Tracer experiment revealed that (E)-3″-hydroxygeranylhydroquinone is not an intermediate of the shikonin/alkannin and shikonofuran biosynthetic pathways in Lithospermum erythrorhizon

Lithospermum erythrorhizon (Boraginaceae) produces shikonin/alkannin, an enantiomeric pair of red naphthoquinone pigments with diverse biological activities. For the industrial production of shikonin/alkannin derivatives, a cell suspension culture system of L. erythrorhizon has been established. To produce shikonin/alkannin derivatives more efficiently in cultured cells, it is essential to understand the shikonin/alkannin biosynthetic pathway, which has not been fully elucidated. A previous study suggested that a conversion of (Z)- to (E)-3″-hydroxygeranylhydroquinone (3″-OH-GHQ) is a branching point of the shikonin/alkannin biosynthetic pathway and the shikonofuran biosynthetic pathway in L. erythrorhizon cell cultures. However, it is not clear whether (E)-3″-OH-GHQ is an intermediate of both pathways. This study performed a feeding assay with three deuterium-labeled compounds including (E)-3″-OH-GHQ and its (Z)-isomer, and showed that (E)-3″-OH-GHQ was not involved in the shikonin/alkannin and shikonofuran biosynthetic pathways.