The application of high resolution nuclear magnetic resonance to biological systems

The use of magnetic resonance spectroscopy in the evaluation of epilepsy

Magnetic resonance spectroscopy (MRS) is indicated in the imaging protocol of the patient with epilepsy to screen for metabolic derangements such as inborn errors of metabolism and to characterize masses that may be equivocal on conventional magnetic resonance imaging for dysplasia versus neoplasia. Single-voxel MRS with echo time of 35 milliseconds may be used for this purpose as a quick screening tool in the epilepsy imaging protocol. MRS is useful in the evaluation of both focal and generalized epilepsy.

Lipopolysaccharide core region of Hafnia alvei: structure elucidation using chemical methods, gas chromatography-mass spectrometry, and NMR spectroscopy

Sugar and methylation analysis with the use of gas chromatography-mass spectrometry and 1H NMR spectroscopy proved that the core oligosaccharides isolated from lipopolysaccharides of eight Hafnia alvei strains have the identical hexasaccharide skeleton. However, 1H, 31P heterocorrelated spectra showed that the phosphorylation pattern is not the same. The branched heptose for the ATCC 13337, 1187, 2, 1191, 1196, 1220, and 481L strains is phosphorylated as in the following formula, where P = -O-P(O)(O-)2 and P-PEtN = [-O-P(O)(O-)]2-O(CH2)2NH3+ [formula: see text] A different phosphorylation pattern was found for the 1211 strain, where the branched heptose residue is 6-substituted by a monophosphorylethanolamine group, ...-->3(-->7)(PEtN-->6)-alpha-LD-Hepp-(1-->3)..., where PEtN = -O-P(O)(O-)-O(CH2)2NH3+.

1H- and 19F-NMR approaches to the study of the structure of proteins larger than 25 kDa

The three-dimensional solution structures of proteins larger than about 25 kDa cannot at present be determined by multi-dimensional nuclear magnetic resonance (NMR) methods. However, for proteins that are larger than 25 kDa, for which X-ray structural information is not available, there are a variety of mostly one-dimensional NMR methods that still represent some of the most informative approaches to obtaining structural answers to questions of biochemical interest. This paper provides recent illustrative examples of 1H- and 19F-NMR experiments that describe ways to focus on proteins by region, by amino acid type, or by individual amino acid. Methods to focus on a particular region of a protein include exploiting domain mobility, using transferred nuclear Overhauser enhancements, the use of difference spectroscopy, the use of paramagnetic species, and domain fragmentation. Particular types of amino acid can be identified using selective deuteration, by incorporation of fluorinated amino acid analogues, by using photochemically induced dynamic nuclear polarization, and from the pH dependence of histidine residues. Individual amino acids can be identified by mutagenesis and, in special circumstances, by chemical shift. Many of the examples given are of plasma proteinases and their protein inhibitors, but other classes of protein are also discussed, including antibodies and DNA-binding proteins.

A 1H NMR probe for mobility in the reactive center loops of serpins: spin-echo studies of native and modified forms of ovalbumin and alpha 1-proteinase inhibitor

It has recently been proposed that the expression of inhibitory activity in serine protease inhibitors (serpins) is a function of the mobility of the extended alpha-helical reactive center loop [Stein, P.E., Leslie, A.G.W., Finch, J.T., Turnell, W.G., McLaughlin, P.J., & Carrell, R.W. (1990) Nature 347, 99-102]. We have employed solution 1H NMR methods, including the Carr-Purcell-Meiboom-Gill (CPMG) and Hahn spin-echo pulse sequences, to try to identify such regions by virtue of their anticipated longer T2 relaxation times in two of the best characterized members of the serpin superfamily, ovalbumin and alpha 1-proteinase inhibitor. The CPMG spectra of native ovalbumin reveal the presence of long-lived resonances from the methyl protons of alanine residues and the CH3 protons of leucine or valine residues as well as the acetyl and ring methine protons of the carbohydrate moieties. Following reaction of ovalbumin with subtilisin Carlsberg to generate plakalbumin [where excision from within the reactive center loop homologue of a hexa- or heptapeptide, with sequence (E)-A-G-V-D-A-A, occurs], its CPMG spectrum retained almost all of the originally present long-lived resonances. Concurrent with the retention of these mobile resonances in plakalbumin is the appearance of two additional resonances consistent with the formation of new C and N termini. On the basis of the proposed mobility of the reactive center loop, it had been expected that removal of the alanine-rich hexapeptide would result in loss of some or all of the long-lived alanine methyl resonances.(ABSTRACT TRUNCATED AT 250 WORDS)

