Alginate as immobilization matrix and stabilizing agent in a two-phase liquid system: application in lipase-catalysed reactions

Alginate was evaluated as an immobilization matrix for enzyme-catalyzed reactions in organic solvents. In contrast to most hydrogels, calcium alginate was found to be stable in a range of organic solvents and to retain the enzyme inside the gel matrix. In hydrophobic solvents, the alginate gel (greater than 95% water) thus provided a stable, two-phase liquid system. The lipase from Candida cylindracea, after immobilization in alginate beads, catalysed esterification and transesterification in n-hexane under both batch and continuous-flow conditions. The operational stability of the lipase was markedly enhanced by alginate entrapment. In the esterification of butanoic acid with n-butanol, better results were obtained in the typical hydrophilic calcium alginate beads than in less hydrophilic matrices. The effects of substrate concentration, matrix area, and polarity of the substrate alcohols and of the organic solvent on the esterification activity were examined. The transesterification of octyl 2-bromopropanoate with ethanol was less efficient than that of ethyl 2-bromopropanoate with octanol. By using the hydrophilic alginate gel as an immobilization matrix in combination with a mobile hydrophobic phase, a two-phase liquid system was achieved with definite advantages for a continuous, enzyme-catalysed process.

Environmental regulation of alcohol metabolism in thermotolerant methylotrophic Bacillus strains

The thermotolerant methylotroph Bacillus sp. C1 possesses a novel NAD-dependent methanol dehydrogenase (MDH), with distinct structural and mechanistic properties. During growth on methanol and ethanol, MDH was responsible for the oxidation of both these substrates. MDH activity in cells grown on methanol or glucose was inversely related to the growth rate. Highest activity levels were observed in cells grown on the C1-substrates methanol and formaldehyde. The affinity of MDH for alcohol substrates and NAD, as well as Vmax, are strongly increased in the presence of a Mr 50,000 activator protein plus Mg(2+)-ions [Arfman et al. (1991) J Biol Chem 266: 3955-3960]. Under all growth conditions tested the cells contained an approximately 18-fold molar excess of (decameric) MDH over (dimeric) activator protein. Expression of hexulose-6-phosphate synthase (HPS), the key enzyme of the RuMP cycle, was probably induced by the substrate formaldehyde. Cells with high MDH and low HPS activity levels immediately accumulated (toxic) formaldehyde when exposed to a transient increase in methanol concentration. Similarly, cells with high MDH and low CoA-linked NAD-dependent acetaldehyde dehydrogenase activity levels produced acetaldehyde when subjected to a rise in ethanol concentration. Problems frequently observed in establishing cultures of methylotrophic bacilli on methanol- or ethanol-containing media are (in part) assigned to these phenomena.

Polymorphism of class I alcohol dehydrogenase in French, Vietnamese and Niger populations: genotyping by PCR amplification and RFLP analysis on dried blood spots

The polymorphism of human alcohol dehydrogenase (ADH) can contribute to the explanation of the important ethnic differences towards alcohol metabolism. Its assessment at the genomic DNA level with a procedure, excluding labelled probes, consisting of PCR (Polymerase chain reaction) amplification on dried blood spots and analysis of allele-specific RFLP (Restriction fragment length polymorphism) profiles, is well adapted to extensive studies in population samples. It can emphasize the importance of ADH as a genetic marker of population. Three ethnic groups (French Caucasians, Vietnamese Orientals, Black Africans from Niger) were studied. ADH2 and ADH3 genotypes were in equilibrium according to the Hardy-Weinberg law. Important differences were noted in the distribution of ADH2 and ADH3 alleles.

