Sequence of reactions which follows enzymatic oxidation of allylglycine

The pathway following flavoprotein-catalyzed oxidation of allylglycine (2-amino-4-pentenoate) has been studied and found to be dependent on the incubation conditions. In N-2-hydroxyethyl-N'-2-ethanesulfonic acid (Hepes) buffer, the oxidation product 2-iminium-4-pentenoate predominantly reacts to form 2-amino-2,4-pentadienoate, a strong noncovalent inhibitor of D-amino-acid oxidase. However, in pyrophosphate buffer, the more rapid reaction is hydrolysis to form 2-keto-4-pentenoate, which has been found to be a substrate for L-lactic dehydrogenase. 2-Keto-4-pentenoate is in rapid equilibrium with 2-hydroxy-2,4-pentadienoate, which is also a strong noncovalent inhibitor of D-amino-acid oxidase. In both systems, these metastable intermediates react in subsequent slower steps to yield trans-2-keto-3-pentenoate, which accumulates in the incubation. Syntheses of trans-2-amino- and trans-2-keto-3-pentenoate are described. Comparisons between the reactivities of acetylenic and olefinic species have been made based on the differences between this pathway and that following oxidation of propargylglycine [Marcotte, P., and Walsh, C. (1978), Biochemistry 17 (preceding paper in this issue)].

Sequence of reactions which follows enzymatic oxidation of propargylglycine

The nonenzymatic reactions which follow enzymatic oxidation of the gamma-delta acetylenic amino acid propargylglycine (2-amino-4-pentynoate) have been studied. The product which accumulates in solution has been identified as 2-amino-4-hydroxy-2,4-pentadienoate gamma-lactone, formed by intramolecular attack of the carboxylate anion on the electrophilic fourth carbon of 2-iminium-3,4-pentadienoate. This previously unknown substance was characterized by its reactions in acid and base and by its nuclear magnetic resonance spectrum. The lactone is preceded in the pathway by 2-amino-2-penten-4-ynoate, a transient electron-rich species which binds tightly to D-amino-acid oxidase and induces a charge-transfer complex with the electron-deficient bound flavin coenzyme. The aminediene lactone is converted by base treatment to 2-amino-4-keto-2-pentenoate, which is also a strong inhibitor of D-amino-acid oxidase and induces a charge-transfer complex.

The formation of dihydrodiols by the chemical or enzymic oxidation of benz[a] anthracene and 7,12-dimethylbenz[a] anthracene

When benz[a] anthracene was oxidised in a reaction mixture containing ascorbic acid, ferrous sulphate and EDTA, the non-K-region dihydrodiols, trans-1,2-dihydro-1,2-dihydroxybenz[a] anthracene and trans-3,4-dihydro-3,4-dihydroxybenz[a] anthracene together with small amounts of the 8,9- and 10,11-dihydrodiols were formed. When oxidised in a similar system, 7,12-dimethylbenz[a] anthracene yielded the K-region dihydrodiol, trans-5,6-dihydro-5,6-dihydroxy-7,12-dimethylbenz[a] anthracene and the non-K-region dihydrodiols, trans-3,4-dihydro-3,4-dihydroxy-7,12-dimethylbenz[a] anthracene, trans-8,9-dihydro-8,9-dihydroxy-7,12-dimethylbenz[a] anthracene, trans-10,11-dihydro-10,11-dihydroxy-7,12-dimethylbenz[a] anthracene and a trace of the 1,2-dihydrodiol. The structures and sterochemistry of the dihydrodiols were established by comparisons of their UV spectra and chromatographic characteristics using HPLC with those of authentic compounds or, when no authentic compounds were available, by UV, NMR and mass spectral analysis. An examination by HPLC of the dihydrodiols formed in the metabolism, by rat-liver microsomal fractions, of benz[a] anthracene and 7,12-dimethylbenz[a] anthracene was carried out. The metabolic dihydriols were identified by comparisons of their chromatographic and UV or fluorescence spectral characteristics with compounds of known structures. The principle metabolic dihydriols formed from both benz[a] anthracene and 7,12-dimethylbenz[a] anthracene were the trans-5,6- and trans-8,9-dihydrodiols. The 1,2- and 10,11-dihydrodiols were identified as minor products of the metabolism of benz [a] anthracene and the tentative identification of the trans-3,4-dihydriol as a metabolite was made from fluorescence and chromatographic data. The minor metabolic dihydriols formed from 7,12-dimethylbenz[a] anthracene were the trans-3,4-dihydrodiol and the trans-10,11-dihydriol but the trans-1,2-dihydrodiol was not detected in the present study.