Nuclear magnetic resonance techniques for studying structure and function of ribosomes

The following conclusions can be drawn from the use of NMR techniques for studies of ribosomes: 1. The majority of ribosomal proteins are rigidly fixed within the particles, and the most mobile components in the isolated ribosome are L7/L12 proteins from the large subunit. 2. Interaction of EF-G with ribosomes results in some changes in ribosomal domains, and, particularly, immobilization of L7/L12 proteins takes place. The changes may pertain to the translocation reaction, since complexes with ribosomes, EF-G, and GTP are functional. The results of these studies using 1H NMR show that structural studies with this technique are limited as only a few proteins express their resonances in the 1H NMR spectra (S1, L7/L12). At the same time such studies are not exhaustive, since only the simplest samples were studied (ribosomes, the ribosomal complex with EF-G). Complexes with other ligands (tRNA, EF-Tu) have not yet been studied. It is also possible to enhance the resolution of 1H NMR techniques with the help of deuterated factors, ribosomes, and proteins, and to adapt the use of NMR to other nuclei (e.g., the use of fluorinated labels or incorporation of fluoroamino acids into the proteins). Many other approaches using NMR in biology have still to be explored. Therefore it is hoped that the use of NMR techniques will prove to be very useful in studies of the different functional steps of protein biosynthesis.

C-13 NMR spectral studies of the thyroid hormone transport protein, transthyretin and the pancreatic insulin storage moiety, the zinc-insulin hexamer

The DEPT spectral editing technique was used to separate the CH, CH2 and CH3 resonances in the C-13 NMR spectra of transthyretin and the porcine zinc insulin hexamer. The advantages of this technique in terms of spectral simplification and sensitivity enhancement for 13C NMR of proteins is discussed. Spin-lattice relaxation time and nuclear Overhauser effect measurements of the backbone C-alpha and aliphatic sidechain carbons provided information about the dynamics of the proteins in solution and the relative mobility of some sidechain groups.

1H nuclear magnetic resonance studies of an integral membrane protein: subunit c of the F1F0 ATP synthase

The membrane-traversing subunit c parallel from the F0 part of the ATP synthase molecule has been studied in chloroform/methanol by high-resolution 1H n.m.r. Various one-dimensional and two-dimensional techniques have been used for assignment purposes, some NOE connectivities were established and some 3JHN alpha coupling constants were measured from spin--echo experiments. The effects of varying pH, solvent composition, lanthanide concentration and temperature have been investigated. Evidence is presented that the molecule has extensive alpha-helical segments, and the hairpin structure suggested by other groups is supported by our n.m.r. data. Only one ionizable group, assigned to the C-terminal carboxyl, is observed to titrate in the pH range 2 to 10; so the conserved residue, Asp61, which binds dicyclohexylcarbodiimide, presumably has (at least in this solvent system) an abnormally high pK value.

Measurement of solute proton spin-lattice relaxation times in water using the 1,3,3,1 sequence

1H NMR spin-lattice relaxation times (T1) of the N-CH3 proton resonances of phosphocreatine (PCr) and creatine (Cr) in water solutions were obtained using the 1,3,3,1 pulse sequence. These T1 values were equivalent to those obtained in D2O and water using either the conventional inversion-recovery experiment or the 1,3,3,1 pulse sequence. Thus, the 1,3,3,1 sequence of proton NMR can provide an independent means along with phosphorous NMR for assess PCr and for the study of the creatine kinase reaction (PCr + ADP in equilibrium ATP + Cr) in aqueous solutions and perhaps in biological preparations.

