Some comparisons of pig and sheep liver cytosolic aldehyde dehydrogenases

1. The pig enzyme was purified to homogeneity and was found to be a tetramer of apparently identical subunits. 2. The pig enzyme was found to contain 1 mol NADH/mol enzyme which is tightly bound, which is not directly involved in catalysis and which so far has not been removed from the enzyme so as to produce an active apoenzyme. 3. The pig enzyme seems to contain only one functioning active site/tetramer. 4. The pig and sheep enzymes are compared in respect of NADH binding, substrate specificity, immunological response and surface charge.     

Studies on the changes in protein fluorescence and enzymic activity of aspartate aminotransferase on binding of pyridoxal 5′-phosphate

1. The alpha and beta subforms of aspartate aminotransferase were purified from pig heart. 2. The alpha subform contained 2mol of pyridoxal 5'-phosphate. The apo-(alpha subform) could be fully reactived by combination with 2mol of cofactor. 3. The protein fluorescence of the apo-(alpha subform) decreased non-linearly with increase in enzyme activity and concentration of bound cofactor. 4. It is concluded that the enzyme activity/mol of bound cofactor is largely independent of the number of cofactors bound to the dimer. 5. The beta subform had approximately half the specific enzyme activity of the alpha subform, and contained an average of one active pyridoxal 5'-phosphate molecule per molecule, which could be removed by glutamate, and another inactive cofactor which could only be removed with NaOH. 6. On recombination with pyridoxal 5'-phosphate the protein fluorescence of the apo-(beta subform) decreased linearly, showing that each dimeric enzyme molecule contained one active and one inactive bound cofactor. 7. The results are not consistent with a flip-flop mechanism for this enzyme.     

Methionine 500, the site of covalent attachment of an active site-directed reagent of beta-galactosidase.

The site of attachment to beta-galactosidase of the active site-directed inhibitor, beta-D-galactopyranosylmethyl p-nitrophenyl triazene, was determined. When the enzyme is completely inactivated, 1 mol of the galactopyranosylmethyl group is bound per mol of monomer with retention of the tetrameric structure. After reaction with the [14C]methyl reagent, labeled peptides were isolated and analyzed. The radioactive label was found to be covalently bound to methionine residue 500.     

Changes in affinity for calcium ions with the formation of two kinds of phosphoenzyme in the Ca2+,Mg2+-dependent ATPase of sarcoplasmic reticulum

The amount of Ca2+ bound to the Ca2+,Mg2+-dependent ATPase of deoxycholic acid-treated sarcoplasmic reticulum was measured during ATP hydrolysis by the double-membrane filtration method [Yamaguchi, M. & Tonomura, Y. (1979), J. Biochem. 86, 509--523]. The maximal amount of phosphorylated intermediate (EP) was adopted as the amount of active site of the ATPase. In the absence of ATP, 2 mol of Ca2+ bound cooperatively to 1 mol of active site with high affinity and were removed rapidly by addition of EGTA. AMPPNP did not affect the Ca2+ binding to the ATPase in the presence of MgCl2. Under the conditions where most EP and ADP sensitive at steady state (58 microM Ca2+, 50 microM EGTA, and 20 mM MgCl2 at pH 7.0 and 0 degrees C), bound Ca2+ increased by 0.6--0.7 mol per mol active site upon addition of ATP. The time course of decrease in the amount of bound 45Ca2+ on addition of unlabeled Ca2+ + EGTA was biphasic, and 70% of bound 45Ca2+ was slowly displaced with a rate constant similar to that of EP decomposition. Similar results were obtained for the enzyme treated with N-ethylmaleimide, which inhibits the step of conversion of ADP-sensitive EP to the ADP-insensitive one. Under the conditions where most EP was ADP insensitive at steady state (58 microM Ca2+, 30 microM EGTA, and 20 mM MgCl2 at pH 8.8 and 0 degrees C), the amount of bound Ca2+ increased slightly, then decreased slowly by 1 mol per mol of EP formed after addition of ATP. Under the conditions where about a half of EP was ADP sensitive (58 microM Ca2+, 25 microM EGTA, and 1 mM MgCl2 at pH 8.8 and 0 degrees C), the amount of bound Ca2+ did not change upon addition of ATP. These findings suggest that the Ca2+ bound to the enzyme becomes unremovable by EGTA upon formation of ADP-sensitive EP and is released upon its conversion to ADP-insensitive EP.     