Effects of chronic alpha-adrenergic receptor blockade on peripheral nerve conduction, hypoxic resistance, polyols, Na(+)-K(+)-ATPase activity, and vascular supply in STZ-D rats

The effects of alpha-receptor blockade on nerve conduction, hypoxic resistance, ouabain-sensitive Na(+)-K(+)-ATPase, nerve polyols, and capillary density were examined in streptozocin-induced diabetic (STZ-D) rats. Nondiabetic and untreated diabetic control groups were used. Diabetes duration was 2 mo. There were two treated diabetic groups. A "prevention" group received 5 mg/kg prazosin for 2 mo from the induction of diabetes. A "reversal" group was untreated for the 1st mo and was given prazosin for the subsequent month. Conduction was measured in motor nerves supplying tibialis anterior and gastrocnemius muscles and sensory saphenous nerve. Diabetes resulted in 15-29% reductions in conduction velocity (P less than 0.01). In the prevention group, conduction deficits were minimal compared with untreated diabetes (P less than 0.01). In the reversal group, motor conduction was also substantially improved, although sensory conduction was not significantly affected. In vitro measurement of sciatic nerve hypoxic resistance revealed a 49% increase in the time taken for compound action potential amplitude to reach half its initial value with diabetes (P less than 0.01). This was largely prevented by prazosin treatment (P less than 0.01), although treatment had a lesser effect in the reversal group. Treatment had no effect on nerve polyol levels or Na(+)-K(+)-ATPase activity. Functional improvements with prazosin were probably based on increased vasa nervorum perfusion. There was a 20% elevation of endoneurial capillary density (P less than 0.01) in both prevention and reversal groups. We conclude that vascular factors play an important role in the etiology of experimental diabetic neuropathy, and functional changes may be corrected by chronic vasodilator treatment.

Accumulation of acyl-enzyme in DD-peptidase-catalysed reactions with analogues of peptide substrates

Thioester substrates can be used to study the hydrolysis and transfer reactions catalysed by beta-lactamases and DD-peptidases. With the latter enzymes, accumulation of the acyl-enzyme can be detected directly. The efficiency of various amines as acceptor substrates was in excellent agreement with previous results obtained with peptide substrates of the DD-peptidases. The results indicated the presence of a specific binding site for the acceptor substrates. Although most of the results agreed well with a simple partition model, more elaborate hypotheses will be needed to account for all the data presented.

Substrate specificity and stereoselectivity of horse liver alcohol dehydrogenase. Kinetic evaluation of binding and activation parameters controlling the catalytic cycles of unbranched, acyclic secondary alcohols and ketones as substrates of the native and active-site-specific Co(II)-substituted enzyme

1. The steady-state parameters kcat and Km and the rate constants of hydride transfer for the substrates isopropanol/acetone; (S)-2-butanol, (R)-2-butanol/2-butanone; (S)-2-pentanol, (R)-2-pentanol/2-pentanone; 3-pentanol/3-pentanone; (S)-2-octanol and (R)-2-octanol have been determined for the native Zn(II)-containing horse-liver alcohol dehydrogenase (LADH) and the specific active-site-substituted Co(II)LADH. 2. A combined evaluation of steady-state kinetic data and rate constants obtained from stopped-flow measurements, allowed the determination of all rate constants of the following ordered bi-bi mechanism: E in equilibrium E.NAD in equilibrium E.NAD.R1R2 CHOH in equilibrium E.NADH.R1R2CO in equilibrium E.NADH in equilibrium E. 3. On the basis of the different substrate specificities of LADH and yeast alcohol dehydrogenase (YADH), a procedure has been developed to evaluate the enantiomeric product composition of ketone reductions. 2-Butanone and 2-pentanone reductions revealed (S)-2-butanol (86%) and (S)-2-pentanol (95%) as the major products. 4. The observed enantioselectivity implies the existence of two productive ternary complexes; E.NADH.(pro-S) 2-butanone and E.NADH.(pro-R) 2-butanone. All rate constants describing the kinetic pathways of the system (S)-2-butanol, (R)-2-butanol/2-butanone have been determined. These data have been used to estimate the expected enantiomer product composition of 2-butanone reductions using apparent kcat/Km values for the two different ternary-complex configurations of 2-butanone. Additionally, these data have been used for computer simulations of the corresponding reaction cycles. Calculated, simulated and experimental data were found to be in good agreement. Thus, the system (S)-2-butanol, (R)-2-butanol/2-butanone is the first example of a LADH-catalyzed reaction for which the stereochemical course could be described in terms of rate constants of the underlying mechanism. 5. The effects of Co(II) substitution on the different steps of the kinetic pathway have been investigated. The free energy of activation is higher for alcohol oxidation and lower for ketone reduction when catalyzed by Co(II)LADH in comparison to Zn(II)LADH. However, the free energies of binding are affected by metal substitution in such a way that the enantioselectivity of ketone reduction is not significantly changed by the substitution of Co(II) for Zn(II). 6. Evaluation of the data shows that substrate specificity and stereoselectivity result from combination of the free energies of binding and activation, with differences in binding energies as the dominating factors. In this regard, the interactions of substrate molecules with the protein moiety are dominant over the interactions with the catalytic metal ion.

