The biosynthesis of coumarin in Melilotus alba.

Previous investigations have shown that CY4-labeled transcinnamic acid is rapidly converted to o-coumaric acid P-glucoside by shoots of Melilotus alba (white sweet clover) (1) and Hierochlod odorata (sweet grass) (2). This glucoside is then converted to coumarin (l-3), possibly by way of trans-cis isomerization of the glucoside to coumarinic acid P-glucoside. The small amount of radioactivity found in the coumarin was shown to be due to its rapid metabolism to melilotic acid (o-hydroxyhydrocinnamic acid), melilotic acid &glucoside, and other unidentified compounds (1, 3). Experiments to be described, in which labeled trans-cinnamic acid was fed to Me&lotus alba shoots, showed results in agreement with those reported by other workers (l-3). In particular, the “bound” coumarin fraction of white sweet clover, which has been shown to be identical with coumarinic acid @-glucoside (4), was an intermediate in the conversion of trans-cinnamic acid to coumarin. In all experiments previously reported, the trans isomer of cinnamic acid has been specified as precursor. Evidence is presented here which shows that cis-cinnamic acid may also function as a coumarin precursor when administered to white sweet clover shoots. However, the formation of coumarin from cis-cinnamic acid does not appear to involve the glucosides of o-coumaric and coumarinic acids.