method will be applicable generally to the study of hydrolysis of peptides, as well as to other metabolic processes both in vitro and in vivo.

Assimilation of alpha-glutamyl-peptides by human erythrocytes. A possible means of glutamate supply for glutathione synthesis

Human erythrocytes are essentially impermeable to glutamate and yet there is a continual requirement for the amino acid for glutathione synthesis. In addition, the intracellular glutamate concentration is approximately five times that of plasma. We present evidence that glutamate enters the red cell as small peptides which are rapidly hydrolysed by cytoplasmic peptidase(s) and that with the estimated physiological levels of plasma glutamyl-peptides the rate of inward flux would be adequate to maintain the glutamate pool at its observed level. Experimentally, we used 1H spin-echo n.m.r. spectroscopy to follow peptide hydrolysis, since peptide spectra are different from those of the free amino acids and the spin-echo sequence enables the monitoring of reactions in concentrated lysates and whole cell suspensions. Thus, the system was studied under near-physiological conditions. Weighted non-linear regression analysis of progress curves using the integrated Michaelis-Menten equation was used to obtain estimates of Km and Vmax. for the hydrolysis of alpha-L-glutamyl-L-alanine and L-alanyl-alpha-L-glutamate in lysates and whole cell suspensions; the values for lysates were Km = 3.60 +/- 0.29 and 5.4 +/- 0.4 mmol/l and Vmax. = 120 +/- 4 and 46.7 +/- 1.7 mmol/h per 1 of packed cells respectively. In whole cell suspensions the rate of peptide hydrolysis was much slower and dominated by the transmembrane flux-rate. The estimates of the steady-state kinetic parameters for the transport were Kt = 2.35 +/- 0.41 and 11.2 +/- 1.0 mmol/l and Vmax. = 3.26 +/- 0.13 and 19.7 +/- 0.7 mmol/h per 1 of packed cells respectively for the previously mentioned peptides. Using the n.m.r. procedure we failed to detect any glutaminase activity in whole cells or lysates; thus, we exclude the possibility that glutamate gains entry to the cell as glutamine which is subsequently hydrolysed by glutaminase.