The known derivatives from hydroquinone, α and β-arbutin, are used as depigmenting agents. In this work, we demonstrate that the oxy form of tyrosinase (oxytyrosinase) hydroxylates α and β-arbutin in ortho position of the phenolic hydroxyl group, giving rise to a complex formed by met-tyrosinase with the hydroxylated α or β-arbutin. This complex could evolve in two ways: by oxidizing the originated o-diphenol to o-quinone and deoxy-tyrosinase, or by delivering the o-diphenol and met-tyrosinase to the medium, which would produce the self-activation of the system. Note that the quinones generated in both cases are unstable, so the catalysis cannot be studied quantitatively. However, if 3-methyl-2-benzothiazolinone hydrazone hydrochloride hydrate is used, the o-quinone is attacked, so that it becomes an adduct, which can be oxidized by another molecule of o-quinone, generating o-diphenol in the medium. In this way, the system reaches the steady state and originates a chromophore, which, in turn, has a high absorptivity in the visible spectrum. This reaction allowed us to characterize α and β-arbutin kinetically as substrates of tyrosinase for the first time, obtaining a Michaelis constant values of 6.5 ± 0.58 mM and 3 ± 0.19 mM, respectively. The data agree with those from docking studies that showed that the enzyme has a higher affinity for β-arbutin. Moreover, the catalytic constants obtained by the kinetic studies (catalytic constant = 4.43 ± 0.33 s-1 and 3.7 ± 0.29 s-1 for α and β-arbutin respectively) agree with our forecast based on 13 C NMR considerations. This kinetic characterization of α and β-arbutin as substrates of tyrosinase should be taken into account to explain possible adverse effects of these compounds.

Background: Primary malignant tumours of the hand are extremely rare, with chondrosarcoma being the most common. We describe six consecutive cases of chondrosarcoma (CS) of the hand, with a follow-up of 12 months to 10 years after surgical intervention. Methods: Five cases were primary tumors and one was secondary to Ollier’s disease. Mean age at diagnosis was 67.5 years (range 46 to 85 years) and three patients were males. The bone most commonly affected was the proximal phalanx (in 4 patients), followed by the metacarpal bone (3 cases in the other 2 patients). The first symptom was an increase in volume in five cases and a pathological fracture in one. The x-ray study showed signs of malignancy in two cases only. In the other four cases, CT and MRI studies were necessary to complete the diagnosis. Results: Primary amputation was performed in two cases and intralesional curettage and cancellous bone autograft in four. Primary amputation was performed in two cases and intralesional curettage and cancellous bone autograft in four. We found three grade I/III CS and three grade II/III CS in the histological study. Local recurrences occurred in three patients, two being grade II/III CS. One of the three had undergone an initial amputation and underwent re-amputation, another had had curettage of the lesion and was amputated, and the third patient decided not to undergo the procedure due to her age. A pulmonary nodule was suspected at the most recent follow up in a fourth patient, who had been diagnosed with a grade II/III CS. Conclusions: Although CS of the hand bones behaves as a locally aggressive lesion, it rarely metastasizes. Intralesional curettage with a cancellous bone autograft appears to be a good option in low-grade CS to preserve the function, keeping in mind the high rate of recurrence. Wide excision or amputation is recommended when severe joint deformity or soft tissue involvement interferes with hand function.

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4-n-Butylresorcinol (BR) is considered the most potent inhibitor of tyrosinase, which is why it is used in cosmetics as a depigmenting agent. However, this work demonstrates that BR is a substrate of this enzyme. The E m (met-tyrosinase) form is not active on BR, but E ox (oxy-tyrosinase) can act on this molecule, hydroxylating it to o-diphenol. In turn, this is oxidized to an oquinone, which isomerizes to a red p-quinone. Thus, for tyrosinase to act on this compound, a mechanism to generate E ox in the medium is required, which can be achieved by means of hydrogen peroxide or ascorbic acid. A kinetic analysis of the proposed mechanism allows its kinetic characterization: catalytic constant k BR cat (8.49 6 0.20 s 21 ) and Michaelis-constant K BR M (60.26 6 8.76 lM). These findings are compared with those for other monophenolic substrates of tyrosinase. Studies of BR docking to the E m form of the enzyme show that the hydroxyl group in C-1 position is oriented toward the copper atom A (CuA), as in it is L-tyrosine. As regards E ox , BR is oriented with the carbon in C-6 position ready to be hydroxylated. The reaction of BR originates o-quinones, which isomerize to p-quinones, which in turn, could react with thiol compounds, a finding that could have important implications for pharmacology and the cosmetic industry. , initial p-quinone formation rate; V p2Q max , maximum action rate of tyrosinase on 4-n-butylresorcinol

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Fecha de recepción del artículo: 04/09/2017 Fecha de aceptación definitiva: 15/10/2017 RESUMEN Este texto analiza los resultados de una encuesta exploratoria entre estudiantes de la Universidad Rey Juan Carlos con el objeto de analizar en qué medida y de qué forma el posicionamiento político influye en el consumo y acceso a las noticias, con especial interés en el plano digital. Esta perspectiva se establece en relación a aspectos como los medios consultados, la credibilidad otorgada o el modo en que se comparte la información de actualidad. A tal efecto, se ha trabajado sobre la base de una encuesta autoadministrada durante un mes a 297 alumnos de Comunicación. Se ha podido observar una correlación entre la ideología y la mayor atención en Internet, junto a la relevancia de la televisión convencional en combinación con el acceso a las noticias digitales. En este contexto, son los medios sociales, antes que la prensa, el espacio más habitual de acceso a la información de actualidad. También se confirma la idea de credibilidad asociada a la prensa y radio tradicionales.Palabras clave: jóvenes; consumo de noticias; internet; redes sociales; credibilidad; ideología. ABSTRACTThis paper aims to analyze to what extent and the ways in which the political positions of Spanish young people has influence on the consumption and access to news, with special interest in the digital point of view. This perspective is established in relation to aspects such as the media consulted, the credibility and the forms in which information is shared. In this sense, a survey self-administered for one month among 297 Communication students is conducted. It has been possible to observe a correlation between the ideology and the greater attention in Internet, in addition to the relevance of the conventional television in combination with the access digital news. In this context, social media become a most commonly space for accessing digital news instead of digital newspapers. This paper also confirms the idea of credibility associated to both traditional press and radio.