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 γ-tocopherol. These observations contradict the hypothesis that assigns a rate-limiting function to tocopherol cyclase in plastidial α-tocopherol biosynthesis. The stoichiometry of α-tocopherol, 2,3-dimethylphytylquinol and γ-tocopherol formation and the inhibition of α-tocopherol synthesis by increasing γ-tocopherol concentrations suggests the regulation of this pathway by its precursors.

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