Abstract
Vitamin E is one of the 13 vitamins that are essential to animals that do not produce them. To date, six natural organic compounds belonging to the chemical family of tocochromanols—four tocopherols and two tocotrienols—have been demonstrated as exhibiting vitamin E activity in animals. Edible plant-derived products, notably seed oils, are the main sources of vitamin E in the human diet. Although this vitamin is readily available, independent nutritional surveys have shown that human populations do not consume enough vitamin E, and suffer from mild to severe deficiency. Tocochromanols are mostly produced by plants, algae, and some cyanobacteria. Tocochromanol metabolism has been mainly studied in higher plants that produce tocopherols, tocotrienols, plastochromanol-8, and tocomonoenols. In contrast to the tocochromanol biosynthetic pathways that are well characterized, our understanding of the physiological and molecular mechanisms regulating tocochromanol biosynthesis is in its infancy. Although it is known that tocochromanol biosynthesis is strongly conditioned by the availability in homogentisate and polyprenyl pyrophosphate, its polar and lipophilic biosynthetic precursors, respectively, the mechanisms regulating their biosyntheses are barely known. This review summarizes our current knowledge of tocochromanol biosynthesis in plants, and highlights future challenges regarding the understanding of its regulation.
Highlights
Natural compounds exhibiting vitamin E activity in animal cells belong to the chemical family of tocochromanols, a group of organic molecules with a polar chromanol ring and lipophilic polyprenyl side chain that varies according to the type of tocochromanol [1]
The present review summarizes our current knowledge on tocochromanol metabolism in plants, including the core tocochromanol biosynthetic pathway that is well delineated (Section 2); the transcriptional regulation of γ-TMT expression, which is the gene encoding the biosynthetic enzyme of the most potent vitamin E form of α-tocopherol in animals (Section 3); the regulation of homogentisate (HGA) biosynthesis, which is the polar precursor of tocochromanols (Section 4); and the regulation of polyprenyl pyrophosphate biosynthesis, which is the lipophilic precursor of tocochromanols (Section 5)
Much less is known about the mechanisms regulating vitamin E biosynthesis in plants
Summary
Natural compounds exhibiting vitamin E activity in animal cells belong to the chemical family of tocochromanols, a group of organic molecules with a polar chromanol ring and lipophilic polyprenyl side chain that varies according to the type of tocochromanol [1]. The first article published on vitamin E relates the finding of an unknown nutritional factor that prevents embryo resorption during rodent gestation [2] Several years later, this “factor X”, as it was originally named, was identified as α-tocopherol [3]. It has been shown that diet supplementation with vitamin E decreased the miscarriage rate of pregnant woman by approximately 50% [12] These results demonstrate the importance of adequate vitamin E intake for proper reproduction. A recent study showed that Plasmodium falciparum, a non-photosynthetic parasite that causes malaria, synthesizes both α- and γ-tocopherols during its intraerythrocytic stages to avoid oxidative stress [14,15,16] Besides this exception, tocochromanol metabolism has been primarily studied in angiosperms and in the vitamin. This work complements the very recent review that highlighted the biosynthetic origins and transports of polar and lipophilic tocochromanol biosynthetic precursors in plants [17]
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