Cold denaturation of myoglobin

The stability of the structure of sperm whale metmyoglobin has been studied in various solutions, in the temperature range -8 degrees C to 100 degrees C, by scanning microcalorimetry, light absorption, circular dichroism, nuclear magnetic resonance spectroscopy and viscosimetry. It has been shown that in 10 mM-sodium acetate solutions (pH 3.5 to 3.9) the protein molecule undergoes a reversible conformational transition into a non-compact disordered state not only when the solution is heated above room temperature but also when it is cooled. In this state the protein does not have a tertiary structure, although it retains some residual ellipticity, which may be caused by the fluctuating alpha-helical conformation of the unfolded polypeptide chain. The disruption of the native protein structure both on cooling (cold-denaturation) and on heating (heat-denaturation) proceeds in an "all-or-none" manner, with a significant and similar increase of the protein heat capacity, but with inverse enthalpic and entropic effects: the enthalpy and entropy of the protein molecule decrease during cold-denaturation and increase during heat-denaturation.

Contributions of nuclear magnetic resonance to renal biochemistry

31P NMR as a descriptive technique is of interest to nephrologists. Particular contributions of 31P NMR to our understanding of renal function may be enumerated.: "Free" metabolite levels are different from those classically accepted; in particular, ADP and Pi are low with implications for the control of renal metabolism and Pi transport, and, via the phosphorylation potential, for Na+ transport. Renal pH is heterogeneous; between cortex, outer medulla, and papilla, and between cell and lumen, a large pH gradient exists. Also, quantitation between cytosol and mitochondrion of the pH gradient is now feasible. In acute renal failure of either ischemic or nonischemic origin, both ATP depletion and acidification of the renal cell result in damage, with increasing evidence for the importance of the latter. Measurements of renal metabolic rate in vivo suggest the existence of a prodromal phase of acute renal failure, which could lead to its detection at an earlier and possibly reversible stage. Human renal cancers show a unique 31P NMR spectrum and a very acidic environment. Cancer chemotherapy may alter this and detection of such changes with NMR offers a method of therapeutic monitoring with significance beyond nephrology. Renal cortex and medulla have a different T1 relaxation time, possibly due to differences in lipid composition. It seems that NMR spectroscopy has much to offer to the future understanding of the relationship between renal biochemistry and function.

31P NMR relaxation studies of the activation of the coenzyme phosphate of glycogen phosphorylase. The role of motion of the bound phosphate

Spin-lattice and spin-spin relaxation rates (1/T1 and 1/T2) have been determined for the catalytically essential coenzyme phosphate at the active site of glycogen phosphorylase in both activated (R state) and inactive (T state) conformations of the enzyme. Dipolar contributions to 31P relaxation due to exchangeable protons on the phosphate group have been determined by measurement of relaxation rates at different concentrations of H2O and D2O, and field dependence studies have been performed to estimate the contribution of chemical shift anisotropy to the remaining 31P relaxation in D2O. At 109 MHz, dipolar relaxation from exchangeable protons was found to account for 50% of the spin-lattice relaxation for activated phosphorylase in 75% H2O, the remainder being due to chemical shift anisotropy. The spin-lattice relaxation rates in D2O for R-state glycogen phosphorylase are very similar to those measured for other proteins of very different size such as actin (Brauer, M., and B. D. Sykes, 1981, Biochemistry. 20:6767-6775), alkaline phosphatase (Coleman, J. E., I. D. Armitage, J. F. Chlebowski, J. D. Otvos, and A. J. M. S. Uiterkamp, 1979), and phosphoglucomutase (Rhyu, G. I., W. J. Ray, Jr., and J. L. Markley, 1984, Biochemistry. 23:252-260). In inactive (T state) phosphorylase the spin-lattice relaxation rates were almost an order of magnitude slower, while the spin-spin relaxation rates were essentially identical. These results have been analyzed by calculating the theoretically expected 31P relaxation rates in the presence of internal motions that are included in the relaxation calculation using the model-free approach of Lipari and Szabo (1982, J. Am. Chem. Soc. 104:4564-4559). The analysis suggests the coenzyme phosphate is relatively immobilized in the activated enzymic conformation, but in the inactive (Tstate) conformation it is considerably more mobile with a rotational correlation time one to two orders of magnitude smaller. Since the spin-lattice relaxation rate for the active R-state (immobilized) phosphate is similar to that observed in other phosphoenzymes of different size it is suggested that a librational motion on the nanosecond time scale may constitute a common spin-lattice relaxation pathway for phosphates in macromolecules. The consequences of phosphate motion in terms of recent suggestions concerning the environment and the catalytic role of the coenzyme phosphate are discussed.

