VIII. On the movements of the flame in the explosion of gases

(1.) On the Rate of Movement of the Flam, and the produced in theExplosion of Gases. Humphry Davy was the first to observe the rate at which an explosion of gases was propagated in a tube, and he also made the first rough experiment on the tem­perature reached in an explosion. When gas from the distillation of coal (which he found more inflammable than fire-damp) was mixed with eight times its volume ofair, and was fired in a glass tube 1 foot long and 1/4 inch in diameter, the flame took more than a second to traverse the tube. When cyanogen mixed with twice its volume of oxygen was fired in a bent tube over water, the quantity of water displaced showed that the gases had expanded fifteen times their original bulk. Bunsen, in 1867, made the first careful measurement of the rate at which an explosion is propagated in gases, and he also made the first systematic researches on the pressure and temperature produced by the explosion of gases in closed vessels. His results led him to the remarkable conclusion that there was a discontinuous combustion in explosions. When electrolytic gas, or when carbonic oxide with haltits volume of oxygen, is fired, only one-third of the mixture is burnt, according to Bunsen, raising the temperature of the whole to about 3000° C. No further chemical action then occurs until the gaseous mixture falls, by cooling, below 2500° C. Then a further combustion begins, and so on<italic>per Saltum</italic>. These deductions were criticised by Berthelot, who pointed out that they assumed the constancy of the specific heats of steam and of carbonic acid at high temperatures.

A chromatographic preparation of ox growth hormone

1. A method is described for the chromatographic preparation of ox growth hormone. It involves chromatography of an extract of anterior pituitary lobes on DEAE-cellulose, followed by rechromatography on a dextran gel of low cross-linkage (Sephadex G-100). 2. The product is highly active in growth-hormone assays, and is obtained in good yield. It was homogeneous by several criteria, but showed some heterogeneity on starch-gel electrophoresis. 3. The molecular weight of the hormone was estimated from its behaviour on gel-filtration columns under various conditions. Evidence that the hormone may dissociate into sub-units under some conditions is presented.

The effect of Mg2+ on cardiac muscle function: Is CaATP the substrate for priming myofibril cross-bridge formation and Ca2+ reuptake by the sarcoplasmic reticulum?

Kinetics are established for the activation of the myofibril from the relaxed state [Smith, Dixon, Kirschenlohr, Grace, Metcalfe and Vandenberg (2000) Biochem. J. 346, 393-402]. These require two troponin Ca2+-binding sites, one for each myosin head, to act as a single unit in initial cross-bridge formation. This defines the first, or activating, ATPase reaction, as distinct from the further activity of the enzyme that continues when a cross-bridge to actin is already established. The pairing of myosin heads to act as one unit suggests a possible alternating mechanism for muscle action. A large positive inotropic (contraction-intensifying) effect of loading the Mg2+ chelator citrate, via its acetoxymethyl ester, into the heart has confirmed the competitive inhibition of the Ca2+ activation by Mg2+, previously seen in vitro. In the absence of a recognized second Ca2+ binding site on the myofibril, with appropriate binding properties, the bound ATP is proposed as the second activating Ca2+-binding site. As ATP, free or bound to protein, can bind either Mg2+ or Ca2+, this leads to competitive inhibition by Mg2+. Published physico-chemical studies on skeletal muscle have shown that CaATP is potentially a more effective substrate than MgATP for cross-bridge formation. The above considerations allow calculation of the observed variation of fractional activation by Ca2+ as a function of [Mg2+] and in turn reveal simple Michaelis-Menten kinetics for the activation of the ATPase by sub-millimolar [Mg2+]. Furthermore the ability of bound ATP to bind either cation, and the much better promotion of cross-bridge formation by CaATP binding, give rise to the observed variation of the Hill coefficient for Ca2+ activation with altered [Mg2+]. The inclusion of CaADP within the initiating cross-bridge and replacement by MgADP during the second cycle is consistent with the observed fall in the rate of the myofibril ATPase that occurs after two phosphates are released. The similarity of the kinetics of the cardiac sarcoplasmic reticulum ATPase to those of the myofibril, in particular the positive co-operativity of both Mg2+ inhibition and Ca2+ activation, leads to the conclusion that this ATPase also has an initiation step that utilizes CaATP. The first-order activation by sub-millimolar [Mg2+], similar to that of the myofibril, may be explained by Mg2+ involvement in the phosphate-release step of the ATPase. The inhibition of both the myofibril and sarcoplasmic reticulum Ca2+ transporting ATPases by Mg2+ offers an explanation for the specific requirement for phosphocreatine (PCr) for full activity of both enzymes in situ and its effect on their apparent affinities for ATP. This explanation is based on the slow diffusion of Mg2+ within the myofibril and on the contrast of PCr with both ATP and phosphoenolpyruvate, in that PCr does not bind Mg2+ under physiological conditions, whereas both the other two bind it more tightly than the products of their hydrolysis do. The switch to supply of energy by diffusion of MgATP into the myofibril when depletion of PCr raises [ATP]/[PCr] greatly, e.g. during anoxia, results in a local [Mg2+] increase, which inhibits the ATPase. It is possible that mechanisms similar to those described above occur in skeletal muscle but the Ca2+ co-operativity involved would be masked by the presence of two Ca2+ binding sites on each troponin.

