Note. Xylose Production from Durum Wheat Bran: Enzymic versus Chemical Methods

Both acidic and enzymic hydrolytic processes of durum wheat bran to obtain high xylose syrups are discussed in terms of yields and operative conditions. Mild sulphuric hydrolysis gave the best results (400mg of aldopentoses per gram of pre-extracted wheat bran within 4–6 h) whereas hydrochloric and especially phosphoric acids led to low yields. Preliminary work with a highly active recombinant xylanase (endoxylanase) led to deep solubilisation of arabinoxylans, mainly leading to oligosaccharides and their acyl (essentially feruloyl) derivatives. Mild sulphuric hydrolysis still seems to be the treatment of choice for xylose recovery from durum wheat bran.

Effects of plant-derived naphthoquinones on the growth of Pleurotus sajor-caju and degradation of the compounds by fungal cultures

The growth of the white-rot basidiomycete Pleurotus sajor-caju in malt-agar plates was inhibited by three naturally occurring, plant-derived naphthoquinones: juglone, lawsone, and plumbagin. The latter two compounds exerted the most potent antifungal activity, and lawsone killed the mycelium at concentrations higher than 200 ppm. Plates containing juglone and lawsone presented large decolorized areas extending from area of fungal growth, suggesting an extracellular enzymatic degradation of these quinones. Screening of culture plates for extracellular enzymatic activities revealed the presence of both laccase and veratryl alcohol oxidase in most plates, the diffusion of both enzymes matching the decolorized area. In agitated cultures, the presence of juglone was found to stimulate the production of veratryl alcohol oxidase in a significant manner. This is the first time degradation of plant derived naphthoquinones by a white-rot fungus is reported.

Effect of 3-hydroxyanthranilic acid on mushroom tyrosinase activity

Tyrosinase is a copper containing protein which catalyzes the hydroxylation of monophenols and the oxidation of diphenols to o-quinones. The monophenolase activity of tyrosinase is characterized by a typical lag time. In this paper the influence of 3-hydroxyanthranilic acid on monophenolase activity of tyrosinase is reported. 3-Hydroxyanthranilic acid reduced the lag time of tyrosinase when the enzyme acted on N-acetyl-L-tyrosine and on 4-tert-butylphenol. In the presence of 3-hydroxyanthranilic acid, the reaction product 4-tert-butyl-o-benzoquinone, derived from 4-tert-butylphenol oxidation, was formed at a higher rate than in its absence. The results reported in this paper indicate that 3-hydroxyanthranilic acid could affect the enzymic activity of mushroom tyrosinase probably by acting as a diphenol substrate. A K(m) value of 0.78 mM was calculated for 3-hydroxyanthranilic acid as substrate. When tyrosinase acted on 4-tert-butylphenol, K(m) for 3-hydroxyanthranilic acid as a cofactor was estimated to be 37.5 microM. No effect was observed on the diphenolase activity of the enzyme acting on 4-tert-butylcatechol in the presence of 3-hydroxyanthranilic acid.

Biosynthesis of the topaquinone cofactor in copper amine oxidases--evidence from model studies

Copper amine oxidases utilize 2,4,5-trihydroxyphenylalanine quinone (topaquinone) as a cofactor in enzymatic catalysis. This cofactor is formed from a tyrosine residue through a self-catalytic mechanism with the participation of the copper ion at the active site. Although pathways have been postulated for topaquinone biogenesis, portions of this scheme are still unclear. We utilized 4-tert-butyl-derived models for the putative intermediates of topaquinone generation and studied the effect of Cu(II) and Zn(II) ions on each autoxidative step from dopa- to topaquinone-like compounds at physiological pH (7.4). Several polyvinyl-alcohol-based soluble resins bearing mono- and di-hydroxyphenolic moieties were also prepared, and their tendency to give hydroxyquinonic structures when incubated at alkaline pH values was investigated. Our results confirm (although indirectly) the formation of dopa and dopaquinone during topaquinone biosynthesis. Moreover, we collected evidence that, following the formation of dopa, the role of the active-site copper ion in topaquinone biogenesis would be limited to the catalysis of the two subsequent quinonization steps (i.e. from dopa to dopaquinone and from topa to topaquinone), thus disfavoring the possibility of a direct intervention of the metal ion in the hydroxylation of dopaquinone. In particular, Cu(II) was shown to influence deeply the autoxidation of 1,2,5-trihydroxy-4-tert-butylbenzene, used as model of topa, both increasing the reaction rate and changing its mechanism. The mechanistic implications of these findings for the biogenesis of topaquinone and its analogs at the active site of various amine oxidases are discussed.