Adenine nucleotides as allosteric effectors of pea seed glutamine synthetase

The effects of adenine nucleotides on pea seed glutamine synthetase (EC 6.3.1.2) activity were examined as a part of our investigation of the regulation of this octameric plant enzyme. Saturation curves for glutamine synthetase activity versus ATP with ADP as the changing fixed inhibitor were not hyperbolic; greater apparent Vmax values were observed in the presence of added ADP than the Vmax observed in the absence of ADP. Hill plots of data with ADP present curved upward and crossed the plot with no added ADP. The stoichiometry of adenine nucleotide binding to glutamine synthetase was examined. Two molecules of [gamma-32P]ATP were bound per subunit in the presence of methionine sulfoximine. These ATP molecules were bound at an allosteric site and at the active site. One molecule of either [gamma-32P]ATP or [14C]ADP bound per subunit in the absence of methionine sulfoximine; this nucleotide was bound at an allosteric site. ADP and ATP compete for binding at the allosteric site, although ADP was preferred. ADP binding to the allosteric site proceeded in two kinetic phases. A Vmax value of 1.55 units/mg was measured for glutamine synthetase with one ADP tightly bound per enzyme subunit; a Vmax value of 0.8 unit/mg was measured for enzyme with no adenine nucleotide bound at the allosteric site. The enzyme activation caused by the binding of ADP to the allosteric sites was preceded by a lag phase, the length of which was dependent on the ADP concentration. Enzyme incubated in 10 mM ADP bound approximately 4 mol of ADP/mol of native enzyme before activation was observed; the activation was complete when 7-8 mol of ADP were bound per mol of the octameric, native enzyme. The Km for ATP (2 mM) was not changed by ADP binding to the allosteric sites. ADP was a simple competitive inhibitor (Ki = 0.05 mM) of ATP for glutamine synthetase with eight molecules of ADP tightly bound to the allosteric sites of the octamer. Binding of ATP to the allosteric sites led to marked inhibition.     

The presence of desulfo xanthine dehydrogenase in purified and crude enzyme preparations from rat liver

Crude and purified xanthine dehydrogenase preparations from rat liver were examined for the existence of a naturally occurring inactive form. Reduction of the purified enzyme by xanthine under anaerobic conditions proceeded in two phases. The enzyme was inactivated by cyanide, which caused the release of a sulfur atom from the molybdenum center as thiocyanate. The amount of thiocyanate released was almost in parallel with the initial specific activity. The active and inactive enzymes could be resolved by affinity chromatography on Sepharose 4B/folate gel. These results provided evidence that the purified enzyme preparation from rat liver contained an inactive form. A method for the determination of the active and inactive enzymes in crude enzyme preparations from rat liver was devised based on the fact that only active enzyme could react with [14C]allopurinol and both active and inactive enzymes could be immunoprecipitated quantitatively by excess specific antibody to xanthine dehydrogenase. The amount of [14C]alloxanthine (derived from [14C]allopurinol) bound to the active sulfo enzyme in crude rat liver extracts was about 0.5 mol/mol of FAD. As this content is closely similar to that in the purified enzyme, these results suggest the existence of an inactive desulfo form in vivo.     

Partial characterization of the polyisoprenoid carrier of N-acetylglucosamine in Glycine max (soya bean).

Radiolabelled anhydrotrypsin was bound by alpha 2M (alpha 2-macroglobulin) sufficiently tightly to resist separation during gel electrophoresis; 2 mol of anhydrotrypsin were bound/mol of alpha 2M, but the interaction differed in important respects from that between active proteinases and alpha 2M. Anhydrotrypsin was bound by the electrophoretically 'fast' form of alpha 2M, although much less effectively than by the 'slow' form. The inactive enzyme was displaced from alpha 2M by trypsin inhibitor, the order of effectiveness being aprotinin > soya-bean trypsin inhibitor > benzamidine. Saturation of alpha 2M with anhydrotrypsin did not prevent subsequent binding and inhibition of active trypsin by the alpha 2M, and the anhydrotrypsin was not displaced during this reaction. Anhydrotrypsin bound by alpha 2M retained its ability to react with antibodies against trypsin, whereas bound trypsin did not.     

Affinity labeling of histidine and lysine residue in the adenosine deaminase substrate binding site.

