Aldehyde oxidase catalysed oxidation of various quinolinium and
related cations has been studied in vitro. Oxidation products were
identified by comparison of their spectral and chromatographic
characteristics with those of authentic compounds. The N-heterocyclic
cations and quinolones used required synthesis.
Incubation of N-methylquinolinium, N-methyl-7,8-benzoquinolinium
and N-phenylquinolinium yielded the corresponding 2- and 4-quinolones
simultaneously. The ratio of 2- to 4-quinolone formation was found to
be species dependent; the proportion of 4-quinolone was greater with
guinea pig enzyme than with rabbit enzyme.
Incubation of N-methyl-4-methylquinolinium, N-methyl-4-phenylquinolinium
and N-methylphenanthridinium produced the expected 2-quinolones.
Cations substituted adjacent to the ring nitrogen, i. e. N-methyl-2-
methylquinolinium, N-methyl-2-phenylquinolinium and N-phenyl-2-phenylquinolinium,
were oxidised to the corresponding 4-quinolones.
Kinetic constants were determined spectrophotometrically. The Km
values obtained with rabbit enzyme ranged from 1.6 x 10-3 M for N-methylquinolinium
to <10-5 M for N-phenyl-2-phenylquinolinium. Quaternary
compounds were found to be better substrates than their non-quaternary
counterparts, except for N-methylisoquinolinium and N-methylphenanthridinium.
In general, guinea pig aldehyde oxidase was shown to have a greater affinity
for N-heterocyclic cations than rabbit enzyme.
The substrate binding site has been discussed in the light of the
results outlined below.
Oxidation of N-methyl-4-phenylquinolinium (to the 2-quinolone) was
competitively inhibited by N-methyl-2-phenylquinolinium (which yields the
4-quinolone), indicating that both these cations interact at the same
active site. The ratio of 2- to 4-quinolone production from N-methylquinolinium
was constant under various conditions, including purification
of the enzyme but changed at high pH or in the presence of N-methylphenanthridinium.
Inhibition studies indicated that both quaternary and non-quaternary
compounds act at the same site on the enzyme. Km and Vmax values for
phthalazine, N-methyl-2-phenylquinolinium and N-methylquinolinium were
determined over the pH range 5.4 to 10.2. In each case, results indicated
that the enzyme has an ionisable group at the active site with a pK ca. 8.
Aldehyde oxidase was shown to catalyse the dehydrogenation of the
pseudobases 3,4-dihydro-4-hydroxy-3-methyl-2-quinazolinone and 3,4-dihydro-
4-hydroxy-3-methylquinazoline.
| Identifer | oai:union.ndltd.org:BRADFORD/oai:bradscholars.brad.ac.uk:10454/3580 |
| Date | January 1984 |
| Creators | Taylor, Susan M. |
| Contributors | Stell, J. Godfrey P., Beedham, Christine |
| Publisher | University of Bradford, Postgraduate School of Studies in Pharmaceutical Chemistry |
| Source Sets | Bradford Scholars |
| Language | English |
| Detected Language | English |
| Type | Thesis, doctoral, PhD |
| Rights | <a rel="license" href="http://creativecommons.org/licenses/by-nc-nd/3.0/"><img alt="Creative Commons License" style="border-width:0" src="http://i.creativecommons.org/l/by-nc-nd/3.0/88x31.png" /></a><br />The University of Bradford theses are licenced under a <a rel="license" href="http://creativecommons.org/licenses/by-nc-nd/3.0/">Creative Commons Licence</a>. |
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