Properties of Thermus aquaticus beta-NADH oxidase immobilised on various supports

beta-NADH oxidase purified from Thermus aquaticus was covalently immobilised on various solid supports. The preparations obtained were compared with the soluble enzyme for activity and kinetic properties. Activated glutaryl-PVA was found to be the best support. The immobilised enzyme was less stable at high temperatures than the soluble enzyme. No differences could be detected in the presence of organic solvents.

Dopaquinone hydroxylation through topaquinone cofactor in copper amine oxidases: a simplified chemical model

A simple model, 4-tert-butyl-1,2-benzoquinone, was chosen to study the hydroxylation step of the tyrosine-derived Dopaquinone residue at the active site of copper amine oxidases in the self-catalytic generation of the Topaquinone cofactor. This hydroxylation step was studied both in the presence and absence of free copper(II), and was found to be dependent on pH value but not on the presence of metal ions. It is therefore proposed that, hydroxide ion and not water should be the true reactive species in this key biosynthetic step of the Topaquinone cofactor, and that the active site Cu2+ is implied, at this point of cofactor biosynthesis, in the quinonisation of Topa rather than in the hydroxylation of Dopaquinone.

Autoxidation of 4-methylcatechol: a model for the study of the biosynthesis of copper amine oxidases quinonoid cofactor

The autoxidation of 4-methylcatechol under quasi-physiological conditions, leading to 2-hydroxy-5-methyl-1,4-benzoquinone, was investigated. The effects of pH and metal ions were examined. An electrophilic attack of dioxygen to the 4-methylcatechol monoanion to form a transient peroxo species is proposed. It was concluded that such a non-enzymic conversion is likely for this model compound and for its physiological counterpart, a specific tyrosyl residue incorporated in the protein chain at the active site of copper amine oxidases.

Purification and characterization of an NAD(P)H:quinone oxidoreductase from Glycine max seedlings

An NAD(P)H:(quinone acceptor) oxidoreductase (EC 1.6.99.2) was purified from Glycine max seedlings by means of chromatographic procedures. After 1371-fold purification, the enzyme showed a single band in IEF corresponding to an isoelectric point of 6.1. A single band was also found in native-PAGE both by activity staining and Coomassie brilliant blue staining. The molecular mass determined in SDS-PAGE was 21900 Da, while in HPLC gel-filtration it was 61000 Da. The NAD(P)H:quinone oxidoreductase was able to use NADH or NADPH as the electron donor. Among the artificial quinones which are reduced by this enzyme, 6-hydroxydopa- and 6-hydroxydopamine-quinone are of particular interest because of their neurotoxic effects.

A dyed substrate for the assay of endo-1,4-beta-glucanases

A new dyed substrate was prepared for the rapid determination of endo-1,4-beta-glucanases. Carboxymethylcellulose was coupled with Ruthenium red to obtain a violet powder, which was stable enough to allow for reproducible assays. The spontaneous reaction is based on ionic interactions between the negatively charged carboxymethylcellulose and the complex cation of the dye. The enzymic assay is based on spectrophotometric measurement at 535 nm of the enzyme-released dyed fragments with low-molecular-weight filterable through a 0.22 microns filter. The release of coloured fragments from this dyed substrate was proportional to its solubilization. The absorbance was directly proportional to the enzyme amount in the range 0.5-5.5 mU enzyme (A535 = 0.1-0.95). This enzymic assay is advantageous for both rapid time of analysis and sensitivity.

Improved chromatographic purification of peroxidase and beta-glucosidase from Hordeum vulgare seedlings

Peroxidases (E.C. 1.11.1.7., hydrogen donor oxidoreductase) utilize hydrogen peroxide or substituted peroxides for the oxidation of a large number of substrates. Peroxidases are widely distributed and have been isolated from many higher plants (1). The wide distribution of the enzyme suggests that it could be of great biological importance, but the physiological functions and metabolic control of these enzymes are still poorly understood. The simultaneous presence of amine oxidase and peroxidase in cell walls suggests that the peroxide generated on oxidation of the amines could be utilized by the peroxidase (2,3). Recently we have purified an amine oxidase from Hordeum vulgare (4) and we have attempted to purify the peroxidase in order to study in vitro the reconstituted coupled system. beta-glucosidase (beta-D-glucoside glucohydrolase E.C. 3.2.1.21.) is capable of transforming glucosides in glucose and the corresponding aglycone or disaccharides as cellobiose, sophorose, gentiobiose. This enzyme is widely distributed in plants, fungi, bacteria, yeasts and animals (5,6). In the homogenate of Hordeum vulgare seedlings we also found beta-glucosidase activity and also attempted to purify beta-glucosidase. This enzyme copurified with peroxidase up to the last step. We report here the isolation of peroxidase and beta-glucosidase from Hordeum vulgare seedlings: some molecular and kinetic properties are given.