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PubMed1989

PQQ: biosynthetic studies in Methylobacterium AM1 and Hyphomicrobium X using specific 13C labeling and NMR.

Using 13C labeling and NMR spectroscopy we have determined biosynthetic precursors of pyrroloquinoline quinone (PQQ) in two closely related serine-type methylotrophs, Methylobacterium AM1 and Hyphomicrobium X. Analysis of the 13C-labeling data revealed that PQQ is constructed from two amino acids: the portion containing N-6,C-7, 8, 9 and the two carboxylic acid groups, C-7' and 9', is derived-intact -from glutamate. The remaining portion is derived from tyrosine; the phenol side chain provides the six carbons of the ring containing the orthoquinone, whereas internal cyclization of the amino acid backbone forms the pyrrole-2-carboxylic acid moiety. This is analogous to the cyclization of dopaquinone to form dopachrome. Dopaquinone is a product of the oxidation of tyrosine (via dopa) in reactions catalyzed by monophenol monooxygenase (EC 1.14.18.1). Starting with tyrosine and glutamate, we will discuss possible biosynthetic routes to PQQ.

Amino Acids↗
PubMed1980

Biochemical similarities among strains of Aureobasidium pullulans (de Bary) Arnaud.

Seventy-seven properties of Aureobasidium pullulans including utilization of various carbon sources, decomposition of the lignin-cellulose complex and the respective enzymes were checked in the present communication. According to these properties the group of 43 strains was separated in three parts, out of which two groups were found to belong to varieties A. pullulans var. pullulans and A. pullulans var. melanigenum. The third group formed a marginal part. The two varieties differed in numerous biochemical markers, particularly in the absence of monophenol monooxygenase in the variety A. pullulans var. pullulans.

Carbohydrate Metabolism↗
PubMed1976

Boophilus microplus: characterization of enzymes introduced into the host.

A number of enzymes, presumably secreted by larvae of B. microplus under natural feeding conditions, have been investigated in the skin of previously unexposed calves 4 h after infestation at the attachment site. Carboxylic ester hydrolase activity was demonstrated in the dermis, immediately adjacent to the mouthparts, or in the attachment cone, depending on substrate and reaction pH. The carboxylic ester hydrolase acting on naphthol AS-D acetate (2-acetoxy-3-naphthoic-O-toluidide) at pH 7-1 was characteristically found in the dermis and not in the attachment cone. The use of specific inhibitors showed that this enzyme was primarily a B-esterase or carboxylesterase with possibly a small portion of C-esterase or acetylesterase. It is postulated that carboxylic ester hydrolase could contribute to the dilation observed in the subepidermal capillaries adjacent to the attachment sites of unexposed animals, through the formation of plasma kinins. Other enzymes demonstrated in the dermis, adjacent to the mouthparts, were triacylglycerol lipase, as an aggregated deposit, and small amounts of aminopeptidase (microsomal) and monophenol monooxygenase. Aminopeptidase (microsomal) was also demonstrated in the attachment cone or adjacent epidermis, according to the substrate used. No activity was found in the host tissue, in association with the attachment site, for either alkaline or acid phosphatase, acetylcholinesterase or cholinesterase, peroxidase or amine oxidase (flavin-containing), despite the intense histochemical reaction for the latter in the tissues of larvae.

Acetylcholinesterase↗
PubMed1974

Genetic control of melanization: isolation and analysis of amelanotic variants from cultured melanotic melanoma cells.

MELANOMA CELLS EXPRESS A PHENOTYPE THAT IS EASY TO RECOGNIZE: the synthesis of melanin. We used this marker to isolate clones of amelanotic variants from large populations of wild-type melanotic clones. Cloudman mouse melanoma (S91, clone M-3, CCL 53.1) was chosen as the parental line because the cells are highly pigmented, grow well as clones in soft agar, and fuse readily with Sendai virus. Subclones (10(7)) of this line were screened without prior mutagenesis, and nine amelanotic variants were isolated. The mutagen ethylmethanesulfonate increased the frequency of variants by three to four orders of magnitude. Wild-type cells had both basal and melanocyte stimulating hormone-inducible tyrosinase activities. The four amelanotic variants that we have examined to date all behaved similarly: they lost basal tyrosinase (EC 1.14.18.1; monophenol monooxygenase) activity but retained melanocyte stimulating hormone-inducible activity; they contained Stage-II melanosomes but no melanized melanosomes; they exhibited growth characteristics similar to those of wild-type cells in culture but produced fewer tumors in mice.

Animals↗
PubMed1974

Cholera toxin mimics melanocyte stimulating hormone in inducing differentiation in melanoma cells.