Postischemic free radical production in the venous blood of the regionally ischemic swine heart. Effect of deferoxamine

BACKGROUND

We tested the hypothesis that secondarily produced free radicals can be detected in venous coronary effluent without the need for direct exposure of postischemic tissue to the spin trapping agent alpha-phenyl-tert-butylnitrone (PBN).

METHODS AND RESULTS

The left anterior descending coronary artery (LAD) of pigs was ligated for 15, 30, 40, or 60 minutes, and the tissue was subsequently reperfused for 60 minutes. Venous effluent (6.5 ml) from the risk area was withdrawn sequentially at 1.5-minute intervals during reperfusion. The effluent blood was immediately infused (4.5 ml/min) with an isotonic saline solution containing 120 mM PBN: Preischemic control effluent samples were collected in an identical fashion. Plasma from each sample was extracted in organic solvent and subsequently analyzed by electron spin resonance (ESR) spectroscopy. Another group of pigs received an infusion of the metal chelator deferoxamine mesylate (25 mg/kg/hr) into the right atrium starting 1 hour before the 40-minute ligation and continuing throughout ligation and reperfusion. We were able to demonstrate the postischemic production of ESR signals for PBN adduct(s) from untreated hearts having spectral characteristics similar to an alkoxyl adduct (PBN-RO.; hyperfine splitting constants for beta-hydrogen [alpha H] = 2.0-2.25 G; nitrogen [alpha N] = 13.5-13.75 G). The reperfusion time course of PBN adduct production had a unique pattern: 1) multiple low-level bursts during the initial 15 minutes of reperfusion, and 2) a prominent PBN adduct signal during a relatively late time (20-25 minutes) of reperfusion. Total postischemic PBN adduct production rose with increasing duration (15-60 minutes) of ischemia and was associated with a progressive elevation of total lactate dehydrogenase in the effluent. Infusion of deferoxamine markedly diminished PBN adduct production as well as total release of lactate dehydrogenase.

CONCLUSIONS

These data suggest the potential feasibility of using an ex vivo ESR spin trapping technique in blood-perfused models of cardiovascular injury and that chelatable free iron contributes to the production of alkoxyl radicals.

Is hyperglycemia risky enough to justify the increased risk of hypoglycemia linked with tight diabetes control?

There is an ongoing debate about the possible disadvantages of human insulin use with respect to a possibly lower awareness of hypoglycemia than is associated with animal insulin usage. Participants in this debate have not, however, discussed a major contributory factor to this life-threatening acute complication of diabetes, the pressure on patients to achieve normal levels of blood glucose. This pressure stems from the view that hyperglycemia is the major causative factor in the long-term diabetic complications. However, the evidence that supranormal levels of tissue and plasma glucose contribute to the diabetic tissue damage is not as strong as the arguments on behalf of this position. Indeed, elevated glycemia may be no more than a crude index of other, unknown metabolic derangements which may be causative agents in diabetes-associated tissue damage. Intensive efforts to "normalize" glycemia lack experimental and clinical justification, distract attention from other possible mechanisms, and may impose an unnecessary risk on the insulin-dependent diabetic population since intensive "normalization" of glycemia lowers hypoglycemia awareness, and thus increases risk of hypoglycemia, irrespective of the type of insulin used.

