Crystallization and preliminary X-ray diffraction studies on cytosolic (class 1) aldehyde dehydrogenase from sheep liver

The cytosolic (Class 1) aldehyde dehydrogenase (AlDH) from sheep liver has been crystallized in a form suitable for X-ray diffraction studies. The crystals, grown by vapour diffusion using 6.5 to 7.5% methoxypolyethylene glycol 5000 as precipitant, at pH 6.5, are orthorhombic with cell dimensions a = 80.7, b = 92.5, c = 151.6 A, space-group P2(1)2(1)2(1), and one dimer in the asymmetric unit. The crystals diffract to at least 2.8 A resolution. Although unmodified AlDH crystallized readily, a key factor in obtaining diffraction-quality crystals was the covalent attachment of an active site reporter group, provided by 3,4-dihydro-3-methyl-6-nitro-2H-1,3-benzoxazin-2-one.

The effect of p-(chloromercuri)benzoate modification of cytosolic aldehyde dehydrogenase from sheep liver. Evidence for a second aldehyde binding site

p-Chloromercuribenzoate (PCMB) at stoichiometric levels reacts with a thiol group of the binary NAD+ complex of sheep liver cytoplasmic aldehyde dehydrogenase (E.NAD+) faster than with the corresponding thiol group of either the free enzyme or the binary enzyme. NADH complexes. High concentrations of propionaldehyde have a protective effect against modification of the enzyme with PCMB in steady-state assays. This protection arises from a reduction in the concentration of the E.NAD+ binary complex rather than competition for a common binding site. PCMB has three major effects on aldehyde dehydrogenase. First, rapid reaction with a high-affinity thiol group in the E.NAD+ binary complex causes activation of the steady-state rate. The activation results from an increase in the rate of NADH release from the enzyme. This modification simultaneously protects against dilution-induced dissociation of enzyme tetramers. Second, premodification of the high-affinity thiol group leads to inhibition of the steady-state rate at high propionaldehyde concentrations, because of the increased affinity of the free enzyme for propionaldehyde with the resultant formation of an enzyme-aldehyde dead-end complex. Third, when higher ratios of PCMB to enzyme (> 3:1) are used, one or more other thiol groups are also modified, causing enzyme dissociation and subsequent inactivation. Since modification of the high-affinity thiol by PCMB causes activation, clearly it cannot be the active site acylation center involved in propionaldehyde oxidation. The different amplitudes of the proton burst at high and low propionaldehyde concentrations for the PCMB modified enzyme provide support for a second binding site for propionaldehyde on the enzyme.

Arthroscopic Bankart suture repair

The purpose of this paper was to report our experience with an arthroscopic technique of repair for the Bankart lesion following shoulder instability. Twenty-seven patients (average age, 21.7 years) were followed for an average of 36 months after arthroscopic suture stabilization of anterior shoulder instability. Patients were excluded if instability was multidirectional or voluntary and if there was radiographic evidence of a significant loss of glenoid bone stock. Clinical evaluation using a functional grading system showed that 10 patients were rated as excellent, 5 good, and 12 poor. Fourteen patients returned to their previous level of activity. There were 12 patients rated as failed; all had recurrent instability of the shoulder. Success was associated with a period of immobilization of 3 weeks or longer and a history of acute injury, especially subluxation. Failures were associated with shorter immobilization periods after surgery and in patients who had recurrent dislocations. The younger patient, who may not have complied with the immobilization protocol, also seemed to be associated with failure. Contact sports seems to leave a patient at high risk for recurrence. We recommend caution in the use of arthroscopic procedures for the competitive athlete in whom a second surgery and rehabilitation might mean loss of more sports participation.

Activation of aldehyde dehydrogenase at physiological temperatures

The release of NADH from the enzyme.NADH complexes was rate limiting at 37 degrees, for the oxidation of propionaldehyde by sheep liver cytosolic aldehyde dehydrogenase. Marked substrate activation was observed at this temperature as was activation by p-(chloromercuri)benzoate. Activation of enzymic activity may be of importance in vivo.