On the role of the N-terminal group in the allosteric function of glucosamine-6-phosphate deaminase from Escherichia coli

Glucosamine-6-phosphate deaminase (EC 3.5.99.6) from Escherichia coli is an allosteric enzyme of the K-type, activated by N-acetylglucosamine 6-phosphate. It is a homohexamer and has six allosteric sites located in clefts between the subunits. The amino acid side-chains in the allosteric site involved in phosphate binding are Arg158, Lys160 and Ser151 from one subunit and the N-terminal amino group from the facing polypeptide chain. To study the functional role of the terminal amino group, we utilized a specific non-enzymic transamination reaction, and we further reduced the product with borohydride, to obtain the corresponding enzyme with a terminal hydroxy group. Several experimental controls were performed to assess the procedure, including reconditioning of the enzyme samples by refolding chromatography. Allosteric activation by N-acetylglucosamine 6-phosphate became of the K-V mixed type in the transaminated protein. Its kinetic study suggests that the allosteric equilibrium for this modified enzyme is displaced to the R state, with the consequent loss of co-operativity. The deaminase with a terminal hydroxy acid, obtained by reducing the transaminated enzyme, showed significant recovery of the catalytic activity and its allosteric activation pattern became similar to that found for the unmodified enzyme. It had lost, however, the pH-dependence of homotropic co-operativity shown by the unmodified deaminase in the pH range 6-8. These results show that the terminal amino group plays a part in the co-operativity of the enzyme and, more importantly, indicate that the loss of this co- operativity at low pH is due to the hydronation of this amino group.

Ca2+ buffering in the heart: Ca2+ binding to and activation of cardiac myofibrils

The measurement of cardiac Ca(2+) transients using spectroscopic Ca(2+) indicators is significantly affected by the buffering properties of the indicators. The aim of the present study was to construct a model of cardiac Ca(2+) buffering that satisfied the kinetic constraints imposed by the maximum attainable rates of cardiac contraction and relaxation on the Ca(2+) dissociation rate constants and which would account for the observed effects of (19)F-NMR indicators on the cardiac Ca(2+) transient in the Langendorff-perfused ferret heart. It is generally assumed that the Ca(2+) dependency of myofibril activation in cardiac myocytes is mediated by a single Ca(2+)-binding site on troponin C. A model based on 1:1 Ca(2+) binding to the myofilaments, however, was unable to reproduce our experimental data, but a model in which we assumed ATP-dependent co-operative Ca(2+) binding to the myofilaments was able to reproduce these data. This model was used to calculate the concentration and dissociation constant of the ATP-independent myofilament Ca(2+) binding, giving 58 and 2.0 microM respectively. In addition to reproducing our experimental data on the concentration of free Ca(2+) ions in the cytoplasm ([Ca(2+)](i)), the resulting Ca(2+) and ATP affinities given by fitting of the model also provided good predictions of the Ca(2+) dependence of the myofibrillar ATPase activity measured under in vitro conditions. Solutions to the model also indicate that the Ca(2+) mobilized during each beat remains unchanged in the presence of the additional buffering load from Ca(2+) indicators. The new model was used to estimate the extent of perturbation of the Ca(2+) transient caused by different concentrations of indicators. As little as 10 microM of a Ca(2+) indicator with a dissociation constant of 200 nM will cause a 20% reduction in peak-systolic [Ca(2+)](i) and 30 microM will cause approx. 50% reduction in the peak-systolic [Ca(2+)](i) in a heart paced at 1.0 Hz.

Potentiation of mitochondrial Ca2+ sequestration by taurine

The effects of taurine (2-aminoethanesulphonic acid) and its analogues, 2-aminoethylarsonic acid, 2-hydroxyethanesulphonic (isethionic) acid, 3-aminopropanesulphonic acid, 2-aminoethylphosphonic acid, and N,N-dimethyltaurine, were studied on the transport of Ca2+ by mitochondria isolated from rat liver. Taurine enhanced Ca2+ uptake in an apparently saturable process, with a Km value of about 2.63 mM. Taurine behaved as an uncompetitive activator of Ca2+ uptake, increasing both the apparent Km and Vmax values of the process. This effect was not modified in the presence of cyclosporin A (CsA). N,N-Dimethyltaurine also stimulated Ca2+ uptake at higher concentrations, but there was no evidence that the process was saturable over the concentration range used (1-10 mM). Aminoethylarsonate was a weak inhibitor of basal Ca2+ uptake, but inhibited that stimulated by taurine in an apparently competitive fashion (Ki = 0.05 mM). The other analogues had no significant effects on this process. Taurine either in the presence or the absence of CsA had no effect on Ca2+ release induced by 200 nM ruthenium red. Thus, the mechanism of taurine-enhanced Ca2+ accumulation appears to involve stimulation of Ca2+ uptake via the uniport system rather than inhibition of Ca2+ release via the ion (Na+/Ca2+ and/or H+/Ca2+) exchangers or by taurine modulating the permeability transition of the mitochondrial inner membrane. Overall, these findings indicate an interaction of taurine with an as yet unidentified mitochondrial site which might regulate the activity of the uniporter. The unique role of taurine in modulating mitochondrial Ca2+ homeostasis might be of particular importance under pathological conditions that are characterised by cell Ca2+ overload, such as ischaemia and oxidative stress.