Isoenzymes of horse liver alcohol dehydrogenase active on ethanol and steroids. cDNA cloning, expression, and comparison of active sites

Horse liver alcohol dehydrogenase occurs as isoenzymes: E is active on ethanol but not steroids; S is active on ethanol and steroids. The cDNAs for these isoenzymes were cloned; both were 1.8-kilobase long and contained complete coding sequences. Both enzymes were expressed in Escherichia coli, and the purified proteins had properties similar to those of the natural enzymes. The amino acid sequence deduced from the open reading frame of the E-type cDNA agreed with the amino acid sequence of the E isoenzyme determined by protein sequencing and x-ray crystallography. When compared with the E-type cDNA, the coding region of the S-type cDNA contains 24 substitutions and 3 deletions, giving rise to an amino acid sequence for the S. isoenzyme that differs from that of the E isoenzyme at 10 positions: nine conservative substitutions and one deletion, of Asp-115. These changes can be accommodated in the three-dimensional structure of the E isoenzyme, and models of the E and S isoenzymes complexed with a 3 beta-hydroxy-5 beta-steroid were built. The modeling shows that Leu-116 apparently sterically hinders binding of steroids in the E isoenzyme, and deletion in the S isoenzyme of Asp-115 moves Leu-116 and relieves the hindrance. The human gamma and rat liver enzymes are also active on steroids, but they have a different constellation of amino acid residues in the substrate pocket. Thus, there are multiple bases for the activity on steroids.

[Prescription of lithium in alcoholic patients]

The first, non-standardized studies on the effects of lithium against alcoholism involved a small number of subjects and were encouraging. Subsequent studies have not cast any doubt on the usefulness of lithium in alcoholics with disorders of mood. No effect of lithium on the urge to drink and on the frequency of relapses has yet been demonstrated, but lithium is known to reduce the cognitive and suggestive effects of alcohol abuse. However, the cell lithium/plasma lithium ratio must be taken into account when the cognitive effects of alcohol are evaluated. From a biological viewpoint, lithium and alcohol share some common targets: serotonin metabolism and uptake, membrane fluidity and membrane phospholipids organization.

Role of ethanol-inducible cytochrome P450 (P450IIE1) in catalysing the free radical activation of aliphatic alcohols

Incubation of rat liver microsomes with 1-propanol and 1-butanol in the presence of NADPH and of the spin trapping agent 4-pyridyl-1-oxide-t-butyl nitrone (4-POBN) allowed the detection of free radical intermediates tentatively identified as 1-hydroxypropyl and 1-hydroxybutyl radical, respectively. Microsomes isolated from rats treated chronically with ethanol (EtOH) or with the combination of starvation and acetone treatment (SA), exhibited a two-fold increase in the ESR signal intensity as compared to untreated controls, whereas no increase was observed in phenobarbital-induced (PB) microsomes. Consistently, in reconstituted membrane vesicles, ethanol-inducible cytochrome P450IIE1 was twice as active as phenobarbital-inducible P450IIB1 in producing 1-butanol free radicals. In the microsomal preparations from EtOH and SA pretreated rats the addition of antibodies against cytochrome P450IIE1, but not of preimmune IgGs, lowered the ESR signal of 1-butanol radicals by more than 50%. The same antibodies decreased the free radical production by untreated microsomes by 35-40%, but were ineffective on microsomes from PB-treated animals. This indicated that cytochrome P450IIE1 is the major enzyme responsible for the free radical activation of alcohols in control and ethanol-fed rats. The generation of 1-hydroxybutyl radicals by EtOH microsomes was inhibited by 40, 48 and 68%, respectively, by the addition of isoniazid, tryptamine and octylamine, compounds known to specifically affect the NADPH oxidase activity of this isoenzyme. This effect was not due to the scavenging of the alcohol radical since none of these compounds affected the ESR signals originated from 1-butanol in a xanthine-xanthine oxidase system. When added to reconstituted membrane vesicles isoniazid, tryptamine and octylamine also decreased 1-butanol radical formation by P450IIE1 by 54, 38 and 66%, respectively. Such an inhibition corresponded to the effect exerted by the same compounds on O2- release from P450IIE1 containing vesicles. These results indicate that the capacity of cytochrome P450IIE1 to reduce oxygen is related to its ability to generate alcohol free radicals and suggest that ferric cytochrome P450-oxygen complex might act as oxidizing species toward alcohols.