Glucose tolerance factor potentiation of insulin action in adipocytes from rats raised on a torula yeast diet cannot be attributed to a deficiency of chromium or glucose tolerance factor activity in the diet

The nature of the dietary component responsible for adipocytes having the ability to respond to Glucose Tolerance Factor (GTF) was investigated. Rats were raised on either a control diet or one of three diets differing only in the protein source (torula yeast, brewer's yeast, or casein). Only in adipocytes from rats fed the torula yeast diet did a GTF fraction prepared from brewer's yeast potentiate the action of suboptimal concentrations of insulin in the incorporation of label from D-[1-14C]-glucose and D-[U-14C]-glucose into CO2 and fatty acids. It was concluded that this potentiation was not the result of a deficiency of GTF activity in torula yeast, because a GTF fraction prepared from torula yeast had similar insulin potentiating activity. Differences in response among diets were not owing to differences in levels of amino acids or owing to concentrations of 22 (Al, As, B, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mo, Na, Ni, P, Pb S, Se, Si, Sn, Sr, Zn) of the 23 trace elements investigated. The level of Mn was low in all diets, but particularly low in the torula yeast diet. Mn deficiencies have previously been implicated in perturbations of glucose metabolism, so that it is possible that this deficiency may be responsible for the effects attributed to the torula yeast diet.

Structural bone-grafting for early atraumatic avascular necrosis of the femoral head

Between 1970 and 1987, nineteen patients, thirty-one to fifty-five years old, had twenty core-decompression procedures with corticocancellous bone-grafting for Stage-I or II atraumatic avascular necrosis of the femoral head. A tibial autogenous graft was used in three hips; a fibular autogenous graft, in seven hips; and a fibular allograft, in ten hips. Treatment was considered to have failed when there was clinical or roentgenographic evidence of progression of the necrosis. Eighteen patients who had a minimum follow-up of two years (average, eight years; range, two to nineteen years) were asymptomatic, with no evidence of progression of the necrosis or collapse of the affected segment. In two hips, the necrotic segment of the femoral head collapsed within one year after the operation, and a replacement arthroplasty was carried out.

Steady-state and pre-steady-state kinetics of propionaldehyde oxidation by sheep liver cytosolic aldehyde dehydrogenase at pH 5.2. Evidence that the release of NADH remains rate-limiting in the enzyme mechanism at acid pH values

The kcat value for the oxidation of propionaldehyde by sheep liver cytosolic aldehyde dehydrogenase increased 3-fold, from 0.16 s-1 at pH 7.6 to 0.49 s-1 at pH 5.2, in parallel with the increase in the rate of displacement of NADH from binary enzyme.NADH complexes. A burst in nucleotide fluorescence was observed at all pH values consistent with the rate of isomerization of binary enzyme.NADH complexes constituting the rate-limiting step in the steady state. No substrate activation by propionaldehyde was observed at pH 5.2, but the enzyme exhibited dissociation/association behavior. The inactive dissociated form of the enzyme was favored by low enzyme concentration, low pH, and low ionic strength. Propionaldehyde protected the enzyme against dissociation.

The use of pH indicators to identify suitable environments for freezing samples in aqueous and mixed aqueous/nonaqueous solutions

The colours of frozen solutions containing pH indicators are shown to provide a test for changes in pH in the solvent environment which occur on freezing. Yeast alcohol dehydrogenase loses activity on freezing in phosphate buffer (a buffer in which pH indicator colour changes shows a marked decrease in pH on freezing) but when frozen in bis-tris, Hepes, or N-glycylglycine buffers (all of which show little change in the colour of universal pH indicator and hence of pH on freezing) is stable on freezing. The effects of freezing in different buffer systems on the rate of decomposition of NADPH, and on the rate hydrolysis of 4-nitrophenyl acetate, are rationalised in terms of the pH shifts in these buffers which were determined using universal pH indicator. It is proposed that a major reason for the instability of samples on freezing is the pH changes which occur when some systems are frozen. From the results a general scheme for selecting the best environment for safely freezing samples is proposed which is based on the use of pH indicators.

Effect of pyrophosphate ions and alkaline pH on the kinetics of propionaldehyde oxidation by sheep liver cytosolic aldehyde dehydrogenase

Pyrophosphate ions activate the steady-state rate of oxidation of propionaldehyde by sheep liver cytosolic aldehyde dehydrogenase at alkaline pH values. The steps in the mechanism governing the release of NADH from terminal enzyme. NADH complexes have been shown to be rate-limiting at pH 7.6 [MacGibbon, Buckley & Blackwell (1977) Biochem J. 165, 455-462]. These steps are shown to be also rate-limiting at more alkaline pH values, and it is through an acceleration of these steps that pyrophosphate ions exert their activation effect.

