In this article we will discuss about the metabolic redesign of vitamin E.
Vitamin E is a generic term used to describe a group of eight lipophilic compounds in the tocopherol and tocotrienol family. The vitamin E-family in plants comprises mainly of tocotrienols and tocopherols.
Tocotrienols are the major form of vitamin E in seeds of most monocots but presence limited among dicots. In contrast, tocopherol occurs predominantly than tocotrienols in plants and is the principle vitamin E component of seeds as well as leaves of majority of dicots.
Vitamin E (tocopherol and tocotrienols) is a powerful antioxidant and are probably involved in the protection of plant cells against oxidative stress such as peroxides which can breakdown polyunsaturated fatty acids in seed oils and in membranes.
The antioxidant potential of vitamin E also contribute to the nutritive value of food products and animal feeds derived from food especially cereal grains. Vitamin E is synthesized in plants, but cannot be produced in animals. Thus, it is essential component of dietary supply.
Vitamin E synthesis in plants is restricted to the plastids of seeds and the chloroplast of leaves. The most committed step in tocopherol synthesis is the condensation of homogentisic acid (HGA) and phytyl diphosphate (PDP). This process is catalysed by the enzyme HGA phytyl transferase (HPT).
Similarly, tocotrienols are synthesised from the condensation of homogentisic acid (HGA) and geranyl geranyl diphosphate (GGDP). This reaction is catalysed by HGA geranyl geranyl transferase (HGGT) (Fig. 17.6). Dicotyledonous plants do not display any traceble activity of HGGT, whereas monocot contains a divergent form of HPT i.e., HGGT.
Production of Tocotrienols in Transgenic Plants:
Production of monocot specific tocotrienols in dicoteledonous plants, barley HGGT cDNA was expressed in Arabidopsis thaliana driven by CaMV35S promoter. Leaves of transgenic plants were found to accumulate large amount of tocotrienols, which were absent from leaves of non-transformed plants.
Comparison between transgenic and non-transgenic plants reveals (to fifteen fold) enhancement of tocotrienols i.e., 95-550 n mol/g dry weight in non-transformed plants to 1,700 to 2,200 n mol/g dry wt. in transgenic plants.
Some of the earliest attempt to alter amount of vitamin E in transgenic plants exhibit only a moderate success, which include over expression of polyhydroxy phenyl pyruvate dioxygenase and homogentisic acid phytyl transferase (HPT) results in marginal increase of 1.5 to 4 fold.
The probable limitation for vitamin E accumulates is due to the restricted availability of the vitamin E precursors like phytyl diphosphate, P-4 hydroxyphenyl purite or homogentisic acid (Fig. 17.7). In addition, over expression of N-1 deoxy-o-xylulose-5 phophate synthase results in moderate increase in tocopherols in about two to four fold.
Enhancement of Tocotrienols in Transgenic Corn:
The endosperm of seeds are enriched in tocotrienols but its content is absent in the seed embryos of corn hybrids. In this plant, tocotrienols reduced considerably to about < 15-20% of the total content of tocotrienols and tocopherols in extracts from whole corn seeds.
Thus, in transgenic studies, the barley cDNA was expressed in corn seeds. The transgene was driven by the strong-embryo specific promoter i.e., corn 16 kDa oleofin gene.
In transgenic plants tocotrienols accounted for as much 74% of total content of tocotrienols and tocopherols of embryos. Analysis of whole seed extracts reveals that the vitamin E from the tocotrienol content was increased by 20-fold in transgenic corn when compared with non-transformed one. These results demonstrated the ability to enhance the potential antioxidant vitamin E of the crops by introducing an enzyme that redirects metabolic flux.