Cholera toxin (choleragen) and melanocyte stimulating hormone alter within hours the morphology of melanoma cells in culture, and they slow the growth of serum-stimulated cells. After 7-10 days, cells exposed to choleragen or hormone show increased size and a fibroblastic growth pattern. Tyrosinase (EC 1.14.18.1; monophenol monooxygenase) activity increases after 3 days in the presence of 10(-8) M hormone or 10(-10) M choleragen. Binding studies with (125)I-labeled choleragen indicate that although a melanoma cell can bind a maximum of 10(6) molecules of cholera toxin, only about 4000 binding sites must be occupied to achieve maximum stimulation of tyrosinase activity. Melanocyte stimulating hormone and choleragen probably have different membrane-binding sites. After exposure to choleragen for 5 min, membrane adenylate cyclase (EC 4.6.1.1) activity increases dramatically upon further incubation of intact cells for several hours at 37 degrees and falls slowly to basal values over a period of more than 10 days. Hormone stimulation of adenylate cyclase is rapidly reversed by washing the cells, but subsequent restimulation of cyclase by the hormone is impaired. These studies indicate that cAMP mediates the effects of melanocyte stimulating hormone on growth and morphology as well as on tyrosinase activity. Cholera toxin may permanently activate the available adenylate cyclase molecules, and the protracted decay of stimulation that follows may reflect the biological turnover of adenylate cyclase molecules in these cells.

Adenylyl Cyclases↗
PubMed1985

Stability of tryptophan during food processing and storage. 1. Comparative losses of tryptophan, lysine and methionine in different model systems.

The stability of tryptophan was evaluated in several different food model systems using a chemical method (high pressure liquid chromatography after alkaline-hydrolysis) and rat assays. Losses of tryptophan were compared with the losses of lysine and methionine. Whey proteins stored in the presence of oxidizing lipids showed large losses of lysine and extensive methionine oxidation but only minor losses of tryptophan as measured chemically. The observed decrease in bioavailable tryptophan was explained by a lower protein digestibility. Casein treated with hydrogen peroxide to oxidize all methionine to methionine sulphoxide showed a 9% loss in bioavailable tryptophan. When casein was reacted with caffeic acid at pH 7 in the presence of monophenol monooxygenase (tyrosinase; EC 1.14.18.1), no chemical loss of tryptophan occurred, although fluorodinitrobenzene-reactive lysine fell by 23%. Tryptophan bioavailability fell 15%, partly due to an 8% reduction in protein digestibility. Alkali-treated casein (0.15 M-sodium hydroxide, 80 degrees, 4 h) did not support rat growth. Chemically-determined tryptophan, available tryptophan and true nitrogen digestibility fell 10, 46 and 23% respectively. Racemization of tryptophan was found to be 10% (D/(D+L)). In whole-milk powder, which had undergone "early' or "advanced' Maillard reactions, tryptophan, determined chemically or in rat assays, was virtually unchanged. Extensive lysine losses occurred. It was concluded that losses of tryptophan during food processing and storage are small and of only minor nutritional importance, especially when compared with much larger losses of lysine and the more extensive oxidation of methionine.

Animals↗
PubMed1979

Ultrastructure and cytochemical localization of laccase in two strains of Leptosphaerulina briosiana (Pollaci) Graham and Luttrell.

Substrate specificity tests were used to identify the presence of laccase in two strains of Leptosphaerulina briosiana (Poll.) Graham and Luttrell, an ascomycete which causes leaf spot in alfalfa. Cytochemical localization of monophenol monooxygenase (laccase) as well as the ultrastructures of the two strains were investigated. Laccase was observed in the outer layers of the cell walls of both strains. The ultrastructures of vegetative hyphae of both strains were typical of those found in most ascomycetes.

Ascomycota↗
PubMed1985

Primary structure of tyrosinase from Streptomyces glaucescens.

The complete amino acid sequence of Streptomyces glaucescens tyrosinase is reported. The molecule consists of 273 amino acids and has a Mr of 30 900 including two copper atoms. The primary structure was determined by a combination of amino acid and DNA sequence analysis. Peptide sequence information was derived from the cyanogen bromide, tryptic, and thermolytic fragments of apotyrosinase by automated Edman degradation and aminopeptidase M and carboxypeptidase C digestions. The nucleotide sequence of the tyrosinase gene cloned into the PvuII site of pBR322 was determined. The enzyme contains no apparent leader peptide despite the fact that it is secreted into the culture medium. As observed for a number of different Streptomyces genes, the tyrosinase gene shows a strong preference (97%) for codons ending in G or C. A comparison of the amino acid sequence of Streptomyces glaucescens tyrosinase with that of Neurospora crassa tyrosinase reveals an overall sequence homology of only 24.2%. However, the sequence homology is much higher in those regions thought to be involved in metal binding of the binuclear active site copper of this monooxygenase.

