Characterisation of plant tocopherol cyclases and their overexpression in transgenic Brassica napus seeds.

Tocopherols, collectively known as vitamin E, are only synthesised in photosynthetic organisms. Tocopherol cyclase (TC) catalyses the formation of the chromanol headgroup of the various tocopherol isoforms. TCs from Arabidopsis and maize (Zea mays) were expressed in Escherichia coli and purified. Analysis of the enzymatic properties revealed similarities but also differences between the two enzymes. Overexpression of chimeric TC gene constructs in developing seeds of transgenic rapeseed plants enhanced and modified the relative abundance of individual tocochromanol species in the seed oil, indicating a regulatory function of the enzyme in prenyllipid metabolism.