A heme-thiolate protein (P-450). The enzyme hydroxylates carbon C-6 of germacra-1(10),4,11(13)-trien-12-oate to give 6α-hydroxygermacra-1(10),4,11(13)-trien-12-oate, which probably spontaneously cyclises to form the lactone ring.
de Kraker, J.W., Franssen, M.C., Joerink, M., de Groot, A. and Bouwmeester, H.J. Biosynthesis of costunolide, dihydrocostunolide, and leucodin. Demonstration of cytochrome p450-catalyzed formation of the lactone ring present in sesquiterpene lactones of chicory. Plant Physiol.129 (2002) 257–268. [PMID: 12011356]