1. Adenosine deaminase was inactivated by 9-(4-bromoacetamidobenzyl)-adenine (I) and 9-(2-bromoacetamidobenzyl)adenine (II), two affinity labels. 2. The stoichiometry of the reaction with reagent II is reported: 1 mol reagent is bound per mol inactive enzyme. Amino acid analysis of the 6 N HCl hydrolyzate of the inactive enzyme identified CM-histidine as the main alkylation product. This is the first evidence of the presence of a histidine in the active site region. 3. The alkylation rate and involved amino acid residues were studied for both reagents I and II, at pH 8 and 5.5. The particular reactivity of a lysine near or in the active site is discussed.     

Chemistry of the inactivation of 4-aminobutyrate aminotransferase by the antiepileptic drug vigabatrin

The chemical modification of pig liver 4-aminobutyrate aminotransferase by the antiepileptic drug 4-aminohex-5-enoate (Vigabatrin) has been studied. After inactivation by 14C-labeled Vigabatrin, the enzyme was digested with trypsin, and automated Edman degradation of the purified labeled peptide gave the sequence FWAHEHWGLDDPADVMTFSKK. Chymotryptic digestion of the tryptic peptide and sequencing of a resulting tripeptide identified the penultimate lysine residue of this peptide as the site of covalent modification. This lysine normally binds the coenzyme. Absorption spectroscopy demonstrated the absence of coenzyme from the tryptic peptide, and mass spectrometry showed its mass/charge ratio to be increased by 128. All of the bound coenzyme released after denaturation of the inactivated enzyme was as pyridoxamine phosphate. The structural nature of the modification is deduced, and mechanisms for its occurrence identified. Initially, 1 mol of radiolabeled inhibitor was bound per mol of monomer of the enzyme, although approximately half was released during denaturation and digestion, while the remainder was irreversibly bound. Coenzyme not released as pyridoxamine phosphate retained the absorbance characteristics of the aldimine, although the enzyme was completely inactive. Mass spectrometry of the sample of purified radiolabeled tryptic peptide revealed the presence of an approximately equal amount of a second fragment that contained no modification and from which the second lysine was absent, indicating that at the time of proteolysis the active site lysine was unaltered in 50% of the enzyme molecules.     

Stoichiometry and stability of the adduct formed between human 4-aminobutyrate aminotransferase and 4-aminohex-5-enoate: sequence of a labelled peptide

The reaction between human 4-aminobutyrate aminotransferase and the anti-epileptic drug 4-aminohex-5-enoate, an irreversible inhibitor of the enzyme, has been studied using the radiolabelled compound. The inactivated enzyme was found to lose radiolabel over a period of a few days at 37 degrees C but even in the presence of the coenzyme, pyridoxal phosphate, no enzyme activity returned. At 4 degrees C the radiolabelled inhibitor remained stably bound. The amount of enzyme-bound 4-aminohex-5-enoate was significantly less than would be expected if one mol of inhibitor was bound per mol of active site. Reversed phase chromatography of a tryptic digest of the labelled enzyme showed that, apart from material eluting at the front of the chromatogram, all of the radioactivity was in a single fraction. This fraction contained a peptide, the sequence of which indicated that it included the lysine that binds the coenzyme and that the major release of radioactivity occurred in an Edman degradation cycle corresponding to this residue.     

Subsite differences between the active centres of papaya peptidase A and papain as revealed by affinity chromatography. Purification of papaya peptidase A by ionic-strength-dependent affinity adsorption on an immobilized peptide inhibitor of papain.

An affinity column consisting of the specific peptide inhibitor of papain, Gly-Gly (O-benzyl)Tyr-Arg, attached to Sepharose was found to bind the active thiol proteinase papaya peptidase A specifically, but only at an ionic strength significantly higher than the one at which papain is bound. When a mixture of active papaya peptidase A and its irreversibly oxidized contaminant was applied to the column, the active enzyme was bound whereas the inactive material was not. The bound enzyme was released by deionized water and found to contain 1 mol of SH group/mol of protein. The different conditions required for the binding of the two enzymes to the immobilized peptide was shown to reflect different ionic-strength-dependences of the affinity of the two enzymes for the peptide in solution. Whereas the affinity of papain for the inhibitor appears to be insensitive to ionic strength over the range studied, that of papaya peptidase A is ionic-strength-dependent and always lower than that of papain. A rate assay is devised for papaya peptidase A with N-benzyloxycarbonylglycine p-nitrophenyl ester as the substrate at pH 5.5. After calibration against an active-site titration the assay yields the thiol-group concentration without interference from inactive contaminants. For the papaya peptidase A-catalysed hydrolysis of N-benzyloxycarbonylglycine p-nitrophenyl ester at pH 5.5 kcat. was found to be 16.7s-1, which is about 3 times the value found for the same reaction catalysed by papain.     

