Improved purification, amino acid analysis and molecular weight of homogenous D-amino acid oxidase from pig kidney

Fluorescence properties of reduced flavins and flavoproteins

Fluorescence lifetimes and polarized emission properties of reduced flavin were measured using several model compounds and flavoproteins. Depending on the conditions of solvent and temperature or reduction method the lifetimes vary between 1 and 15 ns. The longer lifetime values are found in several forms of reduced lactate oxidase, in which a good correlation exists between fluorescence intensity and lifetime. In practically all flavoproteins the fluorescence is heterogeneous. Several mechanisms are proposed to explain the observed heterogeneity in lifetimes. The reduced models in glycerol at subzero temperature exhibit high degrees of polarization of the fluorescence, whereas distinct depolarization is encountered in several reduced flavoproteins suggesting a certain mobility of the flavin chromophor.

Suicide inactivation of the flavoenzyme D-lactate dehydrogenase by alpha-hydroxybutynoate

The acetylenic alpha-hydroxy acid 2-hydroxy-3-butynoate (alpha HB) is a substrate and an irreversible inactivator of the FAD-containing flavoenzyme D-lactate dehydrogenase from Megasphaera elsdenii. On the average, the enzyme undergoes five catalytic turnovers with alpha HB in air at pH 7.0 before being inactivated. Irreversible inactivation is due to the conversion of the flavin to a pink adduct with visible absorption peaks at 522, 382, and 330 nm and weak fluorescence with an emission maximum at 635 nm. The adduct is stable and can be released from the enzyme and purified. It retains a structure analogous to FAD since it binds to the FAD-specific apo-D-amino acid oxidase. It can be further converted to an FMN analogue with phosphodiesterase which binds to the FMN-specific apoflavodoxin. Experiments were conducted to test whether inactivation was initiated by an alpha HB allene carbanion or the dehydrogenation product of alpha HB. Kinetic studies proved inconclusive in that a rapid equilibrium between an oxidized enzyme--allene carbanion pair and reduced enzyme--keto acid pair would make these two species kinetically equivalent. The olefinic substrate 2-hydroxy-3-butenoate, however, produced no flavin adduct. Since the keto acid derived from the oxidation of this alpha-hydroxy acid is expected to be as reactive as 2-keto-3-butynoate, it is concluded that an allene carbanion produced by abstraction of the alpha-hydrogen of alpha HB is the reactive species which covalently adds to the flavin.

Properties of D-amino acid oxidase covalently modified upon its oxidation of D-propargylglycine

Upon oxidation of D-propargylglycine by D-amino acid oxidase, the enzyme is converted by covalent alkylation to catalytic species with different properties from those of native enzyme. At least five distinct modified enzyme species are present in the preparation, as determined by gel electro-focusing. Individual characterization of the components has not yet been attempted. The combined kinetic and spectral properties of the preparation have been studied. The modified enzymes have a marked preference for hydrophobic amino acids: the rates of oxidation decrease in the series D-phenylalanine, D-methionine, D-norleucine, D-norvaline, D-alpha-aminobutyrate, D-alanine. In addition, the observed Kms of the amino acids are increased, especially those of the smaller substrates (D-alanine and D-alpha-aminobutyrate). A primary kinetic isotope effect is observed upon oxidation of amino acids by the modified enzymes, evidence that this catalysis exhibits a different rate-determining step from catalysis by native enzyme. The modified apoenzyme exhibits intense absorbance at 318--320 nm, not present in native enzyme. This chromophore can be partially (75%) removed by treatment of the modified enzyme with hydrazine. However, the activity of native enzyme is not substantially restored by this process, suggesting the existence of superficial alkylations in addition to the modification responsible for the observed changes in kinetic parameters.

Light-mediated reduction of flavoproteins with flavins as catalysts

It has been found that small amounts of free flavins greatly accelerate the photochemical reduction of flavoproteins both to the radical and fully reduced oxidation states. This catalytic effect has been shown to be due to the rapid photochemical reduction of the free flavin to its fully reduced state, followed by its reaction with the flavoprotein to yield flavoprotein radical and by its reaction with flavoprotein radical to yield fully reduced flavoprotein. Evidence is presented that the same route may occur with flavoproteins in the absence of added flavins. In this case the photoreduction is mediated by the small equilibrium concentration of free flavin coenzyme present in a flavorprotein solution. Hence, it is suggested that flavoprotein reduction with EDTA as photosubstrate does not involve an excited state of the holoprotein, nor contact of EDTA with the enzyme, but exchange of electrons between enzyme flavin and free reduced flavin.

Effect of hydrophobic probes on the higher structure of D-amino acid oxidase

1. The holoenzyme of D-amino acid oxidase [D-amino acid: O2 oxidoreductase (deaminating), EC 1.4.3.3] was found to combine with 1-anilinonaphthalene-8-sulfonate without liberation of its coenzyme, FAD. No energy transfer interaction was found to occur between the bound dye and FAD of the holoenzyme. On the other hand, when the apoenzyme was bound to the dye and then to FAD, energy transfer interaction between the bound dye and bound FAD was observed. In both cases, the dye competes with the substrate, D-alanine. It is concluded that the dye bound to the holoenzyme is oriented in such a special manner that the mutual orientation factor between the dye and FAD becomes very small in magnitude. 2. When the apoenzyme combined with the dye, the monomer-dimer equilibrium of the apoenzyme shifted towards the dimer. On the other hand, 4-monobenzoylamido-4'-aminostilbene-2,2'-disulfonate combined with the apoenzyme to induce monomerization.

Isolation, characterization and partial sequence of cyanogen bromide fragments and thiol peptides from pig kidney D-amino-acid oxidase

A partial characterization of the primary structure of D-amino-acid oxidase (D-Amino-acid:oxygen oxidoreductase (deaminating), EC 1.4.3.3.) from hog kidney has been achieved by a CNBr cleavage of the 14C-carboxymethylated protein. Four fragments have been isolated and purified and their alignment made possible by overlapping with methionine-containing peptides derived from tryptic digestion of the 14C-carboxymethylated protein. A partial sequencing of the CNBr fragments has been carried out by the automated Edman procedure and by manual sequence analysis. Chymotryptic peptides containing the 5 alkylated thiols of the monomer enzyme (Curti, B., Ronchi, S., branzoli, U., Ferri, G. and Williams, Jr., C. H. (1973) Biochim. Biophys. Acta 327, 266-273) have been isolated and their sequence determined. The present results do not show any significant homologies with the known sequences of other flavoproteins.