Human liver aldehyde reductase: pH dependence of steady-state kinetic parameters

The pH dependence of steady-state parameters for aldehyde reduction and alcohol oxidation were determined in the human liver aldehyde reductase reaction. The maximum velocity of aldehyde reduction with NADPH or 3-acetyl pyridine adenine dinucleotide phosphate (3-APADPH) was pH independent at low pH but decreased at high pH with a pK of 8.9-9.6. The V/K for both nucleotides decreased below a pK of 5.7-6.2, as did the pKi of competitive inhibitors NADP and ATP-ribose, suggesting that the 2'-phosphate of the nucleotide has to be deprotonated for binding to the enzyme. The pK of the 2'-phosphate of NADPH appears to be perturbed in the ternary complexes to 5.2-5.4. The V/K for NADPH, the V/K for 3-APADPH, and the pKi of ATP-ribose also decreased above a pK of 9-10, suggesting interaction of the 2'-phosphate of the nucleotide with a protonated base, perhaps lysine. Since protonation of a residue with a pK of 8 (evident in V/K for DL-glyceraldehyde and V/K for L-gulonate versus pH profiles) appears to be essential for aldehyde reduction, and deprotonation for alcohol oxidation, this residue appears to act as a general acid-base catalyst. An additional anion binding site with a pK of 9.94 facilitates the binding of carboxylic substrates such as D-glucuronate. With NADPH as the coenzyme the primary deuterium isotope effects on V and V/K for NADPH were close to unity and pH independent, suggesting that the hydride transfer step is not rate determining over the experimental pH range. With 3-APADPH as the coenzyme, the maximum velocity, relative to NADPH was three- to four-fold lower. Isotope effects on V, V/K for 3-APADPH, and V/K for D-glucuronate were pH independent and equal to 2.2-2.8, indicating that the chemical step of the reaction is relatively insensitive to pH. These data suggest that substrates bind to both the protonated and the deprotonated forms of the enzyme, though only the protonated enzyme catalyzes aldehyde reduction and the deprotonated enzyme catalyzes alcohol oxidation. On the basis of these results a scheme for the chemical mechanism of aldehyde reductase is postulated.

The conversion of primaquine into primaquine-aldehyde, primaquine-alcohol, and carboxyprimaquine, a major plasma metabolite

Although efficacy and toxicity of primaquine (PQ) depend on bioconversion, the process is poorly understood, even for carboxyprimaquine (CPQ), the major plasma metabolite. Earlier work to clarify drug metabolism showed that PQ could be converted quantitatively into CPQ, in vitro, with human erythroleukemic K562 cells or nonleukemic bone marrow supplemented with calf serum. We have now found--using systems with serum only, as well as with K562, bone marrow, and adult or embryonic liver cells--that the bioconversion of the side chain of PQ involves a branched pathway with at least three separate enzymes and two derivatives other than CPQ. An oxidase activity in serum converted PQ first into a novel side chain aldehyde (Y). Aldehyde dehydrogenase transformed PQ-aldehyde into CPQ in cell-free systems and in K562, bone marrow, and adult liver cells. Embryonic hepatocytes or bone marrow treated with 1,3-bis(2-chloroethyl)-1-nitrosourea did not produce CPQ; instead, they made a metabolite (Xc) that we could synthetize via PQ-aldehyde and identify as PQ-alcohol. PQ-alcohol replaced CPQ as the final product whenever alcohol-dehydrogenase prevailed over aldehyde dehydrogenase. These enzymes operated in intact cells and controlled the biotransformation of PQ absolutely. Unless both dehydrogenase were absent, inhibited, or deprived of coenzyme, potentially cytotoxic PQ-aldehyde intermediate did not accumulate. Some of the unique tissues schizonticidal and gametocidal effects of PQ may depend on the distribution pattern and relative activities of PQ oxidase, aldehyde dehydrogenase, and alcohol dehydrogenase in human subjects and in parasites.