Interaction of the D-isomer of gamma-methylene glutamate with an active site thiol of gamma-glutamylcysteine synthetase

gamma-Glutamylcysteine synthetase has a thiol group in the vicinity of its glutamate-binding site. During efforts to find a covalently bound inhibitor, interaction of the enzyme with gamma-methylene glutamate was examined because this analog of glutamate, which has an alpha,beta-unsaturated moiety, would be expected to bind at the glutamate site and might react with an active site thiol. gamma-Methylene glutamate, which is not a significant substrate, inhibits the enzyme competitively toward glutamate. Preincubation of the enzyme with gamma-methylene DL-glutamate led to substantial inactivation which was dependent upon the presence of Mg2+ or Mn2+; glutamate protected against inactivation. Inactivation was observed with the D-isomer of gamma-methylene glutamate, but not with the corresponding L-isomer. The inactivated enzyme contains close to 1 mol of gamma-methylene glutamate/mol of enzyme. Studies in which enzyme inactivated by treatment with [14C]gamma-methylene glutamate was hydrolyzed indicate that gamma-methylene glutamate reacts with an active site thiol.     

Reductive trapping of substrate to methylamine oxidase from Arthrobacter P1

Methylamine oxidase (EC 1.4.3.6) from Arthrobacter P1 was inactivated by NaCNBH3 in the presence of [14C]benzylamine, leading to the incorporation of 1 mol of radiolabeled substrate/mol of enzyme subunit at complete inactivation. By contrast, no labeling of enzyme was observed using [3H]NaCNBH3 as reductant. These results are analogous to those previously reported for the eukaryotic enzyme, bovine serum plasma amine oxidase [(1987) J. Biol. Chem. 262, 962-965]. The observed pattern of labeling is consistent with the presence of dicarbonyl cofactor at the active site of methylamine oxidase. Further, these studies suggest that our reductive trapping technique, in which the pattern of radiolabeling of an enzyme is compared using C-14 substrate vs tritiated reductant, may serve as a general assay for covalently bound dicarbonyl structures.     

Modification of acetylcholinesterase with the fluorescent thiol reagent S-mercuric-N-dansylcysteine

11s acetylcholinesterase (EC 3.1.1.7) from Torpedo californica electroplax, purified by a combination of affinity and gel chromatography was found to react stoichiometrically with S-mercuric-N-dansylcysteine. Approximately four mols of reagent per mol of enzyme were incorporated when the modification was carried out in 1.0 mM Tris-C1, pH 7.5, either in the presence or absence of 0.1 M NaCl. Prior incubation of the enzyme with 1.0 x 10(-4) M Zn2+ allowed the incorporation of about six mols of reagent per mol of enzyme. Binding of the reagent produced shifts in the emission and excitation wavelength maxima which were similar for all reaction conditions; however the enhancement of fluorescence intensity which accompanied binding of reagent was dependent on the ionic composition of the reaction medium. The modified enzyme remained active towards the active site titrant 7-(dimethylcarbamoyloxy)-N-methylquinolinium and retained its sensitivity towards inactivation by Zn2+. The results suggest that acetylcholinesterase as prepared contains several accessible thiol groups, and that the bound reagent may prove to be a useful probe of ligand-induced conformational changes in acetylcholinesterase.     

Purification and general properties of glutamate decarboxylase from Clostridium perfringens

1. A seven-step procedure for preparing highly purified glutamate decarboxylase from Clostridium perfringens is described. 2. The homogeneity of the pure enzyme was established by sucrose-density-gradient centrifugation and starch-gel electrophoresis. 3. The isoelectric point of the pure enzyme is about pH4.5 and the molecular weight is 290000. 4. The pH optimum for activity is 4.7. The pure enzyme is specific for l-glutamate; beta-hydroxyglutamate is decarboxylated at a lower rate. 5. Evidence is presented that each mol of enzyme contains 2mol of firmly bound pyridoxal 5-phosphate. 6. Resolution does not occur at acid pH; by dilution with neutral or alkaline buffers the enzyme is inactivated and the coenzyme is released. 7. Reconstitution of active enzyme was obtained by protecting the apoenzyme with thiol compounds.     