Evidence for reactivity of serine-74 with trans-4-(N,N-dimethylamino)cinnamaldehyde during oxidation by the cytoplasmic aldehyde dehydrogenase from sheep liver

A nucleophilic group in the active site of aldehyde dehydrogenase, which covalently binds the aldehyde moiety during the enzyme-catalyzed oxidation of aldehydes to acids, was acylated with the chromophoric aldehyde trans-4-(N,N-dimethylamino)cinnamaldehyde (DACA). Acyl-enzyme trapped by precipitation with perchloric acid was digested with trypsin, and the peptide associated with the chromophoric group was isolated and shown to be Gln-Ala-Phe-Gln-Ile-Gly-Ser-Pro-Trp-Arg. After redigestion with thermolysin, the chromophore was associated with the C-terminal hexaresidue part. If the chromophore is attached to this peptide, serine would be expected to bind the aldehyde and lead to the required acylated derivative. Differential labeling experiments were performed in which all free thiol groups on the acylated enzyme were blocked by carboxymethylation. The acyl chromophore was then removed by controlled hydrolysis and the protein reacted with [14C]iodoacetamide. No 14C-labeled tryptic peptides were isolated, suggesting that the sulfur of a cysteine cannot be the acylated residue in the precipitated acyl-enzyme.

Identification of peptides from autolysates of Saccharomyces cerevisiae that exhibit glucose tolerance factor activity in a yeast assay

1. Cationic fractions were isolated from a low chromium (less than 0.2 ppm) commercial yeast extract in an attempt to purify the material responsible for glucose tolerance factor (GTF) activity observed in a standard yeast assay system. 2. Following previously described procedures a fraction with GTF activity but containing negligible chromium was isolated, which on further purification was found to be composed of many separate small basic peptides. 3. Much of the activity of the yeast GTF material in the yeast assay could be attributed to the presence of basic peptides and free amino acids acting as nitrogen sources for the yeast. 4. Additional activity was present in the yeast GTF sample, which was not due to a synergistic effect of the mixed amino acids and peptides although the component of the yeast extract responsible for this activity was not identified. 5. The results show that the GTF fractions isolated according to most previously published procedures are highly impure, and conclusions drawn about the nature of GTF based on these isolates must remain open to question. 6. The activity due to the presence of peptides and amino acids is a major cause of lack of specificity of the yeast systems as an assay for GTF.

Evidence that the cytoplasmic aldehyde dehydrogenase-catalysed oxidation of aldehydes involves a different active-site group from that which catalyses the hydrolysis of 4-nitrophenyl acetate

Acylation of the aldehyde dehydrogenase.NADH complex by acetic anhydride leads to the production of acetaldehyde and NAD+. By monitoring changes in nucleotide fluorescence, the rate constant for acylation of the active site of the *enzyme.NADH complex was found to be 11 +/- 3 s-1. The rate of acylation by acetic anhydride at the group that binds aldehydes on the oxidative pathway is clearly rapid enough to maintain significant steady-state concentrations of the required active-site-acylated *enzyme.NADH intermediate despite the rapid hydrolysis of this *enzyme.acyl.NADH intermediate (5-10 s-1) [Blackwell, Motion, MacGibbon, Hardman & Buckley (1987) Biochem. J. 242, 803-808]. Hence reversal of the normal oxidative pathway can occur. However, although acylation of the aldehyde dehydrogenase.NADH complex by 4-nitrophenyl acetate also occurs rapidly with a rate constant of 10.9 +/- 0.6 s-1, even under the most extreme trapping conditions only very small amounts of acetaldehyde are detected [Loomes & Kitson (1986) Biochem. J. 235, 617-619]. Furthermore enzyme-catalysed hydrolysis of 4-nitrophenyl acetate is limited by the rate of deacylation of a group on the enzyme (0.4 s-1), which is an order of magnitude less than deacylation of the group at the active site (5-10 s-1). It is concluded that the enzyme-catalysed 4-nitrophenyl ester hydrolysis involves a group on the enzyme that is different from the active-site group that binds aldehydes on the normal oxidative pathway.

