Enhanced laccase production in white-rot fungus Rigidoporus lignosus by the addition of selected phenolic and aromatic compounds.

The white rot fungus Rigidoporus lignosus produces substantial amounts of extracellular laccase, a multicopper blue oxidase which is capable of oxidizing a wide range of organic substrates. Laccase production can be greatly enhanced in liquid cultures supplemented with various aromatic and phenolic compounds. The maximum enzyme activity was reached at the 21st or 24th day of fungal cultivation after the addition of inducers. The zymograms of extracellular fluid of culture preparation in the presence of inducers, at maximum activity day, revealed two bands with enzymatic activity, called Lac1 and Lac2, having different intensities. Lac2 band shows the higher intensity which changed with the different inducers. Laccase induction can be also obtained by adding to the culture medium olive mill wastewaters, which shows a high content of phenolic compounds.

A high sensitivity amperometric biosensor using laccase as biorecognition element.

An amperometric flow biosensor, using laccase from Rigidoporus lignosus as bioelement was developed. The laccase was kinetically characterized towards various phenolics both in solution and immobilized to a hydrophilic matrix by carbodiimide chemistry. A bioreactor connected to an amperometric flow cell by a FIA system was filled with the immobilized enzyme and the operational conditions of this biosensor were optimized as regards pH. Under the adopted experimental conditions, the immobilized enzyme oxidizes all the substrate molecules avoiding the need of cumbersome calibration procedures. The biosensor sensitivity, which was found to be 100 nA/microM for some of the tested substrates, resulted to be constant for more than 100 working days. This biosensor permits the detection of phenolics in aqueous solutions at concentrations in the nanomolar range and was successfully used to detect phenolics in wastewaters from olive oil mill without sample preparation.

Photosensitizing effect of some nonsteroidal antiinflammatory drugs on natural and artificial membranes: dependence on phospholipid composition.

Previous studies have clarified the molecular mechanism of photosensitization on red blood cell membranes induced by some drugs belonging to the class of nonsteroidal antiinflammatory drugs: ketoprofen, naproxen, and diflunisal. This process involves the participation of photodegradation products, free radicals, and reactive oxygen species. The aim of the present paper is to investigate the photohemolytic process using red blood cells of mammalian species, with different membrane phospholipid compositions. Human and bovine red blood cell membranes were selectively enriched with phosphatidylcholine and sphingomyelin. For this purpose, a new approach for phospholipid investigation was undertaken. Moreover, the phototoxic effect was tested with liposomes at different phospholipid compositions. A structure-function relationship between the erythrocyte membrane phospholipid composition and the photohemolytic process induced by the sensitizers can be proposed. Indeed, the different contents of the photoperoxidable double bond and the variable architecture of the membrane bilayer, due to the different phosphatidylcholine and sphingomyelin contents, strongly influence the resistance of the cell to an osmotic shock induced by photogenerated transient species or by the lytic activity of drug photoproducts. The higher content of sphingomyelin, its asymmetric disposition at the outer surface of membrane bilayers, the high level of saturated acyl fatty chains, and the presence of photoperoxidable trans double bonds in the hydrophilic region greatly decrease the fluidity of bilayers and enhance the resistance of the membrane to phototoxic damage. On the other hand, an increase in the content of phosphatidylcholine, which is rich in species with unsaturated acyl fatty chains, decreases the membrane resistance, because these latter can be easily oxidized by drug-photogenerated reactive oxygen species.

The structure of Rigidoporus lignosus Laccase containing a full complement of copper ions, reveals an asymmetrical arrangement for the T3 copper pair.

Laccase is a multicopper blue oxidase that couples the four-electron reduction of oxygen with the oxidation of a broad range of organic substrates, including phenols and arylamines. The enzyme is the object of intense biotechnological research, due to its employment in bioremediation of soils and water as well as in other biotechnological applications. We report here the cDNA and protein sequences, the post-translational modifications, the crystallization and X-ray structure determination of a laccase from the white-rot fungus Rigidoporus lignosus. The amino acid residues sequence deduced from cDNA clearly identified a pre-sequence of 21 residues representing the signal for extra-cellular localization. Mass spectrometry analysis performed on the salvage enzyme, confirmed the deduced sequence and precisely mapped two glycosylation sites at Asn337 and Asn435, determining the nature of the bound glycosidic moieties. The crystal structure was determined at 1.7A resolution from perfectly hemihedrally twinned crystals, by molecular replacement technique. While the overall structure closely resembled those reported for other fungal laccases, the analysis of the T2/T3 trinuclear cluster revealed an unprecedented coordination sphere for the T3 copper pair. No bridging oxygen ligand was present between the two T3 copper ions, which were no longer symmetrically coordinated. The observed structure could represent an intermediate along the process of four-electron reduction of oxygen to water taking place at the trinuclear copper cluster.

A high sensitivity amperometric biosensor using a monomolecular layer of laccase as biorecognition element.

Laccases from various sources were tested, and laccase from Rigidoporus lignosus was found to be the most active towards syringaldazine and ABTS, which are typical substrates of this class of enzymes, and towards the phenols found in olive oil mill wastewaters. This laccase was covalently immobilised by carbodiimide chemistry, on a self-assembled monolayer of 3-mercaptopropionic acid deposited on a gold surface. A flow biosensor, using the monolayer of laccase as bioelement and a glassy carbon electrode as amperometric transduction system, was developed. Although the amount of the immobilised enzyme (about 140 ng/cm2 effective surface area) was tiny, the biosensor showed a sensitivity of 3 nA/microM when 1,4-hydroquinone was used as substrate, and a half-life of 35 days. The proposed device permits detection of phenols in aqueous solutions at concentrations in the low micromolar range, i.e. below European Community limits. The biosensor was successfully used to detect phenols in wastewaters from an olive oil mill after minimal sample preparation (incubation of the aqueous sample with sodium borohydride for a few minutes) to suppress the current due to oxidised compounds present in the wastewaters.

Structure-activity relationship on fungal laccase from Rigidoporus lignosus: a Fourier-transform infrared spectroscopic study.

The structure and thermal stability of a laccase from Rigidoporus lignosus (Rl) was analysed by Fourier-transform infrared (FT-IR) spectroscopy. The enzyme was depleted of copper atoms, then part of the apoenzyme was re-metalled and these two forms of the protein were analysed as well. The enzymatic activity, lost by the removal of copper atoms, was restored in the re-metalled apoenzyme and resulted similar to that of native protein. The infrared data indicated that the enzyme contains a large amount of beta-sheets and a small content of alpha-helices, and it displayed a marked thermostability showing the T(m) at 92.5 degrees C. The apoenzyme and the re-metalled apoenzyme did not show remarkable differences in the secondary structure with respect to the native protein, but the thermal stability of the apoenzyme was dramatically reduced showing a T(m) close to 72 degrees C, while the re-metalled protein displayed the T(m) at 90 degrees C. These data indicate that copper atoms, beside their role in catalytic activity, play also an important role on the stabilisation of the structure of Rl laccase. About 35% of the polypeptide chain is buried and/or constitutes a particular compact structure, which, beside copper atoms, is probably involved in the high thermal stability of the protein. Another small part of the structure is particularly sensitive to high temperatures and it could be the cause of the loss of enzymatic activity when the temperature is raised above 45-50 degrees C.