The biogenesis of rosmarinic acid (α-O-caffeoyl-3,4-dihydroxyphenyl-lactic acid), the second most common ester of caffeic acid in the plant kingdom, was studied in Mentha arvense and Mentha piperita. Administration of 14C-labelled compounds showed that, whereas the caffeoyl moiety was formed from phenylalanine via cinnamic acid and p-coumaric acid, the 3,4-dihydroxyphenyl-lactic acid moiety was formed from tyrosine and 3,4-dihydroxyphenylalanine. Time-course studies and the use of labelled rosmarinic acid showed that endogenous rosmarinic acid had a low turnover rate. The caffeoyl moiety did not appear to contribute to the formation of insoluble polymers, as has been suggested for chlorogenic acid in other plants.