Evidence that the slow conformation change controlling NADH release from the enzyme is rate-limiting during the oxidation of propionaldehyde by aldehyde dehydrogenase

The displacement of NADH from the aldehyde dehydrogenase X NADH complex by NAD+ was followed at pH 7.0, and the data were fitted by a non-linear least-squares iterative procedure. At pH 7.0 the decay constants for the dissociation of NADH from aldehyde dehydrogenase X NADH complexes (1.62 +/- 0.09 s-1 and 0.25 +/- 0.004 s-1) were similar to the values previously determined by MacGibbon, Buckley & Blackwell [(1977) Biochem. J. 165, 455-462] at pH 7.6, and apparent differences between these values and those reported by Dickinson [(1985) Biochem. J. 225, 159-165] are resolved. Experiments at low concentrations of propionaldehyde show that isomerization of a binary E X NADH complex is part of the normal catalytic mechanism of the enzyme. Evidence is presented that the active-site concentration of aldehyde dehydrogenase is halved when enzyme is pre-diluted to low concentrations before addition of NAD+ and substrate. The consequences of this for the reported values of kcat. are discussed. A general mechanism for the aldehyde dehydrogenase-catalysed oxidation of propionaldehyde which accounts for the published kinetic data, at concentrations of aldehyde which bind only at the active site, is presented.

Kinetics of inhibition and hysteresis of sheep liver cytoplasmic aldehyde dehydrogenase with glyoxylic acid: further evidence relating to the two-site model for aldehyde oxidation

Despite the fact that it is an aldehyde, glyoxylic acid is not a substrate for sheep liver cytoplasmic aldehyde dehydrogenase; instead it functions as an inhibitor of both the esterase and dehydrogenase activities. From a consideration of the inhibition patterns it is concluded that glyoxylic acid does not bind in the catalytic propionaldehyde-binding domain, thus confirming the two-site model as proposed previously. Since the corresponding neutral methyl ester is a substrate it is suggested that the catalytic binding domain must contain a negatively charged group which prevents the binding of glyoxylic acid. Steady-state and pre-steady-state kinetic studies indicate that glyoxylic acid inhibits the dehydrogenase activity by converting the enzyme into a dead-end form which cannot undergo the catalytically essential conformational change. Incubation of the enzyme with NAD+ and glyoxylic acid for 10 min before the addition of propionaldehyde gave rise to hysteresis effects which can be explained on the basis of a slow isomerization of the enzyme X NAD+ X glyoxylic acid complex.

Activating effect of p-(chloromercuri)benzoate on the cytoplasmic aldehyde dehydrogenase from sheep liver

In the absence of NAD+, up to 12 SH groups on aldehyde dehydrogenase (ALDH) reacted rapidly with p-(chloromercuri)benzoate (PCMB); a slow reaction with more than twice this number of SH groups then occurred. When PCMB was added to an assay mixture at low (less than 100 microM) concentrations of propionaldehyde, the steady-state rate of production of NADH increased with increasing PCMB concentration up to a maximum activity at a [PCMB]/[ALDH] ratio of 1.9 and then decreased as the [PCMB]/[ALDH] ratio increased further. Under some conditions, activation, or inhibition, showed hysteretic effects as the initial slope after mixing changed to a final linear steady state in a first-order manner, the rate constants for which were proportional to the concentration of free PCMB. Activating levels of PCMB had little effect on the NADH and proton burst amplitudes or rate constants and did not affect the rate of dissociation or association of NADH. However, when a 20-fold excess of PCMB concentration over enzyme concentration was premixed with the enzyme, neither a burst nor a steady-state turnover of substrate was observed. It is concluded that activation arises from the tight binding of PCMB with a single thiol group per subunit which is exposed after the binding of NAD+ to the enzyme, followed by a slow conformational change which causes activation by altering the steady-state mechanism so that NADH dissociation becomes largely rate limiting.(ABSTRACT TRUNCATED AT 250 WORDS)