The study of conformational states of proteins by nuclear magnetic resonance

By the use of examples, mainly of rather rigid proteins, we hope to have shown that conformational analysis of proteins is a problem that is not simply related to the conformational analysis of small molecules. The primary difficulties with proteins are (1) the multitude of possible conformers, (2) the complex dynamical behaviour and (3) the degree of co-operativity within the molecules. Any experimentally derived structural description of a protein is an attempt to represent some average of a complex time dependence. N.m.r. techniques have now reached the point where it is possible to use them to describe many detailed structural features of small globular proteins in solution and to detect and to describe conformational changes in such proteins. In addition, analysis is becoming possible of much less ordered regions of polypeptides, such as are found in less compact proteins, of for example myosin, histones and virus coat proteins, or in denatured states. The limits to the detailed conformational analysis of such proteins are likely to be ones of reality rather than method but the description of the properties shown in Table 1 is by its very nature an extremely important problem in conformational analysis of dynamic macromolecules.

The use of p-fluorobenzenesulfonyl chloride as a reagent for studies of proteins by fluorine nuclear magnetic resonance

The reagent p-fluorobenzenesulfonyl chloride modifies the protein side chains of tyrosine, lysine, and histidine and the alpha-NH2 group. The p-fluorobenzenesulfonyl (Fbs-) group, identified by the 19F nuclear magnetic resonance signal, exhibits a different 19F chemical shift for each functional group modified. The Fourier-transformed spectra of the Fbs- group displayed the expected nine-line multiplet in Fbs- amino acids and simple Fbs- peptides but not in the Fbs- proteins, where the resolution was less. Lysozyme, RNase, DNase, and chymotrypsin react with this reagent and each Fbs- protein exhibits a distinctive pattern of 19F NMR signals due to the label, suggesting that the reaction of the reagent varies with the reactivity of the side chains in a protein. The three major 19F signals of the unfolded Fbs-RNase in 8 M urea are due to the Fbs- label on the imidazolium, alpha-NH2, and epsilon-NH2 groups. Based upon results from amino acid and 19F NMR analyses of the tryptic-chymotryptic peptides of Fbs-RNase, portions of the imidazolium and epsilon-NH2 resonances were assigned to the Fbs- label on His-105 and Lys-41, respectively, while the alpha-NH2 resonance was entirely due to the Fbs- label on the alpha-NH2 of Lys-1. Because Fbs-RNase has an unchanged, near-ultraviolet circular dichroism spectrum and because it retains approximately 80% of the RNase activity, the conformation of Fbs-RNase is probably not altered from the folded conformation of the native enzyme. Upon unfolding in 8 M urea or heating at 70 degrees C, Fbs-RNase gave a 19F NMR spectrum differing from that of the folded Fbs-RNase. In the presence of uridylic acid, Lys-41 was the only residue protected from modification by the reagent with a concomitant reduction of the epsilon-NH2 resonance, and the RNase thus modified was fully active. Hence, 19F NMR analysis of protein, via the reaction with p-fluorobenzenesulfonyl chloride, provided not only information about the protein conformation but also direct measurements of the modification status.