Mechanism and kinetics of iron release from ferritin by dihydroflavins and dihydroflavin analogues

Dihydroflavins reductively release iron rapidly and quantitatively from purified horse spleen or horse heart ferritin. The NAD(P)H:flavin oxidoreductase from Beneckea harveyi is used to generate a constant concentration of dihydroflavin permitting a continuous assay for complete iron release. Sepharose-linked dihydroflavins are not competent to release ferritin iron, demonstrating that the dihydroflavin must pass through the channels of the protein shell prior to iron reduction. Several experiments fail to show any specific flavin binding site, though dihydroflavins do display saturation kinetics with very high apparent Km's. The rates of iron release by a number of dihydroflavin analogues show that the electron transfer is significantly rate determining in iron release by dihydroriboflavin, while diffusion of the dihydroflavin through the protein channel is slow in the release of iron by dihydroFMN. The rate of iron release is also dependent on the initial content of iron, having a maximum at 1200 iron atoms per ferritin.

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.

The preparation of dihydrodiols from 7-methylbenz[a]-anthracene

The products formed when the carcinogenic polycyclic hydrocarbon 7-methylbenz[a] anthracene is oxidized with an ascorbic acid-ferrous sulphate mixture have been investigated. All 5 possible dihydrodiols were formed and the isolation of the 3 non-K-region dihydrodiols, trans-1,2-dihydro-1,2-dihydroxy-7-methylbenz[a]anthracene, trans-3,4-dihydro-3,4-dihydroxy-7-methylbenz[a] anthracene and trans-8,9-dihydro-8,9-dihydroxy-7-methylbenz[a] anthracene is described. The purification of the dihydrodiols was carried out by thin-layer (TLC) followed by preparative high pressure liquid chromatography (HPLC). The ultra-violet, spectral and nuclear magnetic resonance (NMR) characteristics of the dihydrodiols are reported and the data used to assign the proposed structures. An explanation for the unusual preferred conformation which the 8,9-dihydrodiol adopts is advanced.

Chemical and enzymatic properties of riboflavin analogues

The chemical and enzymatic properties of 26 analogues of riboflavin are presented. These analogues include both endo- and exocyclically substituted isoalloxazines with redox potentials from -370 to -128 mV. Physical and chemical data such as the electronic absorption spectra, pKas, and redox potentials of the analogues are presented and are discussed with respect to preferred tautomeric and resonance forms. Like riboflavin, most of the analogues are shown to be catalytic oxidants of dihydro-5-deazaflavins. Analogue binding to egg white binding apoprotein has been quantitated and serves to determine the origins of binding site specificity for this protein. Nearly all of the analogues that possess D-ribityl groups are found to be processed to the FAD level by the flavokinase/FAD synthetase system of Brevibacterium ammoniagenes. Most extensively studied are the reactivities of the analogues with the NAD(P)H:flavin oxidoreductase of Beneckea harveyi. Many of the analogues are substrates in this enzymatic redox reaction, and a linear free energy-rate relation (log Vmax vs. E0' of the analogue) is seen that parallels similar relationships in the nonenzymatic oxidation of dihydro-5-deazaflavins. This suggests a common mechanism for the reactions of such diverse flavins as riboflavin, 5-deazariboflavin, and 1-deazariboflavin.

The development of the epithelium and its innervation in the avian extra-pulmonary respiratory tract

The development of the epithelium and its innervation was investigated with the electron microscope in the larynx and trachea of embryo domestic fowls between the 10th and 21st days of incubation. From days 10 to 16 the airways were lined by undifferentiated cells which on day 15 or 16 became arranged in two distinct layers. Differentiation of the epithelial cells commenced on day 17, so that at hatching on day 21 fully differentiated ciliated cells, goblet cells, non-ciliated columnar cells, granular cells and basal cells were arranged into a simple or pseudostratified columnar epithelium. Comparisons are made with the differentiation of the respiratory epithelium in other vertebrates. Intra-epithelial axons were first seen on day 15 and increased rapidly in number. The varicose axon profiles contained both granular and agranular vesicles. The axons lay close to most types of epithelial cell, and some approached near to the surface of the epithelium. A synaptic complex, possibly efferent, involving a granular epithelial cell is described.