Spontaneous alpha-N-6-phosphogluconoylation of a "His tag" in Escherichia coli: the cause of extra mass of 258 or 178 Da in fusion proteins

Several proteins expressed in Escherichia coli with the N-terminus Gly-Ser-Ser-[His]6- consisted partly (up to 20%) of material with 178 Da of excess mass, sometimes accompanied by a smaller fraction with an excess 258 Da. The preponderance of unmodified material excluded mutation, and the extra masses were attributed to posttranslational modifications. As both types of modified protein were N-terminally blocked, the alpha-amino group was modified in each case. Phosphatase treatment converted +258-Da protein into +178-Da protein. The modified His tags were isolated, and the mass of the +178-Da modification estimated as 178.06 +/- 0.02 Da by tandem mass spectrometry. As the main modification remained at +178 Da in 15N-substituted protein, it was deemed nitrogen-free and possibly carbohydrate-like. Limited periodate oxidations suggested that the +258-Da modification was acylation with a 6-phosphohexonic acid, and that the +178-Da modification resulted from its dephosphorylation. NMR spectra of cell-derived +178-Da His tag and synthetic alpha-N-d-gluconoyl-His tag were identical. Together, these results suggested that the +258-Da modification was addition of a 6-phosphogluconoyl group. A plausible mechanism was acylation by 6-phosphoglucono-1,5-lactone, produced from glucose 6-phosphate by glucose-6-phosphate dehydrogenase (EC 1.1.1.49). Supporting this, treating a His-tagged protein with excess d-glucono-1,5-lactone gave only N-terminal gluconoylation.

Improved conditions for the removal of 2-oxoacyl groups from the N-terminus of proteins

Proteins with R-CO-CO-NH- at the N-terminus, rather than the usual R-CH(-NH3+)-CO-NH-, are produced by non-enzymic transamination and also occur in the pyruvoyl enzymes. The oxoacyl group may be specifically removed from a model peptide, in yields of 70-80%, by treating them in 0.1 M phosphate buffer at 37 degreesC for 24 h with 25 mM of the N-phosphonomethyl derivative of phenylene-1,2-diamine. This provides mild conditions for the stepwise removal of N-terminal residues without denaturation.

Conversion of taurine into N-chlorotaurine (taurine chloramine) and sulphoacetaldehyde in response to oxidative stress

N-Chlorotaurine (taurine chloramine), formed by treating taurine with hypochlorous acid, was shown to decompose to sulphoacetaldehyde with a first-order rate constant of 9.9+/-0.5 x 10(-4).h-1 at 37 degrees C in 0.1 M phosphate buffer, pH 7.4. Rat liver homogenates accelerated this decay in a process that was proportional to tissue-protein concentration and saturable, with maximum velocity (Vmax) and Km values of 0.28+/-0.01 nmol/min per mg of protein and 37+/-9 microM respectively. This activity was found to be lost on heat denaturation, but retained after dialysis. There was no detectable formation of sulphoacetaldehyde when taurine itself was incubated with the tissue homogenates under the same conditions. Activation of human neutrophils (1.67 x 10(6) cells/ml) with latex beads resulted in a respiratory burst of oxygen-radical production, the products of which were partially sequestered by 12.5 mM taurine. Under these conditions sulphoacetaldehyde was generated at a constant rate of 637+/-18 pmol/h per ml for over 7 h. A non-activated neutrophil suspension contained constant levels of 1.42+/-0.02 nmol/ml sulphoacetaldehyde, as did activated cells incubated in the absence of taurine, a basal level which may indicate a steady turnover of taurine in these cells. Such formation of chlorotaurine and its decay to the aldehyde may be the first steps in the metabolism of taurine to isethionate (2-hydroxyethanesulphonate) that has been demonstrated by various authors to occur in vivo.

[Reactions of decarboxylated and side transamination during interaction of glutamate decarboxylase from Escherichia coli with substrate analogs, modified through C3 and C4 atoms]

The interaction of glutamate decarboxylase with the aspartate and glutamate analogues modified at C3 and C4 was studied. 3-Arsonoalanine, 3-phosphonoalanine, 2-amino-4-arsonobutyric acid, 2-amino-4-phosphonobutyric acid, a mixture of diastereoisomers of 4-(methylthio) glutamic acid and erythro-4-(methylthio) glutamic acid were shown to be poor substrates for the enzyme. Their decarboxylation was accompanied by transamination of the coenzyme (PLP) to pyridoxamine phosphate (PMP) which reversibly inactivated the enzyme. With arsonoalanine only part of PLP was converted into PMP and another part irreversibly formed a complex. 4-(Methylsulfonyl)-L-glutamic and 4-[(phenyl)(hydroxy)phosphoryl]-L-glutamic acids did not react with the glutamate decarboxylase.

