Light-Dependent Conversions of Endogenous α-Tocopherylquinone and Plastoquinone-D in Spinacia Oleracea Chloroplasts

Plastoquinone, a substituted benzoquinone occurring in the chloroplasts of plants (5), has been shown to be involved in and required for various photosynthetic electron transport reactions. Bishop (2) has shown that PQ-A4 is required to restore photoreductive activity to petroleum ether extracted chloroplasts. Similarly Krogmann (15) demonstrated the requirement for PQ-A in photophosphorylation. In addition to the PQ-A studied by the above workers there have been 3 new plastoquinone-type compounds reported which are similar in ultraviolet absorption spectra but which have different chromatographic characteristics and melting points (12). They have been given the names PQB, -C, and -D5. PQ-A is the compound first isolated and is commonly referred to simply as plastoquinone. Ubiquinone (coenzyme Q) has been established as a functional intermediate in the respiratory electron transport chain of both plant and animal systems (6, 9). It has also been implicated in the photoreactions of the purple bacteria Rhodospirillumx rubrum (4). Vitamin K1, a naphthoquinone, has been shown to be a redox intermediate in the metabolism of Mycobacterium phlei (3). In this organism the quinone is converted to the chroman form (i.e., to a naphthotocopherol). It was demonstrated that the naphthotocopherol became phosphorylated and could then phosphorylate ADP to form ATP. These examples serve to show the significance of quinones in biological electron and energy transfer. The tocopherols and tocopherylquinones constitute another group of widely distributed chromanols and quinones. It was recently shown by the authors that a-tocopherol and a-TQ are contained within the chloroplasts of spinach (Spinacia oleracea) leaf tissue (11). This observation raised the question as to whether or not these compounds function in photosynthetic reactions. Experiments were conducted with spinach chloroplasts in which the amount of the oxidized form (aTQ) of the quinone was measured after various treatments of light exposure, NADP, and o-phenanthroline. The amount of oxidized PQ-D and PQ(A+B) was also measured. This work shows that the amounts of the oxidized form of a-TQ, PQ-(A+B), and PQ-D change as much as threefold after exposing the chloroplasts to light. The changes are reversible and are inhibited by o-phenanthroline. These observations support the hypothesis that a-TQ and PQ-D are functional intermediates in photosynthetic electron transport.