Xanthine dehydrogenases from various sources are proteins of molecular mass 300 kDa and are composed of two identical independent subunits; each subunit contains one molybdopterin, two non-identical 2Fe/2S centres and FAD [ 1,2]. T h e mammalian enzyme exists originally as xanthine dehydrogenase, but is converted into xanthine oxidase during extraction or purification procedures [3]. Xanthine dehydrogenase is characterized by high xanthine/NAD+ activity and low xanthine/Oz activity, whereas xanthine oxidase is characterized by high xanthine/OZ activity and negligible xanthine/NAD+ activity. This conversion occurs either reversibly by thiol oxidation of the protein molecule or irreversibly by proteolytic cleavage [4]. In contrast with the mammalian enzyme, xanthine dehydrogenase from chicken liver is not converted into xanthine oxidase by either thiol oxidant or proteinase treatment. T h e full amino acid sequences of the enzymes from human liver [5], rat liver [6], mouse liver [7], bovine milk [8], chicken liver [9] and Drosophifu [ 101 have been determined by cDNA cloning. All known xanthine-oxidizing enzymes consist of approx. 1330 residues, except for the chicken enzyme which consists of 1358 residues because of insertion of 23 amino acid residues in the N-terminal region. Although the amino acid sequences are highly homologous among the rat, mouse, bovine and human enzymes, with about 90% identity [5-81, weaker homology was observed between the mammalian enzyme and the Drosophila enzyme, with 52% identity [6,10], and between the chicken liver and mammalian enzymes, with 70% identity [9]. By limited proteolysis of the rat enzyme with trypsin, the enzyme is converted into a xanthine oxi-dase type with concomitant cleavage into three fragments (20, 40 and 85 kDa) [6]. These fragments are only dissociated under denaturating conditions such as in the presence of high concentrations of guanidinium chloride [6], suggesting that the three fragments associate closely with each other. Determination of the N-terminal amino acid sequences of each fragment led to the assignment of the 20 kDa fragment to the N-terminal portion, the 85 kDa fragment to the C-terminal portion and the 40 kDa fragment to the intermediate portion. The chicken enzyme was also cleaved into three fragments in a similar way, but the enzyme is not converted into the oxidase [9]. Comparative alignment of the deduced protein sequences has been attempted in order to identify the possible locations of the redox active centres. Sequence comparison [5-91 and chemical-modification studies suggested that each redox centre is located in different domains, i.e. the two iron-sulphur centres are in the 20 kDa domain, the FAD is in the 40 kDa domain and the molybdopterin is in the 85 kDa domain [6,9]. These conclusions are consistent with the results obtained from X-ray-crystallographic analysis of aldehyde oxidoreductase from Desulfovibrio g&as which is another molybdenum-containing iron-sulphur protein [ 1 11. There is high sequence homology between xanthine ox...

A new reaction of oxoammonium salt is demonstrated. Addition of oxoammonium salts (1) to electron-rich olefins (2) such as vinyl ethers and enamines rapidly proceeded to give corresponding 1,2-adducts (3) in high yields. The adducts 3 formed were further transformed by treatment with sodium ethoxide to isolable substitution products (4) in one pot reaction.

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The titled polysilanes, {CH3-O(CH2CH2O)n-CH2CH2CH2-Si-CH3}x (n = 1∼3), are thermochromic both in the solid state and in solution. In the solid state these polysilanes all show a continuous bathochromic shift until nearly the same kink point temperature(190 K), then blue shift slightly. A water-soluble polysilane, n = 3, forms a complex with LiClO4. The thermochromic behavior of the complex is highly dependent on Li content. The complex of higher Li content shows only a blue shift on cooling while complexes of low Li content display red shift initially till Tg and then blue shift. An unexpected hyperchromic effect accompanied by the blue shift observed for the complex of Si:Li = 1:1 on cooling.