Amino Acid Sequence↗
PubMed2006

Crystallographic evidence that the dinuclear copper center of tyrosinase is flexible during catalysis.

At high resolution, we determined the crystal structures of copper-bound and metal-free tyrosinase in a complex with ORF378 designated as a "caddie" protein because it assists with transportation of two CuII ions into the tyrosinase catalytic center. These structures suggest that the caddie protein covers the hydrophobic molecular surface of tyrosinase and interferes with the binding of a substrate tyrosine to the catalytic site of tyrosinase. The caddie protein, which consists of one six-strandedbeta-sheet and one alpha-helix, has no similarity with all proteins deposited into the Protein Data Bank. Although tyrosinase and catechol oxidase are classified into the type 3 copper protein family, the latter enzyme lacks monooxygenase activity. The difference in catalytic activity is based on the structural observations that a large vacant space is present just above the active center of tyrosinase and that one of the six His ligands for the two copper ions is highly flexible. These structural characteristics of tyrosinase suggest that, in the reaction that catalyzes the ortho-hydroxylation of monophenol, one of the two Cu(II) ions is coordinated by the peroxide-originated oxygen bound to the substrate. Our crystallographic study shows evidence that the tyrosinase active center formed by dinuclear coppers is flexible during catalysis.

Amino Acid Sequence↗
PubMed2006

Structural insights into dioxygen-activating copper enzymes.

Copper-containing enzymes that react with O(2) play a key role in many biological processes. Mononuclear, dinuclear and trinuclear copper centers function in O(2) binding, activation and subsequent substrate oxidation. Recent advances in the structural biology of O(2)-activating copper enzymes range from the identification of novel copper centers, such as that of particulate methane monooxygenase, to the elucidation of the details of O(2) binding and reactivity in peptidylglycine alpha-hydroxylating monooxygenase. Structures of phenoxazinone synthase and Fet3 contribute to our understanding of multicopper oxidases. Additionally, details of the tyrosinase structure provide new insight into how dicopper sites confer substrate specificity. A common theme for each of these enzymes is that the protein scaffold plays a major role in dictating the overall function.

Catalytic Domain↗
PubMedDate not supplied

Melanogenesis, tyrosinase expression, and reproductive differentiation in black and white truffles (Ascomycotina).

White and black truffles of the genus Tuber are Ascomycotina as well as Neurospora crassa, which expresses tyrosinase dependently on the reproductive cycle. Tyrosinase expression dependent on reproductive differentiation has been also described in black truffles. We present novel and comparative work on melanogenic activities in black and white truffles that both express true tyrosinases. L-tyrosine 3-monooxygenase and L-DOPA oxidase activities colocalize as histochemically detected and are similarly located in white and black truffles, from the hypothecium through the sporogenic hyphae to asci and spores. Sulfur components of truffle flavours reversibly inhibit tyrosinase. The respiratory phenotype of truffle mitochondria is discussed in relation to reproductive differentiation and melanogenesis.

Ascomycota↗
PubMed2002

The Arabidopsis REF8 gene encodes the 3-hydroxylase of phenylpropanoid metabolism.

The activity of p-coumarate 3-hydroxylase (C3H) is thought to be essential for the biosynthesis of lignin and many other phenylpropanoid pathway products in plants; however, no conditions suitable for the unambiguous assay of the enzyme are known. As a result, all attempts to purify the protein and clone its corresponding gene have failed. By screening for plants that accumulate reduced levels of soluble fluorescent phenylpropanoid secondary metabolites, we have identified a number of Arabidopsis mutants that display a reduced epidermal fluorescence (ref) phenotype. Using radiotracer-feeding experiments, we have determined that the ref8 mutant is unable to synthesize caffeic acid, suggesting that the mutant is defective in a gene required for the activity or expression of C3H. We have isolated the REF8 gene using positional cloning methods, and have verified that it encodes C3H by expression of the wild-type gene in yeast. Although many previous reports in the literature have suggested that C3H is a phenolase, the isolation of the REF8 gene demonstrates that the enzyme is actually a cytochrome P450-dependent monooxygenase. Although the enzyme accepts p-coumarate as a substrate, it also exhibits significant activity towards other p-hydroxylated substrates. These data may explain the previous difficulties in identifying C3H activity in plant extracts and they indicate that the currently accepted version of the lignin biosynthetic pathway is likely to be incorrect.

Arabidopsis↗
PubMed1983

Neurospora tyrosinase: structural, spectroscopic and catalytic properties.

Tyrosinase is a copper containing monooxygenase catalyzing the formation of melanin pigments and other polyphenolic compounds from various phenols. This review deals with the recent progress on the molecular structure of the enzyme from Neurospora crassa and the unique features of the binuclear active site copper complex involved in the activation of molecular oxygen and the binding of substrates. The results of the spectroscopic properties of Neurospora tyrosinase will also be discussed in the light of the structural similarity of the copper complex in the oxygen binding hemocyanins.