The relationship of chromium to the glucose tolerance factor. II

After incubation with CrCl3 X 6H2O (or 51CrCl3 X 6H2O) for 25 days, a sterile growth medium, whole yeast cells harvested after growth on a similar chromium-containing medium for the same period, and the spent growth medium remaining after removal of the yeast were each subjected to the separation procedure reported previously [S. J. Haylock. P. D. Buckley and L. F. Blackwell, J. Inorg. Biochem., in press]. The results obtained showed that most of the eleven chromium-containing fractions isolated previously were artifacts formed as a result of direct reaction between the chromium and components of the medium. An anionic complex (which was the major chromium-containing fraction isolated) was identified as a chromium-glucose complex, but one possessing no biological activity. The biologically active chromium-containing fractions (P-3 and P-4) that were only present after yeast had been grown in the medium were further purified, however, during the purification steps, the biological activity was cleanly separated from the chromium material for both P-3 and P-4. Fraction P-4 was subsequently shown to consist of approximately 90% tyramine, but pure tyramine was not active in the yeast bioassay. Although the structure of the glucose tolerance factor-active component in fraction P-3 could not be determined due to the presence of high concentrations of salt that could not be separated on gel filtration columns, the results show that the glucose tolerance factor from brewer's yeast can no longer be regarded as a chromium complex.

Separation of biologically active chromium-containing complexes from yeast extracts and other sources of glucose tolerance factor (GTF) activity

A procedure has been developed, based on ion-exchange chromatography, that readily allows the separation of eleven apparently homogeneous chromium-containing fractions from a brewer's yeast extract. Four of the fractions are amphoteric and show no glucose tolerance factor (GTF) activity, three are classified as negative (two of which are biologically inactive, while the third one shows a slight degree of GTF activity), whereas the four cationic chromium-containing fractions all show varying degrees of GTF activity. Application of the separation procedure to other biological sources of GTF activity resulted in a spectrum of cationic fractions, over the pH range 1.75 to 12, which suggests that GTF cannot be a single species. The cationic chromium-containing fraction from pork kidney powder and fraction P-3 from yeast appear to contain the most GTF-active material and P-3 shows saturation kinetics as expected for a biologically significant substance.

A two-site model for the esterase and dehydrogenase activities of sheep liver aldehyde dehydrogenase

Although aldehyde dehydrogenase (ALDH) from sheep liver cytosol has a broad specificity, it will not oxidize the aldehyde group of glyoxylic acid which is in fact an inhibitor of the enzyme. The inhibition pattern is non-linear but competitive at high propionaldehyde concentrations (2-20 mM); however, a simple non-competitive pattern is observed at low (less than 100 microM) propionaldehyde concentrations (Ki = 1.6 mM). The esterase activity was unaffected by glyoxylic acid in the absence of NAD+ but a simple competitive inhibition pattern (Ki = 2.5 mM) was observed with respect to 4-nitrophenyl acetate in the presence of NAD+. The data require a two-site model in which ester and aldehyde binding sites are distinct but with a second propionaldehyde molecule, and glyoxylic acid, binding at or near the ester binding site. Consistent with this model is the fact that chloral hydrate was a non-competitive inhibitor of the esterase activity in the presence of NAD+ but a competitive inhibitor in its absence. The enzyme exhibited hysteretic behavior governed by the protonated form of an ionizable group with an apparent pKa of 7.55.

Proton release during the pre-steady-state oxidation of aldehydes by aldehyde dehydrogenase. Evidence for a rate-limiting conformational change

A transient release of protons with an amplitude corresponding to one proton per active site has been observed for the oxidation of propionaldehyde, acetaldehyde, and benzaldehyde by sheep liver cytoplasmic aldehyde dehydrogenase at pH 7.6 with phenol red as indicator. At saturating substrate levels, the rate constants for the proton burst are in each case the same, and for acetaldehyde and propionaldehyde show the same dependence on the concentrations of the substrates, as the rate constants for the transient production of NADH reported previously [MacGibbon, A.K.H., Blackwell, L.F., & Buckley, P.D. (1977) Biochem. J. 167, 469-477]. Although, with propionaldehyde as a substrate, a full proton burst is also observed at pH 6.0, no proton burst is observed at pH 9.0. For 4-nitrobenzaldehyde, there is no burst in NADH production, but a burst in proton release is observed, showing that proton release precedes hydride transfer. No protons were released during the binding of the substrate analogues acetone and chloral hydrate nor on reaction of the enzyme with the inhibitor tetraethylthiuram disulfide (disulfiram). A model is proposed in which the rate-limiting step in the pre-steady-state phase of the reaction is a conformational change which occurs after the binding of aldehydes to the enzyme. As a result of the conformational change, the environment of a functional group on the enzyme, which initially has a pKa of about 8.5, is perturbed to give a final pKa value for the group of less than 5. Computer simulations were used to show that the model accurately reproduces all of the experimental data. The lack of observation of a second transient proton release, as required by the overall stoichiometry, argues that its release occurs in a slow step prior to NADH dissociation.