The involvement of catalase in alcohol metabolism in Drosophila melanogaster larvae

The involvement of catalase (H2O2:H2O2 oxidoreductase, EC 1.11.1.6) in the metabolism of alcohols was investigated by comparing Drosophila melanogaster larvae in which catalase was inhibited by dietary 3-amino-1,2,4-triazole (3AT) to larvae fed a diet without 3AT. 3AT inhibited up to 80% of the catalase activity with concordant small increases in the in vitro activities of sn-glycerol-3-phosphate dehydrogenase, fumarase, and malic enzyme, but with a 16% reduction in the in vivo incorporation of label from [14C]glucose into lipid. When the catalase activity was inhibited to different degrees in ADH-null larvae, there was a simple linear correlation between the catalase activity and flux from [14C]ethanol into lipid. By feeding alcohols simultaneously with 3AT, ethanol and methanol were shown to react efficiently with catalase in wild-type larvae at moderately low dietary concentrations. Drosophila catalase did not react with other longer chain alcohols. Catalase apparently represents a minor pathway for ethanol degradation in D. melanogaster larvae, but it may be an important route for methanol elimination from D. melanogaster larvae.

An unexpectedly efficient catalytic antibody operating by ping-pong and induced fit mechanisms

A transition state analogue was used to produce a mouse antibody that catalyzes transesterification in water. The antibody behaves as a highly efficient catalyst with a covalent intermediate and the characteristic of induced fit. While some features of the catalytic pathway were programmed when the hapten was designed and reflect favorable substrate-antibody interactions, other features are a manifestation of the chemical potential of antibody diversity. The fact that antibodies recapitulate mechanisms and pathways previously thought to be a characteristic of highly evolved enzymes suggests that once an appropriate binding cavity is achieved, reaction pathways commensurate with the intrinsic chemical potential of proteins ensue.

Conversion of retinoid ethers to alcohols by enzymatic activity present in rat liver microsomes

An enzyme present in rat liver microsomes catalyzes the conversion of retinyl methyl ether (RME) to retinol; NADPH is required for activity. The optimum pH for the reaction is 7.4; the KM and Vmax values are 120 microM RME and 14.3 nmol of retinol/mg protein/hr, respectively. As a substrate, the 2,3,6-trimethyl-4-methoxyphenyl analog of RME is as effective as RME. There is, however, no measurable activity for dealkylation of retinyl ethyl ether or retinyl butyl ether. Hepatic enzyme activity for the metabolism of RME is induced by 3-methylcholanthrene but not by phenobarbital or RME itself. The induced activity also requires NADPH as a cofactor. The optimum pH for the induced enzyme is 8.4; the KM and Vmax values are 50 microM RME and 111 nmol of retinol/mg protein/hr, respectively. For this enzyme, RME is a better substrate than the 2,3,6-trimethyl-4-methoxyphenyl analog of RME; retinyl ethyl ether is less effective; and again, there is no measurable activity with retinyl butyl ether as a substrate. Neither constitutive nor induced activity is detectable in microsomes from lung, spleen, stomach, kidney, small intestine, or large intestine. The enzyme activity that cleaves retinoid ethers appears to be similar to other microsomal NADPH-requiring O-dealkylases and different from a reported tetrahydropteridine-requiring dealkylase.

Acyclic monoterpene primary alcohol:NADP+ oxidoreductase of Rauwolfia serpentina cells: the key enzyme in biosynthesis of monoterpene alcohols

Acyclic monoterpene primary alcohol:NADP+ oxidoreductase, a key enzyme in the biosynthesis of monoterpene alcohols in plants, is unstable and has been only poorly characterized. However we have established conditions which stabilize the enzyme from Rauwolfia serpentina cells, and then purified it to homogeneity. It is a monomer with a molecular weight of about 44,000 and contains zinc ions. Various branched-chain allylic primary alcohols such as nerol, geraniol, and 10-hydroxygeraniol were substrates, but ethanol was inert. The enzyme exclusively requires NADP+ or NADPH as the cofactor. Steady-state kinetic studies showed that the nerol dehydrogenation proceeds by an ordered Bi-Bi mechanism. NADP+ binds the enzyme first and then NADPH is the second product released from it. Gas chromatography-mass spectrometric analysis of the reaction products showed that 10-hydroxygeraniol undergoes a reversible dehydrogenation to produce 10-oxogeraniol or 10-hydroxygeranial, which are oxidized further to give 10-oxogeranial, the direct precursor of iridodial. The enzyme has been found to exclusively transfer the pro-R hydrogen of NADPH to neral. The N-terminal sequence of the first 21 amino acids revealed no significant homology with those of various other proteins including the NAD(P)(+)-dependent alcohol dehydrogenases registered in a protein data bank.

