Specificity of xanthine oxidase for nitrogen heteroaromatic cation substrates

Abstract

A variety of pyridinium, quinolinium, and benzoquinolinium cations have been investigated as potential substrates for milk xanthine oxidase at pH 9.9 and (or) pH 10.6. Steady-state kinetic parameters (kc, Km and (or) kc/Km) have been evaluated for all substrates which are enzymically oxidized. Simple N-alkyl pyridinium cations are neither substrates nor inhibitors, although N-aryl pyridinium cations are slowly oxidized to the 4-pyridinones. N-Methylpyridinium cations bearing 3-CONH2, 3-CONHCH3, 3-COCH3, 3-CO2- or 3-CN substituents are readily oxidized at C-6 and this suggests an important hydrogen-bonding interaction between an enzyme donor and the C-3 carbonyl substituent. A variety of N-methylquinolinium cations bearing C-6 substituents are enzymically oxidized at C-2. Analogous substituent effects on kc/Km for these 6-substituted 1-methylquinolinium cations and the corresponding 1-(substituted phenyl)-pyridinium cations is suggestive of the relative productive binding orientations of these two classes of substrate in the active site. N-Methylbenzoquinolinium and 1,10-phenanthrolinium cations are the best cationic substrates found to date, and suggest a relatively large active-site region for the reducing substrate, and important hydrophobic interactions between enzyme and substrate. The overall enzymic specificity observed for these cationic substrates allows a mapping of the general features of the reducing substrate binding site of this enzyme.

Comparison of the nucleic acid covalent binding capacity of two nitro-substituted benzazolo[3,2-a]quinolinium salts upon enzymatic reduction

“…The ability of XO to reductively activate multiple substrates is documented (Bunting et al, 1980), including the covalent binding of nitroarenes via HX/XO (Fu et al, 1994, Ritter et al, 2002. In the present study, the reaction of NBQ-38 with the highest concentration of HX resulted in the highest level of adduct formation.…”

Synthesis and antimicrobial activity of quaternary N-aryl-5,6-benzoquinaldinium derivatives

“…A comparison of our results (Table 3) with previously published data for salts of N-phenylquinoline [9] led to the unambiguous conclusion that introduction of a new benzo-annelated ring increases substantially the biological activity against practically all studied test strains.…”

“…This trend persists even in condensed derivatives such as benzoquinolinium and benzoquinaldinium salts [7,8]. The biological activity of N-aliphatic benzoquinaldinium compounds is well studied [9,10] due to the difficulty of synthesizing the analogous aryl derivatives. Nevertheless, it was found that replacing an aliphatic substituent on the N atom by an aromatic group increases the antimicrobial activity [11].…”

Preparation of N-phenyl-substituted quinolinium derivatives labeled with tritium by chemonuclear synthesis

“…In many cases N-phenyl compounds surpass their aliphatic analogs. In spite of the large number of studies and the broad aspects of biological and medicinal application (antiseptics, biocides, antimalarials, antitumor, antirecidive, and antipyretic preparations) [6][7][8][9][10][11][12][13][14], the search still goes on for new biologically active compounds in the area of quinoline derivatives [15][16][17][18][19][20][21][22][23][24][25][26].…”