A reinvestigation of the purity, isoelectric points and some kinetic properties of the aldehyde dehydrogenases from sheep liver

1. Cytoplasmic aldehyde dehydrogenase was shown to be free of contamination by the mitochondrial enzyme by isoelectric focusing. 2. Both enzymes showed multiple banding in activity stains. The cytoplasmic enzyme gave two very close bands pI = 5.22 +/- 0.03 whereas the mitochondrial enzyme showed seven bands, a pair at pI = 5.22 and five further bands of pI 5.48 +/- 0.09, 5.56 +/- 0.07, 5.65 +/- 0.06, 5.70 +/- 0.03 and 5.76 +/- 0.02. Possible origins of the isoenzymes are discussed. 3. Disulfiram in a fourfold excess reduced the activity of the cytoplasmic enzyme to 9% of the initial value. The residual activity represents the activity of the disulfiram-modified enzyme and is not due to mitochondrial contamination. This casts doubt on the role of an essential thiol group. 4. The mitochondrial enzyme shows a low amplitude (22%) burst in the production of 4-nitrophenoxide ion during the hydrolysis of 4-nitrophenyl acetate at pH 7.6. The burst rate constant was 7.3 +/- 1 s-1 and the steady-state rate constant was 0.2 s-1, values similar to those previously reported for the cytoplasmic enzyme. 5. The mitochondrial enzyme shows a burst in the release of protons during the oxidation of propionaldehyde at pH 7.6. The burst rate constant was 6 s-1 and the amplitude was equal to half the formal enzyme concentration. The significance of these results for the steady-state mechanism is discussed.

Kinetics of sheep-liver cytoplasmic aldehyde dehydrogenase

1. Sheep liver cytoplasmic aldehyde dehydrogenase showed little pH dependence of V or kcat. Some buffer anion effects were noted. 2. The oxidation of aldehydes at pH 7.6 was quantitative but irreversible. The initial velocity data indicated a sequential mechanism for the addition of substrates. Inhibition by NADH and the product analogue 2-bromo-2-phenylacetic acid, together with the known tight binding of NADH to the free enzyme, indicated an ordered mechanism with NAD+ as leading substrate. 3. Values for the rate of binding and dissociation of NAD+ were obtained from the steady-state data. The values obtained were virtually identical with those which could be calculated from the data for the horse liver cytoplasmic enzyme. Close similarities are in general apparent for the horse and sheep liver cytoplasmic enzymes and with other tissue aldehyde dehydrogenases. 4. Apparent substrate activation was observed with high concentrations of both acetaldehyde and propionaldehyde, a limiting value of 0.25s-1 being obtained for kcat. No isotope effect was observed on V using [1-2H]propionaldehyde as substrate suggesting that NADH release might be rate-limiting in the steady-state. 5. The implications of the non-linear steady-state behaviour are discussed.

Purification and properties of sheep-liver aldehyde dehydrogenases

Sheep liver cytoplasmic aldehyde dehydrogenase was purified to homogeneity to give a sample with a specific activity of 380 nmol NADH min(-1) mg(-1). An amino acid analysis of the enzyme gave results similar to those reported for aldehyde dehydrogenases from other sources. The isoelectric point was at pH 5.25 and the enzyme contained no significant amounts of metal ions. On the binding of NADH to the enzyme there is a shift in absorption maximum of NADH to 344 nm, and a 5.6-fold enhancement of nucleotide fluorescence. The protein fluorescence (lambdaexcit = 290 nm, lambdaemisson = 340 nm) is quenched on the binding of NAD+ and NADH. The enhancement of nucleotide fluorescence on the binding of NADH has been utilised to determine the dissociation constant for the enzyme . NADH complex (Kd = 1.2 +/- 0.2 muM). A Hill plot of the data gave a straight line with a slope of 1.0 +/- 0.3 indicating the absence of co-operative effects. Ellman's reagent reacted only slowly with the enzyme but in the presence of sodium dodecylsulphate complete reaction occurred within a few minutes to an extent corresponding to 36 thiol groups/enzyme. Molecular weights were determined for both cytoplasmic and mitochondrial aldehyde dehydrogenases and were 212 000 +/- 8 000 and 205 000 respectively. Each enzyme consisted of four subunits with molecular weight of 53 000 +/- 2 000. Properties of the cytoplasmic and mitochondrial aldehyde dehydrogenases from sheep liver were compared with other mammalian liver aldehyde dehydrogenases.