Pre-steady-state and steady-state kinetic analysis of the low molecular weight phosphotyrosyl protein phosphatase from bovine heart

The complete time course of the hydrolysis of p-nitrophenyl phosphate catalyzed by the low molecular weight (acid) phosphotyrosyl protein phosphatase from bovine heart was elucidated and analyzed in detail. Burst titration kinetics were demonstrated for the first time with this class of enzyme. At pH 7.0, 4.5 degrees C, a transient pre-steady-state "burst" of p-nitrophenol was formed with a rate constant of 48 s-1. The burst was effectively stoichiometric and corresponded to a single enzyme active site/molecule. The burst was followed by a slow steady-state turnover of the phosphoenzyme intermediate with a rate constant of 1.2 s-1. Product inhibition studies indicated an ordered uni-bi kinetic scheme for the hydrolysis. Partition experiments conducted for several substrates revealed a constant product ratio. Vmax was constant for these substrates, and the overall rate of hydrolysis was increased greatly in the presence of alcohol acceptors. An enzyme-catalyzed 18O exchange between inorganic phosphate and water was detected and occurred with kcat = 4.47 x 10(-3) s-1 at pH 5.0, 37 degrees C. These results were all consistent with the existence of a phosphoenzyme intermediate in the catalytic pathway and with the breakdown of the intermediate being the rate-limiting step. The true Michaelis binding constant Ks = 6.0 mM, the apparent Km = 0.38 mM, and the rate constants for phosphorylation (k2 = 540 s-1) and dephosphorylation (k3 = 36.5 s-1) were determined under steady-state conditions with p-nitrophenyl phosphate at pH 5.0 and 37 degrees C in the presence of phosphate acceptors. The energies of activation for the enzyme-catalyzed hydrolysis at pH 5.0 and 7.0 were 13.6 and 14.1 kcal/mol, respectively. The activation energy for the enzyme-catalyzed medium 18O exchange between phosphate and water was 20.2 kcal/mol. Using the available equilibrium and rate constants, an energetic diagram was constructed for the enzyme-catalyzed reaction.

On the reactions of superoxide with keto enols, aci-reductones and ascorbic acid derivatives

The superoxide-mediated base catalyzed autoxidation of alpha-oxo enols is initiated by the deprotonation of the labile hydroxyl group. Thus, the reaction of O2-. (generated from KO2/crown ether in aprotic media) with 3-hydroxycoumarin (1), followed by a CH3I-workup, generates products 2-4 via a deprotonation-oxidation sequence complicated by a competing saponification of the lactone linkage. The related coumarin reductone (alpha-oxo enediol) 8 is rapidly oxidized by O2-., HO- and t-butoxide to the corresponding triketone, which in turn undergoes further oxidation and rearrangement ultimately yielding (upon methyl iodide workup) products 9-14. When the O2-. mediated oxidation is carried out under argon in completely degassed solutions, large amounts (greater than 20%) of monodeprotonation product (detected as 9) accumulate. These results are discussed in light of the differing mechanisms proposed by Sawyer and Afanas'ev for the interaction of O2-. with the reductone ascorbic acid.

[A new reaction catalyzed by Bollum's terminal deoxyribonucleotidyltransferase]

The highly purified preparations of Bollum's terminal transferase from calf thymus were shown to catalyze, along with the common reaction of nucleotide addition to the 3'-terminus of an oligonucleotide primer, a "non-common" reaction between dNTP or rNTP on one hand, and various alcohols on the other hand. This reaction was carried out with ethylene glycol, glycerol, ethanol and methanol to produce substances containing one molecule of nucleotide, one molecule of alcohol and non-organic pyrophosphate. The reaction conditions are cacodylate buffer, pH 7, 2, in the presence of Mg2+ or Co2+ ions. The structure was determined for the product of the reaction between glycerol and dATP, which appeared to be 2,3-dihydroxypropyl-ether of 2'-deoxyadenosine-5'-monophosphate.

