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PubMed1979

Stereospecificity of sodium borohydride reduction of tyrosine decarboxylase from Streptococcus faecalis.

Sodium boro[3H]hydride reduction of tyrosine decarboxylase from Streptococcus faecalis followed by complete hydrolysis of the enzyme produces epsilon-[3H]pyridoxyllysine. Degradation of this material to [4'-3H]pyridoxamine and stereochemical analysis with apoaspartate aminotransferase shows that the re side at C-4' of the cofactor is exposed to solvent at pH 5.5 and 7.0. After binding of L-tyrosine at pH 5.5 or tyramine at pH 7.0 to the holoenzyme, sodium boro[3H]hydride reduction proceeds from the si face at C-4' of the substrate . cofactor complex. This indicates one of two conformational changes occurs upon binding of substrate; either rotation about the C-4 to C-4' bond in the cofactor or rotation about the axis through the C-5 and C-5' bond.

Binding Sites↗
PubMed1979

Quaternary structure of methemoglobin. Pulse radiolysis study of the binding of oxygen to the valence hybrid.

The pulse radiolysis of solutions of adult human methemogolbin was used in order to reduce a single heme iron within the protein tetramers. The valence hybrids thus formed were reacted with oxygen. Kinetics of the reactions were studied. The effects of pH and inositol hexaphosphate were examined. The kinetics of the ligation of oxygen to stripped valence hybrids showed a single phase behavior at the pH range 6.5 to 9. As the pH was lowered below 6.5, a second, slower phase became apparent. In the presence of inositol hexaphosphate, above pH 8, the kinetics of oxygen binding was of a single phase. As the pH was lowered, a transition to a second, slower phase was noticed. Below pH 7, the slower phase was the only detectable one. The analysis of the relative contribution of the faster phase to the total reaction as a function of the pH showed a typical transition curve characterized by a pK = 7.5 and a Hill parameter n = 2.9. On this basis, it is concluded that human adult stripped methemoglobin resides in an R quarternary structure, while the presence of IHP stabilizes the T structure at pH below 7.5. This transition between the quaternary structures of methemoglobin cannot be accounted for by the switch between the high spin and the low spin states of the ferric iron. This switch of spin state takes place at pH greater than 8.2.

Heme↗
PubMed1979

Kinetic studies on the reaction of p-hydroxybenzoate hydroxylase. Agreement of steady state and rapid reaction data.

p-Hydroxybenzoate hydroxylase (EC 1.14.13.2) from Pseudomonas fluorescens is a NADPH-dependent, FAD-containing monooxygenase catalyzing the hydroxylation of p-hydroxybenzoate to form 3,4-dihydroxybenzoate in the presence of NADPH and molecular oxygen. The mechanism of this three-substrate reaction was investigated in detail at pH 6.6, 4 degrees C, by steady state kinetics, stopped flow spectrophotometry, and equilibrium binding experiments. The initial velocity patterns are consistent with a ping-pong type mechanism which involves two ternary complexes between the enzyme and substrates. The first ternary complex is formed by random addition of p-hydroxybenzoate and NADPH to the enzyme, followed by the release of the first product (NADP+). The reduced enzyme . p-hydroxybenzoate complex now reacts with oxygen, the third substrate, to form the second ternary complex. The enzyme-bound p-hydroxybenzoate then reacts with the activated oxygen to give 3,4-dihydroxybenzoate which is released regenerating the oxidized enzyme for the next cycle. The binding of p-hydroxybenzoate to the oxidized enzyme to form a 1:1 complex causes large, characteristic spectral perturbations and fluorescence quenching. The dissociation constant for the enzyme . substrate complex was obtained by titrations in which absorbance and/or fluorescence quenching was measured. The binding constants of NADPH to the enzyme with and without p-hydroxybenzoate were determined kinetically by measuring the rate of reduction of the enzyme at different concentrations of NADPH. The reduction of the enzyme proceeds extremely slowly in the absence of p-hydroxybenzoate. The presence of the substrate causes a dramatic stimulation (140,000-fold) in the rate of enzyme reduction. The anaerobic reduction of the enzyme by NADPH in the presence of p-hydroxybenzoate produces a transient charge-transfer intermediate. On the basis of the proposed mechanism, the dissociation constants for p-hydroxybenzoate and NADPH as well as the Michaelis constants for all the three substrates were calculated from the initial velocity data. The agreement obtained between various kinetic parameters from the initial rate measurements and those calculated from the individual rate constants determined in rapid reactions, strongly supports the proposed mechanism for the p-hydroxybenzoate hydroxylase reaction.

4-Hydroxybenzoate-3-Monooxygenase↗
PubMedDate not supplied

[Photooxidation of aspartate transaminase from chicken heart cytosol].