The metabolic activation of 7-methylbenz(a)anthracene in mouse skin

The metabolism of 7-methylbenz(a)anthracene by rat-liver preparations and by mouse skin has been studied using a combination of thin-layer and high pressure liquid chromatography and all five possible trans-dihydrodiols have been detected as metabolites but in different proportions. The roles of these dihydrodiols and of the related vicinal diol-epoxides in the metabolic activation of 7-methylbenz(a)anthracene in mouse skin has been studied using Sephadex LH-20 column chromatography. The results show that the hydrocarbon-nucleic acid products formed in mouse skin in vivo most probably arise from 3,4-dihydro-3,4-dihydroxy-7-methylbenz(a)anthracene 1,2-oxide which, on the basis of this and other evidence, appears to be the reactive intermediate involved in the metabolic activation of 7-methylbenz(a)anthracene in this tissue.

Studies on the intramolecular and intermolecular kinetic isotope effects in pyruvate carboxylase catalysis

A deuterium kinetic isotope effect of 2.1 was observed when [2H3]pyruvate was used as the substrate for pyruvate carboxylase. The effect is on Vmax/Km alone and disappears at infinite substrate concentration. This is interpreted to mean that the slowest step in the overall catalysis is in the half-reaction involving the carboxylation of enzymebiotin by ATP and HCO3-. A tritium intramolecular isotope effect of 4.8 and an intermolecular effect of 1.2 were also observed. The former was interpreted as the isotope effect on the "effective kcat", while the latter the one on V max/Km. With these data, the rate constant for binding of pyruvate was estimated to be 4.5 X 10(6) M-1 min-1, and the deuterium kinetic isotope effect on the catalytic step to be 3.1. Relative values for various rate constants were also obtained. Fluoropyruvate was also shown to be a substrate, reacting six times slower. A deuterium kinetic isotope effect of 1.5 was observed, which remained even at infinite substrate concentration. This is interpreted to mean that the slowest step in the overall catalysis is now the carboxylation of fluoropyruvate.

Vinylglycine and proparglyglycine: complementary suicide substrates for L-amino acid oxidase and D-amino acid oxidase

Proparglyglycine (2-amino-4-pentynoate) and vinylglycine (2-amino-3-butenoate) have been examined as substrates and possible inactivators of two flavo enzymes, D-amino acid oxidase from pig kidney and L-amino acid oxidase from Crotalus adamanteus venom. Vinylglycine is rapidly oxidized by both enzymes but only L-amino acid oxidase is inactivated under assay conditions. The loss of activity probably involves covalent modification of an active site residue rather than the flavin adenine dinucleotide coenzyme and occurs once every 20000 turnovers. We have confirmed the recent observation (Horiike, K, Hishina, Y., Miyake, Y., and Yamano, T. (1975) J, Biochem. (Tokyo), 78, 57) that D-proparglglycine is oxidized with a time-dependent loss of activity by D-amino acid oxidase and have examined some mechanistic aspects of this inactivation, The extent of residual oxidase activity, insensitive to further inactivation, is about 2%, at which point 1.7 labels/subunit have been introduced with propargly[2-14C]glycine as substrate. L-Proparglyclycine is a substrate but not an inactivator of L-amino acid oxidase and the product ahat accumulats in the nonnucleophilic N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid buffer is acetopyruvate. In the presence of butylamine HCl, a species with lambdaman 317 nm (epsilon = 15 000) accumulates that may be a conjugated eneamine adduct. The same species accumulates from D-amino acid oxidase oxidation of D-propargylglycine prior to inactivation; the inactivated apo D-amino acid oxidase has a new peak at 317 nm that is probably a similar eneamine. A likely inactivating species is 2-keto-3,4-pentadienoate arising from facile rearrangement of the expected initial product 2-keto 4 pentynoate. Vinylglycine and proparglyglycine show inactivation specificity, then, for L-and D-amino acid oxidase, respectively.

Inactivation of the phosphoenolpyruvate-dependent phosphotransferase system in various species of bacteria by vinylglycolic acid

The alkenoic hydroxyacid 2-hydroxy-3-butenoic acid (vinylglycolate) specifically inhibited the phosphotransferase system in a variety of bacteria while not affecting respiration-coupled transport systems.