Steady-state and pre-steady kinetic studies on mitochondrial sheep liver aldehyde dehydrogenase. A comparison with the cytoplasmic enzyme

Kinetic studies were carried out on mitochondrial aldehyde dehydrogenase (EC 1.2.1.3) isolated from sheep liver. Steady-state studies over a wide range of acetaldehyde concentrations gave a non-linear double-reciprocal plot. The dissociation of NADH from the enzyme was a biphasic process with decay constants 0.6s-1 and 0.09s-1. Pre-steady-state kinetic data with propionaldehyde as substrate could be fitted by using the same burst rate constant (12 +/- 3s-1) over a wide range of propionaldehyde concentrations. The quenching of protein fluorescence on the binding of NAD+ to the enzyme was used to estimate apparent rate constants for binding (2 X 10(4) litre.mol-1.s-1) and dissociation (4s-1). The kinetic properties of the mitochondrial enzyme, compared with those reported for the cytoplasmic aldehyde dehydrogenase from sheep liver, show significant differences, which may be important in the oxidation of aldehydes in vivo.

Kinetic studies on the esterase activity of cytoplasmic sheep liver aldehyde dehydrogenase

The hydrolysis of 4-nitrophenyl acetate catalysed by cytoplasmic aldehyde dehydrogenase (EC 1.2.1.3) from sheep liver was studied by steady-state and transient kinetic techniques. NAD+ and NADH stimulated the steady-state rate of ester hydrolysis at concentrations expected on the basis of their Michaelis constants from the dehydrogenase reaction. At higher concentrations of the coenzymes, both NAD+ and NADH inhibited the reaction competitively with respect to 4-nitrophenyl acetate, with inhibition constants of 104 and 197 micron respectively. Propionaldehyde and chloral hydrate are competitive inhibitors of the esterase reaction. A burst in the production of 4-nitrophenoxide ion was observed, with a rate constant of 12 +/- 2s-1 and a burst amplitude that was 30% of that expected on the basis of the known NADH-binding site concentration. The rate-limiting step for the esterase reaction occurs after the formation of 4-nitrophenoxide ion. Arguments are presented for the existence of distinct ester- and aldehyde-binding sites.

Pre-steady-state kinetic studies on cytoplasmic sheep liver aldehyde dehydrogenase

Stopped-flow experiments in which sheep liver cytoplasmic aldehyde dehydrogenase (EC 1.2.1.3) was rapidly mixed with NAD(+) and aldehyde showed a burst of NADH formation, followed by a slower steady-state turnover. The kinetic data obtained when the relative concentrations and orders of mixing of NAD(+) and propionaldehyde with the enzyme were varied were fitted to the following mechanism: [Formula: see text] where the release of NADH is slow. By monitoring the quenching of protein fluorescence on the binding of NAD(+), estimates of 2x10(5) litre.mol(-1).s(-1) and 2s(-1) were obtained for k(+1) and k(-1) respectively. Although k(+3) could be determined from the dependence of the burst rate constant on the concentration of propionaldehyde to be 11s(-1), k(+2) and k(-2) could not be determined uniquely, but could be related by the equation: (k(-2)+k(+3))/k(+2) =50x10(-6)mol.litre(-1). No significant isotope effect was observed when [1-(2)H]propionaldehyde was used as substrate. The burst rate constant was pH-dependent, with the greatest rate constants occurring at high pH. Similar data were obtained by using acetaldehyde, where for this substrate (k(-2)+k(+3))/k(+2)=2.3x10 (-3)mol.litre(-1) and k(+3) is 23s(-1). When [1,2,2,2-(2)H]acetaldehyde was used, no isotope effect was observed on k(+3), but there was a significant effect on k(+2) and k(-2). A burst of NADH production has also been observed with furfuraldehyde, trans-4-(NN-dimethylamino)cinnamaldehyde, formaldehyde, benzaldehyde, 4-(imidazol-2-ylazo)benzaldehyde, p-methoxybenzaldehyde and p-methylbenzaldehyde as substrates, but not with p-nitrobenzaldehyde.

