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Figure 8 Synthesis of secondary metabolites through the isoprenoid pathway.

(41). Cholesterol synthesis in mammals requires the coordinated synthesis of HMG-CoA synthase, HGMR, and FPP synthase. However, the degrada-tive turnover of these enzymes appears to be regulated independently of each other (42). This example of the effect of multiple enzymes to change flux in different pathways of terpenoid metabolism has been demonstrated in plants where exposure to specific metabolites can alter the production of either the steroid, glycoalkaloid, or sesquiterpene phytoalexin pathways, depending on the elicitor.

1. Sterol Biosynthesis

The nutritional interest in this class of phytochemicals derives from the fact that sterols have a similar structure to cholesterol and some sterols have a capacity to reduce the levels of total plasma cholesterol and low-density lipoprotein (LDL)-cholesterol (43,44). There has been a dramatic reduction in the morbidity and mortality from cardiovascular diseases with the use of hypolipidemic drugs (statins), and the interest in plant sterols lies in their potential to act as a natural, preventative dietary product.

The sterols are thought to act through their ability to limit the uptake of cholesterol from the diet as well as their potential to increase cholesterol excretion. This knowledge has led to the development of specific foods with cholesterol-reducing claims. These are already marketed in some countries as BenecolĀ® and Take ControlĀ®.

Sterol biosynthesis begins with the conversion of farnesyl diphosphate into squalene, which determines the channeling of the isoprenoid pathway into the branches that produce phytosterols (Fig. 9). The sterol pathway involves a sequence of more than 30 enzyme-catalyzed reactions, all of which are membrane linked. Nothing is known about the catabolism of plant sterols (45).

Up-regulation of HMG-CoA in transgenic tobacco, corn, and tomatoes results in the accumulation of cycloartenol only, rather than sterols. Targeting of enzymes upstream from cycloartenol results in a decrease in the total amount of plant sterols with potentially deleterious effects on membrane stability in the plant and the protection against infection. However, penta-cyclic triterpenes (such as a- and /3-amyrin) are formed by another route from squalene oxide, and inhibition of this route could increase free sterol concentrations. Potential strategies for sterol production are, however, likely to focus on overexpression of terminal enzymes in the pathway (45).

As far as the regulation of enzymes downstream from cycloartenol is concerned, the methylation of cycloartenol is a critical rate-limiting step. Inhibition of two of these enzymes, COI and OBT 14DM, leads to the accumulation of unusual steroids rather than natural A5-sterols (46).

Figure 9 Biosynthesis of plant sterols in photosynthetic phyla.

! COI-cycloeucalenol obtusifoliol isomerase. OBT14DM-obtusifoliol 14-demethylase

Figure 9 Biosynthesis of plant sterols in photosynthetic phyla.

From the food quality perspective, the principal interest is in the biosynthesis of sterol esters, which give a more consistent cholesterol-lowering effect than the free sterols. The sitostanol esters (47) were the first shown to be consistently effective in lowering cholesterol, but sterol esters appear to be equally effective. Ideally, stages in the final phases of biosynthesis need to be studied to ensure that all the sterols are esterified and thereby ensure that they are more bioavailable (see Sec. VI).

2. Carotenoid Biosynthesis

Carotenoids are also derived from the general isoprenoid pathway. The first committed step in the carotenoid pathway is the head-to-head condensation of two geranylgeranyl diphosphate (GGPP) molecules to produce phytoene. The three enzymes responsible for converting GGPP into /3-carotcne are shown in Fig. 10. /3-Carotene is of particular interest because it functions as a metabolically well-regulated precursor of vitamin A. Its importance to the health of children in the developing world has been discussed in Sec.

Although rice synthesizes GGPP, the enzymes required to convert it into /3-carotene are not all present. In a remarkable feat of genetic engineering, three genes encoding these enzymes have been introduced into the endosperm of rice (48). Phytoene synthase and phytoene desaturase were introduced using a construct that did not have a selectable marker. This was introduced through the simultaneous introduction of another construct, which carried the third gene of interest (lycopene /3-cyclase) as well as a selectable antibiotic resistance gene. This cotransformation strategy should

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