Effects of GroESL coexpression on the folding of nicotinoprotein formaldehyde dismutase from Pseudomonas putida F61

The overexpression of fdm, which encodes the formaldehyde dismutase from Pseudomonas putida F61, resulted in the formation of inclusion bodies made up of aggregated enzyme, leaving little activity in the soluble fraction of the transformant cells. On the other hand, coexpression of groESL along with fdm facilitated in vivo solubilization of the enzyme protein in its active form. When coexpressed with groESL, formaldehyde dismutase purified from E. coli had the same crystalline form (i.e., a regular octahedron) as the native enzyme, and like the native enzyme, it bound 1 mol of NAD(H) and 2 mol of zinc in each subunit.     

The flip-flop mechanism of the phosphorylation of yeast inorganic pyrophosphatase

1. An active monomeric form of inorganic pyrophosphatase from baker's yeast was prepared by maleylation of the protein at pH 10.5. 2. The dimeric and monomeric pyrophosphatase bound at non-catalytic sites 0.5 and 1.0 mol of slowly dissociating Pi per mol subunit, respectively. This stoichiometry was not affected on active site blockage with PPi. 3. Added Pi accelerated the dissociation of Pi from the dimeric but not monomeric enzyme. 4. Our results indicate a strong interaction to occur between the non-catalytic sites of two subunits of native pyrophosphatase which results in diminished stability of Pi binding to one of them.     

Interaction of L- and D-3-amino-1-chloro-2-pentanone with gamma-glutamylcysteine synthetase

The optical isomers of 3-amino-1-chloro-2-pentanone, which are the alpha-chloroketone analogs of L- and D-alpha-aminobutyrate, were synthesized and found to be highly potent irreversible inactivators of gamma-glutamylcysteine synthetase. These chloroketones are 20 to 30 times more active than L-2-amino-4-oxo-5-chlorpentanoate. L- and D-Glutamate, in the presence of Mg2+ or Mn2+, protect the enzyme against inactivation. The enzyme is almost completely inhibited by cystamine under conditions in which 0.5 mol of this compound is bound/mol of enzyme. Treatment of the enzyme with cystamne, which produces inhibition that is reversible by dithiothreitol, prevents the interaction of the new chloroketones, L-2-amino-4-oxo-5-chloropentanoate and methionine sulfoximine with the enzyme. The findings suggest that a sulfhydryl group at the active site interacts with the chloroketones and with cystamine and that the chloroketone inhibitors and cystamine bind to the enzyme as glutamine analogs. The data also suggest that a gamma-glutamyl-S-enzyme intermediate may be formed in the reaction catalyzed by this enzyme.     

Multiple phosphorylation of acetyl-CoA carboxylase in chick liver cells. A cyclic AMP-independent process.

When chick liver cells in monolayer culture were incubated with 32Pi in the presence of insulin, acetyl-CoA carboxylase became extensively labeled with 32Pi reaching a stoichiometry of 9 to 10 mol of phosphoryl group per mol of 240,000-dalton enzyme subunit. The covalently bound phosphate was found to be metabolically labile, turning over with a t1/2 of approximately 2 h (enzyme t1/2 approximately equal to 24 h). Addition of Bt2cAMP altered neither the rate nor extent of phosphorylation. Contrary to other reports, the fully phosphorylated acetyl-CoA carboxylase appears to be catalytically active.     

Nickel in the catalytically active hydrogenase of Alcaligenes eutrophus.

Nickel is a constituent of soluble and particulate hydrogenase of Alcaligenes eutrophus. Incorporation of 63Ni2+ revealed that almost the total nickel taken up by the cells was bound to the protein. Chromatography of a crude extract on diethylaminoethyl cellulose demonstrated an association of 63Ni2+ with soluble and particulate hydrogenase, supported by further analysis like polyacrylamide gel electrophoresis. Unspecific binding of 63Ni2+ to the protein was excluded by comparison with a mutant extract free of hydrogenase protein. X-ray fluorescence analysis of the homogeneous soluble hydrogenase indicated the presence of 2 mol of nickel per mol of enzyme, whereas the amount of nickel determined by incorporation of 63Ni2+ was calculated to be approximately 1 mol/mol of enzyme. Cells grown under nickel limitation contained catalytically inactive, but serologically active, soluble and particulate hydrogenase. The immunochemical reactions were only partially identical with the enzyme from nickel-cultivated cells indicating a structural modification of the proteins in the absence of nickel. It is concluded that nickel is essential for the catalytic activity of hydrogenase and not involved as a regulatory component in the synthesis of this enzyme.