A 1H NMR technique for observing metabolite signals in the spectrum of perfused liver

We have developed a 1H NMR technique to selectively edit the spectrum of perfused liver for specific resonances of metabolites that occur in low concentration. The method employs selective DANTE pulses, which avoid exciting the water signal and at the same time control the J modulation effect in the homonuclear spin-echo experiment. By difference spectroscopy, we have suppressed the background signals from lipids and water and have resolved the CH3 resonance of lactate at 1.33 ppm. Moreover, the technique is highly selective and allows us to select the CH3 resonance of alanine at 1.47 ppm in the presence of the CH3 resonance of lactate at 1.33 ppm, even though the latter was much larger before editing. We have applied this technique to study the metabolic effect of ethanol in perfused mouse liver and have observed that the rate of formation of lactate from pyruvate is increased by a factor of 2.8 when ethanol is added.

Measurements of exchange in the reaction catalysed by creatine kinase using 14C and 15N isotope labels and the NMR technique of saturation transfer

31P-NMR measurements of saturation transfer have been used to measure exchange between the gamma-phosphate of ATP and phosphocreatine and between the beta-phosphate of ATP and the beta-phosphate of ADP in the reaction catalysed by creatine kinase in vitro. The calculated exchange fluxes have been compared with measurements of 15N label exchange between creatine and phosphocreatine and 14C label exchange between ATP and ADP. At pH 8.0 the fluxes between phosphocreatine and the gamma-phosphate of ATP and between the beta-phosphates of ATP and ADP, measured by saturation transfer, were the same and equal, within experimental error, to the fluxes between creatine and phosphocreatine, measured by 15N label exchange, and between ADP and ATP, measured by 14C label exchange. At pH 7.0 the flux between phosphocreatine and the gamma-phosphate of ATP, measured by saturation transfer, was equal, within experimental error, to the flux between creatine and phosphocreatine, measured by 15N label exchange. However, at low ADP concentrations (less than 0.2 mM), the flux between ATP and ADP measured by saturation transfer was significantly less than that between phosphocreatine and ATP and, more importantly, less than the ADP-ATP exchange flux measured by 14C label exchange. The saturation transfer and isotope exchange measurements at pH 7.0 have shown that it is valid to equate saturation transfer measurements of exchange between phosphocreatine and ATP in vivo with the potential for net chemical flux through the reaction. The observed discrepancy at pH 7.0 between the 14C and saturation transfer measurements of ATP----ADP exchange can be explained if there is significant loss of saturation in an intermediate in the exchange reaction. Under these conditions analysis of the exchange according to two-site exchange model is invalid. In magnetisation transfer measurements of exchange in other enzyme catalysed reactions, the possible presence of a kinetically significant intermediate and therefore the validity of data analysis using a two-site exchange model should be considered.

Solution structure of mitochondrial cytochrome c. I. 1H nuclear magnetic resonance of ferricytochrome c

The 1H nuclear magnetic resonance spectra of tuna and horse ferricytochromes c have been investigated and the resonances of all amino acid methyl groups have been assigned to specific absorption lines. The assignment procedure involves principally the comparison of one-dimensional nuclear magnetic resonance spectra from a range of homologous ferricytochromes c and does not require a prior knowledge of the secondary or tertiary protein structure. Of the 49 methyl groups of tuna cytochrome c, the assignment of 33 is made without reference to the X-ray crystal structure. The method should therefore be applicable to other proteins of similar size where X-ray structures are unavailable. The assignments will be used to investigate the structure of cytochrome c in solution.

A spectroscopic investigation of the structure and redox properties of Escherichia coli cytochrome b-562

The six-coordinate monohaem ferricytochrome b-562 from Escherichia coli exhibits two haem-linked pH-dependent transitions detected by NMR and optical spectroscopy. Only one of these transitions, that of the Fe(III)-coordinated His-102, is detected by EPR and MCD; the ionisation of a haem propionate is not. Both ionisations are redox-state-dependent and the midpoint redox potential of the protein is markedly pH-dependent. Over the pH range 5.0 to 8.5 the potential drops from 260 mV to 110 mV and at least five single proton ionisations are responsible for this. In addition to the two spectroscopically identified ferricytochrome ionisations, there are at least three unidentified ionisations, two of which occur in the ferrous protein. From a consideration of the X-ray structure, together with NMR data, it seems probable that at least one of these ionisations involves an amino acid carboxylate. The X-ray structure also suggests that the relatively low pKa of His-102 is a result of its proximity to Arg-98. However, an appreciable interaction between these groups requires that the solution conformation differs slightly from the X-ray structure. The fast rate of electron self-exchange, over 4 X 10(6) M-1 X s-1 at 315 K and pH* 7, may be a reflection of the fact that, as shown by the X-ray structure, a large amount of the haem and axial histidine ligand are exposed at the molecular surface with an asymmetric distribution of charged groups surrounding them.