Zinc-activated alcohols in ternary complexes of liver alcohol dehydrogenase

Activation parameters for each reaction step in the kinetic mechanism of liver alcohol dehydrogenase have been measured for the oxidation of ethanol and the reduction of acetaldehyde. In the oxidation process, the highest enthalpy of activation, 9.7 kcal/mol, occurs for the turnover of the liver alcohol dehydrogenase-NAD(+)-ethanol ternary complex. To investigate if this enthalpy requirement represents a change in the ionization state of ethanol bound in the ternary complex, inhibition of ethanol oxidation was determined using the following series of small, electronegative alcohols with pKa values ranging from 12.37 to 15.5: 2,2,2-trifluoroethanol, 2,2,2-trichloroethanol, 2,2,2-tribromoethanol, 2,2-dichloroethanol, 2,2-difluoroethanol, propargyl alcohol, 3-hydroxypropionitrile, 2-chloroethanol, 2-iodoethanol, 2-methoxyethanol, ethylene glycol, and methanol. The observed inhibition patterns were analyzed according to several kinetic inhibition models; in each case, the best fit model was used to determine the substrate competitive inhibition constant. A plot of the logarithm of these inhibition constants is shown to be dependent on the pKa values of the inhibiting alcohols with a slope approaching -1, indicating that inhibition is controlled by a proton loss from the alcohol. The observed competitive inhibition behavior, coupled with crystallographic studies depicting a direct ligation of an alcohol oxygen to the catalytic zinc ion, indicates that inhibition is controlled by the formation of a zinc-bound alkoxide. Because the inhibiting alcohols are structurally homologous to ethanol, a relationship between the inhibition constant and the inhibiting alcohol's pKa can be derived to show that the pKa of an alcohol bound in a ternary complex is also dependent on its pKa as a free alcohol. Ternary complex pKa values have been determined for ethanol and the inhibiting alcohols.

Importance of organic osmolytes for osmoregulation by renal medullary cells

The cells in the renal medulla protect themselves from the extracellular hypertonicity in that region of the kidney by accumulating large amounts of sorbitol, inositol, glycerophosphorylcholine, and betaine. The system is uniquely active in this part of the body, but it represents a throwback to primitive mechanisms by which cells in virtually all organisms, including bacteria, yeasts, plants, and lower animals counteract water stress. In this brief review, we summarize how these "compatible organic osmolytes" help the renal medullary cells to survive, the mechanisms by which the organic osmolytes are accumulated, and how the accumulation is controlled to adjust for changing extracellular NaCl and urea concentrations. The compatible organic osmolytes are all intermediates in important biochemical pathways, and although the medical consequences are not yet fully worked out, it is already apparent that inappropriate accumulation of these solutes has major pathophysiological consequences.

Volatile metabolites and other indicators of Penicillium aurantiogriseum growth on different substrates

Penicillium aurantiogriseum Dierckx was cultivated on six agar substrates (barley meal agar, oat meal agar, wheat meal agar, malt extract agar, Czapek agar, and Norkrans agar) and on oat grain for 5 days in cultivation vessels provided with an inlet and an outlet for air. Volatile metabolites produced by the cultures were collected on a porous polymer adsorbent by passing an airstream through the vessel. Volatile metabolites were collected between days 2 and 5 after inoculation. CO2 production was simultaneously measured, and after the cultivation period ergosterol contents and the numbers of CFU of the cultures were determined. Alcohols of low molecular weight and sesquiterpenes were the dominant compounds found. During growth on oat grain the production of 8-carbon alcohols and 3-methyl-1-butanol was higher and the production of terpenes was lower than during growth on agar substrates. The compositions of the volatile metabolites from oat grain were more similar to those from wheat grain, which was used as a substrate in a previous investigation, than to those produced on any of the agar substrates. Regarding the agar substrates, the production of terpenes was most pronounced on the artificial substrates (Czapek agar and Norkrans agar) whereas alcohol production was highest on substrates based on cereals. The production of volatile metabolites was highly correlated with the production of CO2 and moderately correlated with ergosterol contents, whereas no correlation with the numbers of CFU was found. Thus, the volatile metabolites formed and the ergosterol contents of fungal cultures should be good indicators of present and past fungal activity.