The homogentisate ring-cleavage pathway in the biosynthesis of acetate-derived naphthoquinones of the droseraceae

Photosynthesis experiments with 14 CO 2 established that of 16 Droseraceae species tested Drosophylum lusitanicum incorporated the highest amount of label into plumbagin (2-methyl-5-hydroxy-1,4-naphthoquinone). Tyrosine-[β- 14 C] fed to Drosophyllum was shown to label plumbagin efficiently (20% incorporation). Extensive chemical degradation of the labeled naphthoquinone showed, however, that the incorporation of tyrosine was indirect, the label being distributed throughout the molecule. It was established that plumbagin and the closely related 7-methyljuglone are biosynthesized via the acetate-polymalonate pathway. Tyrosine is broken down to acetate in this tissue via the homogentisate pathway, which was demonstrated by feeding and incorporation of label into plumbagin of intermediates such as homogentisate-[ 14 C], maleyl- and fumarylacetoacetate-[ 14 C]. Simultaneous application of tyrosine-[β- 14 C] and α,α′-bipyridyl, an inhibitor of the homogentisate oxigenase, led to an accumulation of homogentisate-[ 14 C] within the tissue. The degradation of tyrosine to acetate by Drosophyllum is not due to epiphytic bacteria since ring cleavage of tyrosine and formation of plumbagin from breakdown products occurred both within sterile grown plants and sterile cell suspension cultures. In tissue kept in darkness, plumbagin undergoes a slow turnover with a half life of about 400 hr.