1H NMR methods for the noninvasive study of metabolism and other processes involving small molecules in intact erythrocytes

1H NMR methods are described with which resolved resonances can be obtained for many of the small molecules in intact erythrocytes. In one method, the more intense hemoglobin resonances are suppressed by transfer of saturation throughout the hemoglobin spin system by cross relaxation following a selective saturation pulse. In a second method, the hemoglobin resonances are eliminated with the spin-echo pulse sequence by using a between-pulse delay time long enough for complete elimination of the hemoglobin resonances by spin-spin relaxation. Selected examples of the study of erythrocyte biochemistry by 1H NMR are discussed.

Conformation of somatostatin using scalar coupling constants from 270 and 600 MHz simulated proton magnetic resonance spectra

The conformation of the 14 amino acid peptide hormone somatostatin in aqueous solution was investigated through a proton magnetic resonance (PMR) scalar coupling analysis. Experiments were performed at two fields, 270 and 600 MHz, and included double and triple resonance difference scalar decoupling, resolution enhancement and computer simulation. The agreement between simulated and observed spectra at both fields provided support for the correctness of the analysis. The resultant scalar coupling constants, 3J alpha H-NH and 3J alpha B, gave information on the backbone (phi) and side chain (chi 1) torsional angles, respectively, which eliminated either of the proposed conformations of somatostatin as describing a predominant conformer of the molecule in solution under our conditions.

Growth of Lactobacillus casei in deuterated media. Purification of deuterated thymidylate synthase and proton nuclear magnetic resonance studies

The purification of thymidylate synthase from amethopterin-resistant Lactobacillus casei grown in a deuterated medium is described. The deuterated enzyme and the non-deuterated enzyme appear to be identical with respect to specific activity, pH optimum, electrophoretic mobility on polyacrylamide gels and ability to form ternary complexes with 5-fluoro-2'-deoxyuridine 5'-monophosphate (FdUMP) and 5,10-methylenetetrahydrofolate. The deuterated enzyme remained stable at 25 degrees C for 8 h during the acquisition of 400 MHz 1H-NMR spectra and was stored at 5 degrees C for 3 months without loss of catalytic activity. The incorporation of deuterium was essentially complete as demonstrated by a comparison of the NMR spectra of deuterated and non-deuterated enzyme. Proton NMR data obtained with deuterated thymidylate synthase and 1H-FdUMP are in agreement with the fluorine-19 NMR studies of Lewis et al. (Lewis, C.A., Jr., Ellis, P.D. and Dunlap, R.B. (1980) Biochemistry 19, 116-123) which support the formation of a binary complex where the enzyme is covalently linked to carbon 6 of 5-fluoro-5,6-dihydro-2'-deoxyuridylate.

Dipolar NMR relaxation of nonprotonated aromatic carbons in proteins. Structural and dynamical effects

The crystal structure and a 96-ps molecular dynamics simulation used to analyze structural and motional contributions to spin-lattice (T1) relaxation times of phenylalanine and tyrosine C gamma carbons of the pancreatic trypsin inhibitor. The H beta and H delta protons geminal to C gamma are calculated to account for approximately 80% of the dipolar relaxation for each residue. Experimental T1 values for the phenylalanine residues obtained at 25 MHz are observed to be 15-25% longer than estimates based on the rigid crystal structure. It is shown how an increase in T1 can be related to order parameters for the picosecond motional averaging of the important C,H dipolar interactions, and how these order parameters can be calculated from a protein molecular dynamics trajectory.