The transport of acidic amino acids and their analogues across monolayers of human intestinal absorptive (Caco-2) cells in vitro

The X-AG system, a sodium-dependent, acidic amino-acid transport system has been implicated in the transport of L-aspartate and L-glutamate across monolayers of human Caco-2 cells, an in vitro model of intestinal absorption. This system, which shares many properties with the L-glutamate carrier present in the human jejunum, is highly saturable (> 95% at 50 microM), vectorial (apical-to-basolateral >> basolateral-to-apical) and sodium-, pH- and temperature-dependent. L-Aspartate was also transported against a 10-fold reverse concentration gradient. These data are consistent with a major (saturable) carrier-mediated pathway superimposed onto a minor non-saturable (diffusional) pathway. The carrier has an absolute sodium-dependence and the Michaelis constants for the sodium-dependent transport component (Km) for L-aspartate and L-glutamate were 56 +/- 3 microM and 65 +/- 6 microM, respectively. Cross-inhibition studies showed that strong interaction with the carrier was limited to close analogues of the natural substrates. Potent inhibitors included L-aspartate, D-aspartate (Ki, 70 microM), L-glutamate (Ki 180 microM) and threo-beta-hydroxy-DL-aspartate (Ki, 55 microM), while partial inhibitors included alpha-methyl-DL-aspartate, D-glutamate, L-asparagine, L-proline and L-alanine. Replacement of the side-chain -COO- group (aspartate) with -SO-3 (L-cysteate, Ki, 65 microM) or -(H)P(O)O- (DL-3-(hydroxyphosphoryl)alanine, Ki, 60 microM) maintained strong interaction with the carrier while -As(O)(OH)O- (DL-3-arsonoalanine, Ki, 1100 microM) and -P(O)(OH)O- (DL-3-phosphonoalanine, Ki, 3270 microM) were much more weakly bound, with the larger, but probably less ionised, arsono analogue being more tightly bound than the phosphono compound. The corresponding analogues of glutamate (homologous extension of the methylene chain) showed negligible interaction. We conclude that Caco-2 monolayers are a relevant experimental model for the study of the transport of acidic amino acids and their analogues in man.

The removal of 2-oxoacyl residues from the N-terminus of peptides and cystatin in non-denaturing conditions

The N-terminal residue of a protein or peptide may be converted into a 2-oxoacyl group by non-enzymic transamination. This group may then be removed, to obtain the peptide chain shortened by one residue, by treatment with phenylene-1,2-diamine. Hitherto this scission has required a pH of 4-5, but we find that the reaction will proceed well at pH 7 in the presence of concentrated phosphate buffer. We describe a method using reverse-phase HPLC for determining the extent of scission in model peptides; this method also allows products to be isolated and identified. The new scission conditions have been tested by removing the N-terminal residue from cystatin, an inhibitor of cysteine peptidases; electrospray mass spectrometry was used to assess how this protein reacted.

Arsenite release on enzymic transformation of arsonomethyl substrate analogues: a potentially lethal synthesis by glycerol-3-phosphate dehydrogenase

The isosteric arsenical analogue of glycerol 3-phosphate, 3,4-dihydroxybutylarsonic acid, is a good substrate for rabbit muscle glycerol-3-phosphate dehydrogenase. Its oxidation is accompanied by release of arsenite. This release seems to be due to a spontaneous elimination of arsenite by 3-oxoalkylarsonic acids, as it is also observed in (1) the oxidation of 3-hydroxypropylarsonic acid by yeast alcohol dehydrogenase, (2) treatment of 3,4-dihydroxybutylarsonic acid with periodate and (3) nonenzymic transamination of the glutamate analogue 2-amino-4-arsonobutyric acid. Enzymic formation of 3-oxoalkylarsonic acids in cells can therefore be lethal, as arsenite is poisonous to most organisms because of its high affinity for dithiols such as dihydrolipoyl groups.

Sulphate-activated phosphorylase b: the pH-dependence of catalytic activity

The pH-dependence of sulphate-activated phosphorylase b has been studied in the direction of glycogen synthesis. The bell-shaped curve of the pH-dependence of the catalytic constant for the AMP-activated enzyme showed pK values of 6.1 and 7.3, but the curve for the enzyme activated by 0.9 M ammonium sulphate showed a drop of activity on the acid side at much higher pH values. Its bell was centred at pH 7.8 but it was too narrow to be characterized by only two pK values. The narrowness of the curve could be explained by positive co-operativity, but not its unusually steep acid side. We suggest that the fall on the acid side is due to more than one hydronation (addition of H+). The points can be fitted by a curve with two de-activating hydronations and a de-activating dehydronation having identical titration pK values of 7.5, and hence molecular values of 7.0, 7.5 and 8.0. If both 0.9 M ammonium sulphate and 5 mM AMP are added, the bell is as broad as with AMP alone, but is somewhat raised in pH optimum. The results are discussed in the light of new structural data from crystallographic studies on binary complexes of the enzyme.

Synthesis of 3-arsonopyruvate and its interaction with phosphoenolpyruvate mutase

3-Arsonopyruvate was prepared in four steps from glycine. The arsenic-carbon bond was formed by a Meyer reaction between alkaline arsenite and 2-bromo-3-hydroxy-2-(hydroxymethyl)propionic acid; the 3-arsono-2-hydroxy-2-(hydroxymethyl) propionic acid formed was oxidized with periodate to give 3-arsonopyruvate. This proves to be an alternative substrate for phosphoenolpyruvate mutase, giving pyruvate, which was assayed using lactate dehydrogenase. The K(m) is 20 microM, similar to that observed for the natural substrate phosphonopyruvate (17 microM), whereas the kcat. of 0.01 s-1 was much lower than that for phosphonopyruvate (58 s-1). Arsonopyruvate competitively inhibited the action of the mutase on phosphonopyruvate.