Amino Acid Sequence↗
PubMed1989

Microbial enzymes for oxidation of organic molecules.

Enzymatic systems employed by microorganisms for oxidative transformation of various organic molecules include laccases, ligninases, tyrosinases, monooxygenases, and dioxygenases. Reactions performed by these enzymes play a significant role in maintaining the global carbon cycle through either transformation or complete mineralization of organic molecules. Additionally, oxidative enzymes are instrumental in modification or degradation of the ever-increasing man-made chemicals constantly released into our environment. Due to their inherent stereo- and regioselectivity and high efficiency, oxidative enzymes have attracted attention as potential biocatalysts for various biotechnological processes. Successful commercial application of these enzymes will be possible through employing new methodologies, such as use of organic solvents in the reaction mixtures, immobilization of either the intact microorganisms or isolated enzyme preparations on various supports, and genetic engineering technology.

Bacteria↗
PubMed2002

Spectroscopic characterization of the electronic changes in the active site of Streptomyces antibioticus tyrosinase upon binding of transition state analogue inhibitors.

The dinuclear copper enzyme tyrosinase (Ty) from genetically engineered Streptomyces antibioticus has been investigated in its paramagnetic half-met form [Cu(I)-Cu(II)]. The cw EPR, pulsed EPR, and hyperfine sublevel correlation spectroscopy (HYSCORE) experiments on the half-met-Ty and on its complexes with three different types of competitive inhibitor are reported. The first type includes p-nitrophenol, a very poor substrate for the monooxygenase activity of Ty. The second type comprises hydroxyquinones, such as kojic acid and l-mimosine, and the third type of inhibitor is represented by toluic acid. The electronic and structural differences of the half-met-Ty form induced at the cupric site by the different inhibitors have been determined. Probes of structural effects are the hyperfine coupling constants of the non coordinating Ndelta histidyl nitrogens. By using the available crystal structures of hemocyanin as a template in combination with the spectroscopic results, a structural model for the active site of half-met-Ty is obtained and a model for the binding modes of both mono- and diphenols could be proposed.

Binding Sites↗
PubMed2004

O2 activation by binuclear Cu sites: noncoupled versus exchange coupled reaction mechanisms.

Binuclear Cu proteins play vital roles in O(2) binding and activation in biology and can be classified into coupled and noncoupled binuclear sites based on the magnetic interaction between the two Cu centers. Coupled binuclear Cu proteins include hemocyanin, tyrosinase, and catechol oxidase. These proteins have two Cu centers strongly magnetically coupled through direct bridging ligands that provide a mechanism for the 2-electron reduction of O(2) to a mu-eta(2):eta(2) side-on peroxide bridged Cu(II)(2)(O(2)(2-)) species. This side-on bridged peroxo-Cu(II)(2) species is activated for electrophilic attack on the phenolic ring of substrates. Noncoupled binuclear Cu proteins include peptidylglycine alpha-hydroxylating monooxygenase and dopamine beta-monooxygenase. These proteins have binuclear Cu active sites that are distant, that exhibit no exchange interaction, and that activate O(2) at a single Cu center to generate a reactive Cu(II)/O(2) species for H-atom abstraction from the C-H bond of substrates. O(2) intermediates in the coupled binuclear Cu enzymes can be trapped and studied spectroscopically. Possible intermediates in noncoupled binuclear Cu proteins can be defined through correlation to mononuclear Cu(II)/O(2) model complexes. The different intermediates in these two classes of binuclear Cu proteins exhibit different reactivities that correlate with their different electronic structures and exchange coupling interactions between the binuclear Cu centers. These studies provide insight into the role of exchange coupling between the Cu centers in their reaction mechanisms.

Binding Sites↗
PubMed1999

The effect of intrastriatal injection of liposome-entrapped tyrosinase on the dopamine levels in the rat brain.

Parkinson's disease is a neurodegenerative disorder which is mainly characterized by degeneration of the dopaminergic cells in the nigro-striatal system. Due to a lowered L-tyrosine 3-monooxygenase activity, L-tyrosine is not sufficiently transformed to L-DOPA. To date the most common therapy is the administration of the dopamine precursor L-DOPA, with severe collateral effects. Therefore, the substitution of the lacking tyrosine hydroxylase with tyrosinase might be a novel therapeutical approach that would generate specifically L-DOPA from L-tyrosine. We present here evidence that stereotaxic injection of liposome-entrapped tyrosinase is able to significatively increase the levels of dopamine in the rat brain. The catecholamines L-DOPA, dopamine, L-epinephrine, L-norepinephrine were extracted by acid treatment from the brains and detected by HPLC.

Animals↗