Carotenoids de novo

The carotenoids are the most widespread group of pigments in nature, with an estimated yield of 100 million tonnes per annum. They are present in all photosynthetic organisms and responsible for most of the yellow to red colours of fruits and flowers. The characteristic colours of many birds, insects and marine invertebrates are also due to the presence of carotenoids, which have originated in the diet. Animals are unable to synthesise carotenoids de novo, and so rely upon the diet as the source of these compounds. Carotenoids found in the human diet are primarily derived from crop plants, where the carotenoids are located in roots, leaves, shoots, seeds, fruit and flowers. To a lesser extent, carotenoids are also ingested from eggs, poultry and fish. Commercially, carotenoids are used as food colourants and in nutritional supplements (Table 13.1). Over recent years there has been considerable... 

Animals cannot synthesize carotenoids de novo. To deposit carotenoids in the proper tissues in the proper amounts, they must acquire carotenoids from dietary sources and transport them to target sites. Knowledge of the molecular mechanisms of carotenoid transport, however, is still... 

Because plants are able to synthesize carotenoids de novo, the carotenoid composition of plant foods is enriched by the presence of small or trace amounts of biosynthetic precursors, along with derivatives of the main components. Although commonly thought of as plant pigments, carotenoids are also encountered in some animal foods. Animals are incapable of carotenoid biosynthesis thus their carotenoids need to be derived from the diet. Selectively or unselectively absorbed, carotenoids accumulate in animal tissues unchanged or slightly modified into typical animal carotenoids. 

To finish this duscussion on lipophorin biosynthesis we will mention studies on the origins of PLs, hydrocarbons, sterols, and carotenoids. It has been reported that in adult M. sexta and Rhodnius prolixm PL can be transferred from fat body to lipophorin (Van Heusden et al., 1991 Correa et al., 1992). This transfer of PL is independent of de novo synthesis of lipophorin however, the mechanism by which it occurs is unknown. Hydrocarbon transport by lipophorin has been studied only in P. ameri-cana. Katase and Chino (1982) have shown, in in vitro incubations, that a fat body rich in oenocytes, one type of cell in the hemolymph, which is the major site of hydrocarbon biosynthesis (Diehl, 1975), can release labeled hydrocarbon to lipophorin. It was also shown, using in vitro incubations, that the labeled hydrocarbon in lipophorin was delivered to the epidermis, the normal site of hydrocarbon deposition in insects. The sterols and carotenoids that are present in lipophorin must arise from the diet, because insects cannot biosynthesize either sterols or carotenoids de novo. Chino and Gilbert (1971) have shown that sterol can be transferred from the midgut to lipophorin, and the same is most likely true for carotenoids. The mechanism by which hydrocarbons, sterols, and carotenoids are transferred from either oenocytes or midgut epithelial cells to lipophorin is unknown. 

Carotenoids are synthesised de novo by all photosynthetic organisms including higher plants, phytoplankton and phototrophic bacteria as well as by certain other bacteria, yeasts and fungi. Carotenoids are selectively absorbed and metabolically modified in the food chain [17]. Animals cannot synthesise carotenoids de novo, but may contain specific carotenoids derived from dietary sources. 

Our discussion in this chapter is mainly concerned with green plants, but bacteria and fungi are also considered briefly. Carotenoids are also found in many animals, particularly marine animals and birds, although none of these appear to synthesize carotenoids de novo. Carotenoids are terpenoid compounds and, as such, share a common biosynthetic origin with other terpenoid compounds. The early steps in the biosynthesis of these compounds, leading to the formation of isopentenyl pyrophosphate and other prenyl pyrophosphates, are thought to be identical in all systems. Therefore, information related to these early steps that is found in Goodwin (this volume. Chapter 15) is also relevant to the subject of carotenoid biosynthesis. This subject has also been reviewed by Beytia and Porter (1976). 

Carotenoids are similarly present in foods of animal origin. However, animals are unable to synthesise carotenoids de novo, and only convert plant pigments occurring in food into substances of different structure or store them as such. 