The interaction of glutamate decarboxylase from Escherichia coli with substrate analogues modified at C-3 and C-4

The interaction of glutamate decarboxylase with the aspartate analogues 3-arsonoalanine and 3-phosphonoalanine, with the glutamate analogues 2-amino-4-arsonobutyric acid and 2-amino-4-phosphonobutyric acid, and with 4-(methylthio)-L-glutamic acid, both as a mixture of diastereoisomers and as the (2S,4R)-form, was studied. All these analogues were poor substrates for the enzyme and only weak inhibitors. Their decarboxylation was accompanied by transamination of the enzyme-bound pyridoxal phosphate (PLP) to pyridoxamine phosphate (PMP), thus inactivating the decarboxylase. With arsonoalanine only part of the PLP was converted into PMP.

Enolase and the arsonomethyl analogue of 2-phosphoglycerate

(RS)-3-Arsono-2-(hydroxymethyl)propionic acid was synthesized by the action of alkaline arsenite on 3-bromo-2-(bromomethyl)propionic acid. It is a substrate for yeast enolase (EC 4.2.1.11) with a Km of 6.5 mM (for 2-phospho-D-glycerate Km = 0.08 mM). The catalytic constant of the enzyme with the arsonomethyl analogue is 230 times lower than with 2-phosphoglycerate.

Synthesis of 3-arsonoalanine and its action on aspartate aminotransferase and aspartate ammonia-lyase. Comparison with arsenical analogues of malate and fumarate

DL-3-Arsonoalanine has been synthesized by the Strecker synthesis from the unstable compound arsonoacetaldehyde. It inactivates pig heart cytosolic aspartate aminotransferase and inhibits aspartate ammonia-lyase by competing with aspartate (Ki/Km 0.23). The fumarate analogue (E)-3-arsonoacrylic acid and the malate analogue (RS)-3-arsonolactate also inhibit fumarate hydratase, competing with fumarate (Ki/Km 1.8) and malate (Ki/Km 1.6) respectively. Attempted non-enzymic transamination of 3-arsonoalanine gave elimination of arsenite, in contrast with the transamination of 3-phosphonoalanine, which is either successful or leads to loss of phosphate.

A rapid spectrophotometric estimation of individual phosphates and phosphonates: its application to chromatography of sugar phosphates and their phosphonate analogues

A compound containing the -PO3H2 group (phosphoric acid, one of its monoesters, or an alkylphosphonic acid) may be rapidly assayed by the decrease it produces in the absorbance at 450 nm of a buffered acidic solution of Fe3+ and N(-3) [corrected]. The method has been used to follow chromatograms of sugar phosphates and their phosphonomethyl analogues.

Pyridoxal arsenate as a prosthetic group for aspartate aminotransferase

The apoenzyme of aspartate aminotransferase formed a stable, active holoenzyme on treatment with pyridoxal in the presence of arsenate.

Utilization of 2-aminoethylarsonic acid in Pseudomonas aeruginosa

This paper describes the metabolism, transport and growth inhibition effects of 2-aminoethylarsonic acid (AEA) and 3-aminopropylarsonic acid (APrA). The former compound supported growth of Pseudomonas aeruginosa, as sole nitrogen source. The two arsonates inhibited the growth of this bacterium when 2-aminoethylphosphonic acid (AEP) but not alanine or NH4Cl, was supplied as the only other nitrogen source. The analogy between AEA and the natural compound AEP led us to examine the in vitro and in vivo interaction of AEA with the enzymes of AEP metabolism. The uptake system for AEP (Km 6 microM) was found to be competitively inhibited by AEA and APrA (Ki 18 microM for each). AEP-aminotransferase was found to act on AEA with a Km of 4 mM (3.85 mM for AEP). Alanine and 2-arsonoacetaldehyde was generated concomitantly, in a stoichiometric reaction. In vivo, AEA was catabolized by the AEP-aminotransferase since it was able to first induce this enzyme, then to be an efficient substrate. The lower growth observed may have been due to the slowness with which the permease and the aminotransferase were induced, and hence to a poor supply of alanine by transamination.

The origin of multiply sigmoid curves of pH-dependence. The partitioning of groups among titration pK values

An acid, HnA, with n ionizing groups is known to have the same titration curve as an equimolar mixture of n hypothetical monobasic acids, whose dissociation constants are known as the 'titration constants' of the real acid. We show that the pH-dependence of any property of HnA is also represented by the sum of one-site titration curves, characterized by these same titration constants. Since one such property is the degree of dissociation of one of the dissociating groups, a fraction of each group shows each of the various titration pK values, so that the group partitions among them. The n groups therefore share the same n titration pK values but differ in the fractions belonging to each. The one H+ ion per molecule that titrates with each pK is thus made up of the fractions, one from each group, that share this pK value. A group may possess a single pK value, in that it contributes virtually all of this pK and almost nothing to the others, only if either (1) in titrates in a different pH range from the other groups or (2) its affinity for H+ is unaffected by their ionization state.