A p.m.r. isotope-exchange method for studying the kinetic properties of dehydrogenases in intact cells

A method to determine the activity of dehydrogenases in an intact-cell system is described. The method involves the use of n.m.r. to monitor bulk isotope exchange. The approach is illustrated by application to the isotope equilibration of pyruvate and lactate as catalyzed by lactate dehydrogenase in intact erythrocytes. Particular problems peculiar to bulk isotope exchange and its observation by n.m.r. are considered.

A 1H n.m.r. study of isotope exchange catalysed by glycolytic enzymes in the human erythrocyte

The exchange of hydrogen and deuterium atoms between the C-2 position of lactate and solvent was monitored in suspensions of human erythrocytes by using a non-invasive spin-echo p.m.r. method that permits continuous assessment of the rate and the extent of exchange. Exchange rates were measured in cells suspended in buffers made in 2H2O and 1H2O after the addition of L-[2-1H]lactate and L-[2-2H]lactate respectively. The rate of exchange is dependent on the activities of four glycolytic enzymes (fructose bisphosphate aldolase, triose phosphate isomerase, glyceraldehyde phosphate dehydrogenase and lactate dehydrogenase) and on the concentrations of their substrates. The dependence of the exchange on the following substrates was studied: (1) lactate, (2) the triose phosphates and fructose 1,6-bisphosphate and (3) pyruvate. Observation of the exchange in vitro, in a system produced by mixing the isolated enzymes, permits determination of the individual isotope-exchange equilibrium velocities of the enzymes. The dependence of the equilibrium velocity of human erythrocyte lactate dehydrogenase on NAD+ + NADH concentration was measured. Possible applications of these methods are discussed.

Estimation of deuteration levels in whole cells and cellular proteins by 1H n.m.r. spectroscopy and neutron scattering

Methods using conventional Fourier transform 1H n.m.r. spectroscopy at 250 MHz for the determination of the overall deuteration levels of cells cultured in media containing 2H2O or deuterated carbon sources are described. These were developed for Escherichia coli as a model, and extended to Neurospora crassa hyphae and mouse myeloma cells P815. The results were investigated by 1H n.m.r. and neutron scattering measurements on deuterated proteins that were obtained from E. coli. It is concluded that 1H n.m.r. is able to observe the soluble proteins of E. coli in certain cases, that deuteration levels can be determined by 1H n.m.r. for small quantities of proteins in their native state, and that glycerol is a more efficient carbon source than glucose for the deuteration of E. coli proteins.

13C nuclear magnetic resonance studies of the composition and fluidity of several chloroplast monogalactosyldiacylglycerols

The motional properties of four monogalactosyldiacylglycerols isolated from photosynthetic membranes, and containing different fatty acid chain lengths and degrees of unsaturation, have been determined using 13C nuclear magnetic resonance. These properties have been compared with those of a lipid containing only saturated fatty acids. The 13C longitudinal relaxation times (T1) of the carbon atoms of the acyl chains in [2H4] methanol were measured as an index of the rates of motion of the lipid molecules and used to compare the relative fluidity of the acyl chains. The T1 values of the glyceryl and galactosyl carbon atoms in each monogalactosyldiacylglycerol are essentially constant, when allowance is made for concentration differences and the presence of two hydrogens on a methylene carbon versus one on a methine carbon. These results indicate similar rates of motion for the headgroup carbons in each lipid. However, for the acyl chains, the T1 values increase with the introduction of a double bond and increase further with additional unsaturation. This increase in the rate of motion only occurs at carbon atoms beyond the first double bond in an acyl chain. These results differ to those reported for monolayer experiments where changes in packing characteristics are predominantly dependent on the introduction of the first double bond and then vary little between species.

Studies of pyruvate-water isotope exchange catalysed by erythrocytes and proteins

Erythrocyte suspensions in buffer made with 2H2O catalyse the exchange of pyruvate protons. This process can be easly observed by spin-echo proton magnetic resonance. The dominant exchange process is shown to be due to the formation of Schiff-base links between pyruvate and amino groups of haemoglobin. Other proteins with free alpha-amino groups also catalyse the exchange. The pH*-dependence of the exchange rate due to hen-egg-white-lysozyme reflects the dissociation of the alpha-amino group.