Initial reactions of xanthone biodegradation by an Arthrobacter sp.

This study examined the catabolism of xanthone by an Arthrobacter sp. (strain GFB100) capable of growth on xanthone as its main source of carbon and energy. An early catabolic intermediate was 3,4-dihydroxyxanthone. This compound was isolated from the growth medium of a mutant strain of the Arthrobacter sp. which lacked the xanthone-inducible dihydroxyxanthone ring-fission dioxygenase of the wild-type strain. Cell extracts from wild-type xanthone-grown cells oxidized 3,4-dihydroxyxanthone to a yellow ring-fission metabolite. The same yellow compound accumulated in xanthone-grown cultures of a spontaneous mutant which lacked an active, xanthone-inducible, NADPH-linked ring-fission metabolite reductase. The yellow ring-fission metabolite appears to be 4-hydroxy-3-(2'-oxo-3-trans-butenoate)-coumarin, based on its nuclear magnetic resonance spectrum and mass spectral fragmentation pattern, indicating that ring cleavage of 3,4-dihydroxyxanthone was by an extra-diol (meta-fission) mechanism. Enzymatic analyses indicated that growth on xanthone induced a complete gentisate pathway: dioxygenase-catalyzed cleavage of gentisate to maleylpyruvate, isomerization of maleylpyruvate to fumarylpyruvate, and hydrolysis of fumarylpyruvate to fumarate and pyruvate. 4-Hydroxycoumarin was thought to be a likely pathway intermediate linking the early xanthone catabolic steps to the gentisate pathway, since 2-hydroxyacetophenone, a byproduct of 4-hydroxycoumarin hydrolysis, was formed when wild-type cells were cultured with xanthone. Chlorinated 2-hydroxyacetophenones were also obtained from specific chloro-substituted xanthones.