Effect of magnesium ions on the inhibition of S-adenosylmethionine decarboxylase from Escherichia coli by [2-(amino-oxy)ethyl](5'-deoxyadenosin-5'-yl)(methyl)sulphonium

[2-(Amino-oxy)ethyl](5'-deoxyadenosin-5'-yl)(methyl)sulphonium+ ++, the amino-oxy analogue of decarboxylated S-adenosylmethionine, is a potent irreversible inhibitor of Escherichia coli S-adenosylmethionine decarboxylase [Khomutov, Zavalova, Syrku, Artamonova & Khomutov (1983) Bioorg. Khim. 9, 130-131; Artamonova, Zavalova, Khomutov & Khomutov (1986) Bioorg. Khim. 12, 206-212]. We have shown that Mg2+ ions are required for the irreversible inhibition of the decarboxylase, and that S-adenosylmethionine protects against this inhibition.

The preparation and properties of bromoacetylphosphonic acid

Bromoacetylphosphonic acid, Br-CH2-CO-PO3H2, was made by brominating dimethyl acetylphosphonate and de-esterifying with HBr. It proves to be a powerful alkylating agent, reacting rapidly with GSH, with a rate constant of about 6M(-1).s(-1) at pH6.

A synthesis of acylphosphonic acids and of 1-aminoalkylphosphonic acids: the action of pyruvate dehydrogenase and lactate dehydrogenase on acetylphosphonic acid

Acylphosphonic acids, R-CO-PO(OH)2, have been synthesized by the steps [formula: see text] of which the last is new and provides a mild method for de-esterifying acylphosphonic acids. Their reductive amination gives a simple way of making 1-aminoalkylphosphonic acids. Acetylphosphonic acid inhibited NAD+ reduction by pyruvate with the pyruvate dehydrogenases from Escherichia coli and Bacillus stearothermophilus. The inhibition was competitive with pyruvate, with Ki of 6 microM for the E. coli enzyme (pyruvate Km 0.5 mM) and one of 0.4 mM of the B. stearothermophilus enzyme (pyruvate Km 0.1 mM). Acetylphosphonate and its monomethyl ester are substates for pig heart lactate dehydrogenase, with Km values of 15 mM and 10 mM respectively (pyruvate Km 0.05 mM) and specificity constants one thousandth that for pyruvate.

The synthesis of 3-phosphonoalanine, phosphonopyruvic acid and phosphonolactic acid. Scission of the C-P bond during diazotization of phosphonoalanine

3-Phosphonoalanine has been made by the Strecker synthesis from phosphonoacetaldehyde, which is easily prepared from vinyl acetate. It gives phosphonopyruvate by transamination when treated with glyoxylate. Phosphonolactate, an analogue of phosphoglycerate, is prepared by reducing phosphonopyruvate. Diazotization of phosphonoalanine was investigated as a route for making phosphonolactate: addition of NaNO2 to the isoelectric form of phosphonoalanine gave much scission of the C-P bond with release of phosphate; addition of HBr prevented this release and gave largely the bromo acid. The supplement reports the synthesis of arsonolactate, a similar analogue, by treating chlorolactate with alkaline arsenite.

Application of the synthetic method to other amines

Taurine and 2-aminoethylphosphonic acid were synthesized by the method of the main paper [Geoghegan & Dixon (1989) Biochem. J. 260, 295-296], i.e. by treating the corresponding halo compound with 2-aminoethanol and then with periodate.

Synthesis of 2-aminoethylarsonic acid. A new synthesis of primary

2-Aminoethylarsonic acid was prepared from 2-choloethylarsonic acid. The route constitutes a new procedure for making primary amines from haloalkanes; chloride was displaced by treatment with 2-aminoethanol at 70 degrees C, and the product was converted into the required primary amine by treatment with periodate.

2-Aminoethylarsonic acid as an analogue of ethanolamine phosphate. Endowment of ethanolamine-phosphate cytidylyltransferase with CTP pyrophosphatase activity

2-Aminoethylarsonic acid was tested for its ability to act as a substrate for ethanolamine-phosphate cytidylytransferase as a cytidylyl acceptor in place of ethanolamine phosphate. The expected product, like all mixed anhydrides of arsonic acids, should hydrolyse spontaneously with regeneration of the substrate analogue and CMP formation; such CMP production was observed. The limiting velocity with aminoethylarsonic acid is about 90% that with ethanolamine phosphate, and the Michaelis constant is below 20 mM.

Relations between the dissociation constants of dibasic acids

The relationships between the molecular pK values of a dibasic acid, its titration pK values and the pH values at which the concentration of monohydronated species is half-maximal are presented. These enable any pair of the values to be found from any other pair.

pH-activity curves for enzyme-catalysed reactions in which the hydron is a product or reactant

A reaction in which the hydron [International Union of Pure and Applied Chemistry (1986) Chem. Int. 8 (4), 21] is a reactant will have a more acidic pH optimum than the reverse reaction in which it is a product. Simple mechanistic models, made by postulating various ways in which the hydron is supplied for the forward reaction, lead to the prediction that the pK that characterizes the fall in activity on the alkaline side for the forward reaction will be equal to that characterizing the fall on the acid side for the reverse reaction. It is not possible to identify what is responsible for this pK without additional information; as is well known, different possible dispositions of hydrons in transition states cannot usually be distinguished solely by kinetics.