Evidence for two-step binding of reduced nicotinamide-adenine dinucleotide to aldehyde dehydrogenase

The displacement of NADH from cytoplasmic aldehyde dehydrogenase (EC 1.2.1.3) from sheep liver was studied by using NAD+, 1,10-phenanthroline, ADP-ribose, deamino-NAD+ and pyridine-3-aldehyde-adenine dinucleotide as displacing agents, by following the decrease in fluorescence as a function of time. The data obtained could be fitted by assuming two first-order processes were occurring, a faster process with an apparent rate constant of 0.85 +/- 0.20 s-1 and a relative amplitude of 60 +/- 10% and a slower process with an apparent rate constant of 0.20 +/- 0.05 s-1 and a relative amplitude of 40 +/- 10% (except for pyridine-3-aldehyde-adenine dinucleotide, where the apparent rate constant for the slow process was 0.05 s-1). The displacement rates did not change significantly when the pH was varied from 6.0 to 9.0. Kinetic data are also reported for the dependence of the rate of binding of NADH to the enzyme on the total concentration of NADH. Detailed arguments are presented based on the isolation and purification procedures, the equilibrium coenzyme-binding studies and the kinetic data, which lead to the following model for the release of NADH from the enzyme: (formula: see article). The parameters that best fit the data are: k + 1 = 0.2 s-1; k - 1 = 0.05 s-1; k + 2 = 0.8 s-1 and k - 2 = 5 X 10(5)litre-mol-1-s-1. The slow phase of the NADH release is similar to the steady-state turnover number for substrates such as acetaldehyde and propionaldehyde and appears to contribute significantly to the limitation of the steady-state rate.

Correlation of 13C N.M.R. shifts with chemical reactivity in phenethyl systems

Correlations between 13C substituent-induced chemical shifts (scs) for carbon atoms in substituted phenethyl bromides, dimethylphenethylsulphonium bromides and ethylbenzenes have been examined. C1 13C scs were found to correlate with Hammett D values as expected from previous results with 1H scs. The β-carbons showed no correlation while the α-carbons showed an inverse linear relationship with the electron-withdrawing power of the substituents.

The effect of solvent on the barrier to internal rotation in p-substituted nitrosoanilines

The solvent dependence of the barrier to internal rotation about the Ar-NO bond in N,N-dimethyl-p-nitrosoaniline and N,N-diethyl-p- nitrosoaniline has been investigated by a complete N.M.R. line-shape method. Activation parameters are reported for the nitrosoanilines in solution in acetone[D6], chloroform[D], and toluene[D8]. The effect of solvent has been found to be small.

Correlation of N.M.R. shifts with chemical reactivity in phenethyl systems

Correlations between 1H substituent-induced chemical shifts (scs) for α- and β-methylene protons in, and rate constants for base-catalysed E2 elimination from, substituted phenethyl bromides and dimethyl(phenethyl)sulphonium bromides have been obtained. The slopes of the best-fit straight lines were similar in all cases for the β-methylene protons but were found to vary with the leaving group for the α-methylene protons.

c. Internal rotation of the nitroso group in p-substituted nitrosoanilines by the density matrix line-shape method

Density matrix theory has been applied to study the barrier to internal rotation about the Ar-NO bond in N,N-dimethyl-p-nitrosoaniline and N,N-diethyl-p-nitrosoaniline. Theoretical fits to experimental n.m.r. spectra have been obtained in the range - 30°C to +40°C. Activation parameters have been determined and compared with those obtained by more approximate methods.

The self-association of Pyrrolid-2-one in acetonitrile and chloroform: An N.M.R. spectroscopic study

The self-association of pyrrolid-2-one has been studied in CDCl3 and CD3CN at several temperatures by the use of high resolution nuclear magnetic resonance spectroscopy. The enthalpies of self-association of the lactam in the two solvents have been measured and are in reasonable agreement with those for similar systems determined by the use of other methods. The enthalpy of salvation of pyrrolid-2-one in CHCl3 has also been measured. A discussion on the validity of the approximations used in the calculations of the thermodynamic parameters has been included.