Rose-bengal-sensitized photooxidation of aspartate transaminase from chicken heart cytosol results in a loss of enzymatic activity which follow first order kinetics down to 70--75% inactivation. 0.9 Histidine, 0.9 tryptophane residues and 1.5 SH groups per enzyme subunit were found to be modified in the photooxidized transaminase, which retained 26% residual activity. Photodestruction of the coenzyme was about 16%. The rate of enzyme photoinactivation is constant in the pH range 6--8, and drastically decreases with lowering pH from 6 to 4. alpha-Ketoglutarate partially protects the holoenzyme from inactivation. The apoenzyme undergoes photoinactivation at a rate almost twice as rapid as the holoenzyme. Photooxidized apotransaminase retains affinity to pyridoxal phosphate and binds as much coenzyme as the native apoenzyme. Photooxidation induces no significant alterations in the circular dichroism pattern of the enzyme in the 200 to 240 nm range. However, positive circular dichroism is markedly increased in the absorption bands of aromatic amino acids (260--300 nm). The affinity of photooxidized holoenzyme for glutarate and alpha-methyl aspartate is greatly decreased. On the other hand, photooxidized enzyme retains its ability to bind alpha-alanine and to catalize the transamination half-reaction between alpha-alanine and the bound coenzyme. These findings imply that photooxidation disturbs the binding of the distal carboxyl group of dicarboxylic substrates. This may be due to a localized conformational change induced by destruction of a photoreactive histidine residue at the active site. A role of the histidine residue in transamination reaction is discussed.

Alanine↗
PubMedDate not supplied

[Oxidation of fatty-aromatic aldehydes in liver tissues].

A protein fraction, which did not contain NADP [or NADPH]-dependent aldehyde reductase as well as NAD [or NADP]-dependent aldehyde dehydrogenases, but which catalyzed oxidation of fatty-aromatic aldehydes, was isolated from extract of rat liver tissue using ammonium sulfate fractionation combined with gradient syvorptive chromatography on DEAE-Sephadex A-25 [or Molselect DEAE-25], CM-Sephadex C-25 and gel-filtration on Sephadex G-200. Investigations of molecular weight and catalytic properties of the protein fraction obtained enabled to identify it with xanthine oxidase [EC 1.2.3.2]. Aldehyde dehydrogenases as well as xanthine oxidase are involved in oxidation of fatty-aromatic aldehydes to corresponding fatty acids, besides the reduction of the aldehydes to alcohols, catalyzed by aldehyde reductase and alcohol dehydrogenases.

Aldehyde Oxidoreductases↗
PubMed1979

[Light-dependent oxygen consumption by isolated pea chloroplasts under NADP+ photoreduction].

The exchange of oxygen during NADP+ photoreduction by isolated pea chloroplasts was studied. It was found that NADP+ oxidation is accompanied by oxygen photoreduction preceeding at a high rate. A possibility for calculation of the ration between the pseudocyclic electron transport and the total electron transport based on oxygen exchange in the presence of NaN3 and catalase, was established. It was found that the pseudocyclic transport can make up to 30% or more of total electron transport.

Chloroplasts↗
PubMedDate not supplied

[Electron transfer to hemoproteins. II. pH-dependence of the reduction rate of ferricytochrome c by oxymyoglobin].

The pH-dependence of the reduction rate of ferricytochrome C by intact and chemically modified oxymyoglobins has been studied. The modification was performed with respect to histidine residues and alpha-aminogroup of N-terminal valine. Two histidine residues of myoglobin, His A10 and His GH1, are shown to take part in the realization of the "active" contact between the molecules in the course of the reaction. The deprotonation of the first residue contributes to the acceleration and that of the second to the reduction of the reaction. The found orientation of the Mb molecules in the "active complex" implies that at any orientation of cytochrome C the distance between the haemes of the both molecules should be more than 30 A. This makes highly probable that a structure-dependent mechanism of electron transfer in the system under study can be proposed.

Binding Sites↗
PubMed1979

[Stability of glucose-6-phosphate dehydrogenase in human cells cultivated in vitro].

It was shown that the thermal stability of glucose-6-phosphate dehydrogenase in human diploid cells is much higher than in human heteroploid cell lines HeLa and T-9. The purified enzymes from human diploid cells and from HeLa and T-9 cells possess similar thermal stabilities. Mixing of T-9 extracts with the purified enzyme preparations revealed that the non-stability factors of the dehydrogenase are present in the T-9 extracts. An addition of NADP- and NADPH-containing buffers and crystalline NADP to the heteroploid cell extracts stabilizes the enzyme. The thermal stability of the enzyme from "in vitro" cultivated human cells depends on the concentration of the coenzyme. It was also demonstrated that glucose-6-phosphate dehydrogenase stability in HeLa and T-9 extracts is the same at low concentrations of the coenzyme and after addition of crystalline NADP. However, at NADP concentration of 10(-3) M the enzyme stability in HeLa and T-9 extracts is different. It is assumed that the destabilizing factors are the enzymes possessing the nucleotidases activity, which is different in various cell lines.