The phosphonomethyl analogue of 3-phosphoglycerate is a potent competitive inhibitor of phosphoglycerate mutases

The phosphonomethyl analogue of 3-phosphoglycerate (2-hydroxy-4-phosphonobutanoate) is a potent competitive inhibitor of cofactor-dependent phosphoglycerate mutase from yeast and of cofactor-independent phosphoglycerate mutase from wheat germ. For the yeast enzyme Ki is 1.3 mM (Km for substrate is 0.71 mM); for the wheatgerm enzyme Ki is 18 mM (Km for substrate is 0.86 mM). This analogue should be a useful tool for n.m.r. spectroscopic studies on the mechanism of action of the two mutases. The arsonomethyl analogue of 3-phosphoglycerate (4-arsono-2-hydroxybutanoate) was a relatively poor inhibitor.

The mechanism of pyrophosphorolysis of RNA by RNA polymerase. Endowment of RNA polymerase with artificial exonuclease activity

DNA-directed RNA polymerase from Escherichia coli can break down RNA by catalysing the reverse of the reaction: NTP + (RNA)n = (RNA)n+1 + PPi where n indicates the number of nucleotide residues in the RNA molecule, to yield nucleoside triphosphates. This reaction requires the ternary complex of the polymerase with template DNA and the RNA that it has synthesized. It is now shown that methylenebis(arsonic acid) [CH2(AsO3H2)2], arsonomethylphosphonic acid (H2O3As-CH2-PO3H2) and arsonoacetic acid (H2O3As-CH2-CO2H) can replace pyrophosphate in this reaction. When they do so, the low-Mr products of the reaction prove to be nucleoside 5'-phosphates, so that the arsenical compounds endow the polymerase with an artificial exonuclease activity, an effect previously found by Rozovskaya, Chenchik, Tarusova, Bibilashvili & Khomutov [(1981) Mol. Biol. (Moscow) 15, 636-652] for phosphonoacetic acid (H2O3P-CH2-CO2H). This is explained by instability of the analogues of nucleoside triphosphates believed to be the initial products. Specificity of recognition of pyrophosphate is discussed in terms of the sites, beta and gamma, for the -PO3H2 groups of pyrophosphate that will yield P-beta and P-gamma of the nascent nucleoside triphosphate. Site gamma can accept -AsO3H2 in place of -PO3H2, but less well; site beta can accept both, and also -CO2H. We suggest that partial transfer of an Mg2+ ion from the attacking pyrophosphate to the phosphate of the internucleotide bond of the RNA may increase the nucleophilic reactivity of the pyrophosphate and the electrophilicity of the diester, so that the reaction is assisted.

The arsonomethyl analogue of adenosine 5'-phosphate. An uncoupler of adenylate kinase

Adenosine was converted into the arsonomethyl analogue of AMP. The reactions used provide a general route for converting an alcohol, R-CH2-OH, into the arsonomethyl analogue, R-CH2-CH2-AsO3H2, of its phosphate, R-CH2-O-PO3H2. The analogue of AMP proves to be a substrate for rabbit adenylate kinase, which shows a limiting velocity with it of 1/17 that with AMP, a Michaelis constant raised 70-fold to about 10 mM, and hence a specificity constant lowered about 1200-fold. The product of transfer of a phospho group from ATP to the analogue is, like all anhydrides of arsonic acids, unstable to hydrolysis, and so breaks down to yield orthophosphate and regenerate the analogue. Hence adenylate kinase is converted into an ATPase by the presence of the analogue.

The arsonomethyl analogue of 3-phosphoglycerate

4-Arsono-2-hydroxybutanoic acid, the analogue of 3-phosphoglycerate in which -CH2-AsO3H2 replaces -O-PO3H2, was synthesized. It proved to be a substrate for phosphoglycerate kinase. Its Michaelis constant was only slightly higher than that of the natural substrate, but its catalytic constant was about 1300 times smaller.

The adverse consequences of heavy Trichuris infection

Sixty-seven children with heavy Trichuris trichiura infection (Group A) were compared to 73 control children of similar socio-economic status but with light or no T. trichiura (Group B), for nutritional status, rates of concomitant bacterial and protozoal, and symptoms and clinical signs associated with heavy T. trichiura infection. Anoscopy was used to determine heavy T. trichiura infection. Measurements and physical examination were done on Group A on admission to and discharge from hospital; 46% were seen on follow-up visit two to eight months later. Children in Group B were seen only once. There were significant differences for nutritional status (p less than 0.01) and rates of bacterial and protozoal co-infection (p less than 0.01) and a significantly greater rate of invasive amoebiasis in Group A. After treatment, nutritional parameters of Group A children improved significantly, symptoms and clinical signs decreased and there was also a significant decrease in the rate of concomitant bacterial and protozoal infection. Subgroups of Group A children, with and without concomitant enteropathogens, revealed that infection with Entamoeba histolytica and bacterial enteropathogens had had no significant impact on clinical and nutritional status.

