Tocopherol synthesis from homogentisate in Capsicum anuum L. (yellow pepper) chromoplast membranes: evidence for tocopherol cyclase.

The present study shows for the first time appreciable tocopherol cyclase activities in plastidial membrane preparations of Capsicum annuum L. (yellow pepper) fruits. When chromoplast membranes from yellow peppers were incubated with [3H]homogentisate and phytyl pyrophosphate under strictly reducing conditions, all biosynthesis precursors were labelled. The main labelling was found in gamma-tocopherol. These observations contradict the hypothesis that assigns a rate-limiting function to tocopherol cyclase in plastidial alpha-tocopherol biosynthesis. The stoichiometry of alpha-tocopherol, 2,3-dimethylphytylquinol and gamma-tocopherol formation and the inhibition of alpha-tocopherol synthesis by increasing gamma-tocopherol concentrations suggests the regulation of this pathway by its precursors.