Buffers↗
PubMed1979

[Transhydrogenase as an additional site of energy accumulation in the E. coli respiratory chain].

NAD+ reduction catalyzed by transhydrogenase (EC 1.6.1.1) from E. coli membrane particles at the expense of NADPH oxidation is coupled with phenyldicarbaundecaborate (PCB-) absorption by the particles. This process is inhibited by oxidative phosphorylation protonophorous uncouplers and by equilibration of concentrations of the substrates and products of the transhydrogenase reaction. Elimination of the water-soluble part of membrane ATPase results in the inhibition of PCB- absorption at the expense of the transhydrogenase reaction energy. Treatment of the particles by dicyclohexyl carbodiimide increases the transhydrogenase-coupled absorption of PCB-. The transhydrogenase-induced increase of pPCB in the suspension of particles is directly correlated with the ratio of ([NADPH].[NAD+])/([NADP+].[NADH]). When this value is equal to 1, no energy-dependent increase of pPCB was observed. NADP+ reduction at the expense of NADH oxidation leads to a decrease in the amount of PCB- absorbed by the particles at the expense of ATP hydrolysis energy. The experimental data suggest that NADPH oxidation in the course of the transhydrogenase reaction is coupled with the formation of a membrane potential with a positive charge localized inside the particles.

Cell Membrane↗
PubMedDate not supplied

Enhancement of reductive metabolism of p-nitrobenzoate and nitrazepam in isolated perfused rat liver by ethanol.

Reductive metabolism of p-nitrobenzoate (2 mM) was studied in the isolated perfused rat liver, after acute ethanol dosing, with use of a hemoglobin-free perfusion medium. Formation of reduced metabolites under control conditions (0.3 mumol per g of liver per hr) was enhanced fivefold (1.4 mumol/g/hr) in the presence of ethanol (38 mM), thus reaching hepatic reductase activities occurring under anaerobic conditions (1.4 mumol/g/hr). Ethanol failed to increase hepatic nitro reduction when alcohol dehydrogenase was inhibited by pyrazole. Addition of acetaldehyde led to a marked stimulation of nitroreductase activity. Carbon monoxide did not influence the ethanol-mediated enhancement of nitroreductase activity but almost abolished the enhancement caused by anoxia. Reductive azo cleavage of salazosulfamide was not enhanced by ethanol. When nitrazepam was used as the substrate (1 mM) for the isolated perfused rat liver, addition of ethanol (38 mM) led to an enhanced content of 7-amino derivative in the liver and in the perfusate, whereas the formation of 7-acetylamino derivative remained unchanged. The distribution of nitrazepam in liver and perfusate was not altered by ethanol.

Animals↗
PubMed1976

The metabolic activation of the carcinogen 1'-hydroxysafrole in vivo and in vitro and the electrophilic reactivities of possible ultimate carcinogens.

Administration of [2',3'-3H]-1'-hydroxysafrole to rats or mice resulted in the formation of hepatic DNA-, ribosomal RNA-, and protein-bound 3H derivatives. Alkaline digestion of the 3H-protein released 0.1 to 0.3% of the 3H as a derivative that was identified as 3'-methylmercaptoisosafrole by its cochromatography in five solvent systems with the synthetic compound. 1'-Hydroxysafrole was metabolized at a low rate by rat and mouse liver cytosols in a 3'-phosphoadenosine 5'-phosphosulfate-dependent reaction to a derivative (presumably the sulfuric acid ester) that was captured by its reaction with RNA. Likewise, 1'-hydroxysafrole was oxidized at a low rate by rat and mouse liver microsomes to 1'-hydroxysafrole-2',3'-oxide in a reduced nicotinamide adenine dinucleotide phosphate-dependent reaction. Both of these electrophilic metabolites are candidate ultimate carcinogenic derivatives of 1'-hydroxysafrole. The electrophilic reactivities of various safrole derivatives with nucleosides were determined to be in the order of 1'-oxosafrole greater than 1'-acetoxysafrole greater than 1'-acetoxysafrole-2',3'-oxide greater than 1'-hydroxysafrole-2',3'-oxide greater than safrole-2',3'-oxide greater than or equal to 1'-oxosafrole-2',3'-oxide. The major reactions were generally observed with guanosine. A major reaction product of 1'-acetoxysafrole and guanosine 5'-monophosphate yielded 3'-hydroxyisosafrole under very mild acidic conditions. These data further substantiate the previous characterization of this reaction product as O-6-(isosafrol-3'-yl)guanylic acid. The syntheses of 1'-oxosafrole, 2',3'-dehydrosafrole, [2',3'-3H]-1'-hydroxysafrole, and the 2',3'-oxed.

Acetyltransferases↗