Alternative reducing agents for reductive methylation of amino groups in proteins

Dimethylamine borane and trimethylamine borane were compared with sodium cyanoborohydride as reducing agents for the reductive methylation of amino groups in proteins. Dimethylamine borane performed nearly as well as cyanoborohydride; either reducing agent (15 mM) with 20 mM formaldehyde gave extensive methylation of turkey ovomucoid (5 mg/ml) in 2 h at 22 degrees. Trimethylamine borane gave equivalent levels of methylation only with a higher concentration of formaldehyde, suggesting that it is an even milder reducing agent than the other two under these conditions. With all three reducing agents, the pH optimum for methylation covered a range of pH 6-9. Methylations should be performed at the lowest possible pH to avoid side reactions of formaldehyde with the protein. Possible effects of these reducing agents on the disulfide bonds of proteins were studied. No reduction of disulfides of turkey ovomucoid was observed using each of the three reducing agents under conditions for methylation.

Derivation of molecular pK values from pH-dependences

If the titration pK values of a dibasic acid are called pK +/- logp, then its molecular pK values are pK +/- log(p+1/p). If the pH values at which the concentration of its monoprotonated form is half-maximal are called pK +/- logq, then its molecular pK values are pK +/- log(q-4+1/q).

Phosphonate analogues of aminoacyl adenylates

Phosphonomethyl analogues of glycyl phosphate and valyl phosphate, i.e. NH2-CHR-CO-CH2-PO(OH)2, were synthesized and esterified with adenosine to give analogues of aminoacyl adenylates. The interaction of these adenylate analogues with valyl-tRNA synthetase from Escherichia coli was studied by fluorescence titration. The analogue of valyl phosphate has an affinity for the enzyme comparable with that of valine, but that of valyl adenylate is bound much less tightly than either valyl adenylate or corresponding derivative of valinol. The affinity of the analogue of glycyl adenylate was too low to be measured. We conclude that this enzyme interacts specifically with both the side chain and the anhydride linkage of the adenylate intermediate.

Removal by transamination and scission of residues from the peptide representing the copper-transport site of serum albumin

The peptide Asp-Ala-His-NH-Me was subjected to removal of its N-terminal residue by transamination and scission. Despite the high affinity of the peptide for Cu2+ ions, they catalysed its transamination smoothly. Two main transamination products were found, a complication previously observed with another peptide with an N-terminal aspartic residue, but their scission gave a single product, Ala-His-NH-Me. This was subjected to a further cycle of transamination and scission, and gave a single product after each step. For scission of transaminated peptides it proved unnecessary to remove them from transamination reagents provided that transamination was stopped with EDTA before adding the scission reagent.

An arsenical analogue of adenosine diphosphate

An analogue of ADP was made in which the terminal phosphono-oxy group, -O-PO(OH)2, has been replaced by the arsonomethyl group, -CH2-AsO(OH)2. This compound cannot form a stable analogue of ATP because anhydrides of arsonic acids are rapidly hydrolysed, so that any enzyme that phosphorylates ADP and accepts this analogue as a substrate should release orthophosphate in its presence. The analogue proves to be a poor substrate for 3-phosphoglycerate kinase (V/Km is diminished by a factor of 10(2)-10(3)) and a very poor substrate for pyruvate kinase (V/Km is diminished by a factor of 10(5)-10(6)). No substrate action was detected with adenyl kinase and creatine kinase.

The pH-dependence of second-order rate constants of enzyme modification may provide free-reactant pKa values

1. Reactions of enzymes with site-specific reagents may involve intermediate adsorptive complexes formed by parallel reactions in several protonic states. Accordingly, a profile of the apparent second-order rate constant for the modification reaction (Kobs., the observed rate constant under conditions where the reagent concentration is low enough for the reaction to be first-order in reagent) against pH can, in general, reflect free-reactant-state molecular pKa values only if a quasi-equilibrium condition exists around the reactive protonic state (EHR) of the adsorptive complex. 2. Usually the condition for quasi-equilibrium is expressed in terms of the rate constants around EHR: (formula: see text) i.e. k mod. less than k-2. This often cannot be assessed directly, particularly if it is not possible to determine kmod. 3. It is shown that kmod. must be much less than k-2, however, if kobs. (the pH-independent value of kobs.) less than k+2. 4. Since probable values of k+2 greater than 10(6)M-1.S-1 and since values of kobs. for many modification reactions less than 10(6)M-1.S-1, the equilibrium assumption should be valid, and kinetic study of such reactions should provide reactant-state pKa values. 5. This may not apply to catalyses, because for them the value of kcat./Km may exceed 5 X 10(5)M-1.S-1. 6. The conditions under which the formation of an intermediate complex by parallel pathways may come to quasi-equilibrium are discussed in the Appendix.

Aldolase-like imine formation in the mechanism of action of phosphonoacetaldehyde hydrolase

Phosphonoacetaldehyde hydrolase (EC 3.11.1.1), the bacterial enzyme that catalyses the reaction HCO-CH2-PO(OH)2+H2O leads to HCO-CH3+Pi, is inactivated by borohydride if either phosphonoacetaldehyde or acetaldehyde is present. This supports the suggestion that the substrate forms an imine with an amino group of the enzyme. Such imine formation would labilize the C-P bond in the same way that aldolase and related enzymes labilize C-C and C-H bonds (Scheme 1a).