Figure S2.4 shows the structures of 11 -c/.v-retinal and its more stable isomer all-frans-retinal. The reti-nals are related to the alcohol retinol, or vitamin A,. Mammals cannot synthesize these compounds de novo but can form them from dietary carotenoids such as /3-carotene. A deficiency of vitamin A causes night blindness, along with serious deterioration of the eyes and other tissues.

P. F. Wareing and his dormin group had, meanwhile, joined with Cornforth s team in the Shell Research Laboratories and they too succeeded in isolating the active compound for dormancy in shoots of sycamore (Cornforth et al., 1965). It had the same structure as Addicott s abscisin II Confirmation that both groups had isolated the identical substance came in the same year with Cornforth, Milborrow and Ryback s (1965) chemical synthesis of abscisin II. It was a sesquiterpene, the stereoisomer (S)-conformation being native (Figure 2e). Two pathways of synthesis seemed operative, de novo via mevalonate and the isoprene route, or as a degradation product of carotenoids. 

In some species of green algae keto-carotenoids were synthesized when growth was halted by environmental conditions and these compounds may be formed either by the catabolism of chlorophyll or by the result of de novo synthesis. The profile of incorporation of tracer from [2acetate by Haematococcus lacustris into carotenoid and keto-carotenoids indicated that the latter was the major pathway to the keto-compounds, but some breakdown of chlorophyll did occur and the relative contributions of these two pathways varied with the external conditions. In the marine isopod Idotea resecata incorporation of [ Cj-zS-carotene into hydroxy-carotenoids was very low and it was not possible to decide whether the keto-carotenoids present were formed by direct oxidation of jS-carotene to canthaxanthin via echinenone or via hydroxy-compounds. ... 

Vitamin A is essential for life, and although humans and other animals are capable of modifying retinoid compounds, these species are incapable of the de novo synthesis of retinoids. Thus, dietary intake of the retinoids is crucial for the maintenance of health and prevention of vitamin A deficiency. Dietary retinoids consist of two general types (i) carotenoids, including principally jS-carotene as well as potentially hundreds of other related compounds... 

Fruits with a marked de novo biosynthesis of carotenoids, referred to as carotenogenic fraits (i.e., tomato, red pepper, orange, persimmon, etc.). 

It is generally assumed that animals do not have the capability to carry out de novo carotenoid synthesis. However, they occassionally have the ability to modify structurally carotenoids obtained through their diet. The absolute configuration of the carotenoids involved may throw light on such metabolic transformations. 

Due to their inability to synthesize retinol de novo, animals need to obtain retinoids through the diet, mainly in the form of carotenoids, such as P,P-carotene (Figure 1.1). Carotenoids are isoprenoids that are widespread in nature and are typically associated with the yellow, orange, red, or purple colors found in vegetables, fruits, flowers, birds, and crustaceans (Fraser and Bramley 2004). Alternatively, retinol can also be taken up directly in the form of pro-vitamin A carotenoids (Blomhoff and Blomhoff 2006). 

Animals are not capable of de novo synthesis of vitamin A-active substances, neither preformed retinol and its derivatives nor the carotenoid precursor forms. The preformed retinoids are defined and their chemical and structural characteristics are given in other chapters in this volume and in the appendix. 

Inhibition of fatty acid biosynthesis in chloroplasts The de novo synthesis of fatty acids proceeds in the chloroplasts (plastids) (35). The biosynthesis of total fatty acids in isolated maize chloroplasts is inhibited by sethoxydim (Table 7). The Igg-value is lower than 10 molar. Thus in grass weeds the major biochemical target appears to be the fatty acid synthetase. Sethoxydim acts in a similar way to the structurally different herbicide diclofopmethyl (36). The block of chlorophyll and carotenoid... 

The carotenoids represent one of the most important and widespread groups of pigments in nature. The structures of some 500 of them are known, and they are responsible for many of the yellow and red colors of flowers, fruits, birds, insects, and other animals. " Carotenoids are synthesized de novo in all organisms except for animals, where the pigments are of dietary origin." Their universal presence in photosynthetic tissues is only noticeable at the onset of leaf senescence, when the chlorophylls disappear. The dramatic changes in color of ripening fruits also reflect the disappearance of chlorophyll and the concomitant, massive increase in carotenoids. 

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