6-Carotene biosynthesis

PORTER J w and Lincoln r e (1950) Lycopersicon selections containing a high content of carotenes and colourless polyenes II. The mechanism of carotene biosynthesis , yfrc/iiv Biochem Biophys, 27, 390-95. 

YE X, AL-BABILI s, KLOTZ A, ZHANG J, LUCCA p, BEYER p and POTRYKUS I (2000) Engineering provitamin A ( 3-carotene) biosynthesis pathway into (carotenoid-free) rice endosperm . Science, 287, 303-5. 

Juttner, F. (1979). Algal excretion product, geranylacetone-potent inhibitor of carotene biosynthesis in syn-echococcus. Zeitschrift Naturforschung C 34(11) 957-960. 

A great diversity in molecular structure is observed among herbicides which inhibit carotene biosynthesis as is exemplified by the structures of norflurazon, fluridone and difunone (shown below). Nonetheless, many of these compounds, which comprise a subset of the larger group known as bleaching herbicides, appear to inhibit the same step in the biosynthetic pathway to the carotenoids (1 ). The inhibited step is the desaturation of 15-cis phytoene to 15- cis phytofluene (Figure 1) and the build-up of phytoene in plants and in cell-free systems which have been treated with these herbicides is well documented (2-4). 

Data on herbicides are presented and reviewed, which allows the distinction between two different modes of bleaching. The first mode is caused by inhibited carotene biosynthesis exhibited by particular phenylpyridazinones, substituted phenylfuranones or amitrole. Decrease of carotenes leads to subsequent photodestruction of chlorophyll, peroxidation of other membrane components, and decay of electron transport activity. The second mode, represented by p-nitrodl-phenylethers, is associated with peroxidation of membrane-bound polyunsaturated fatty acids concurrently with the breakdown of carotenes, chlorophylls, and decay of photosynthetic electron transport. Short-chain hydrocarbon gases are reliable markers. The action of peroxidizing diphenylethers appears to be related to that of bipyridylium salts, although no light-induced oxygen uptake can be measured. 

During the inhibition of carotene biosynthesis by substituted phenylpyridazinones in algae (19,20) or wheat (21) and barley (22), an accumulation of phyto-ene, an uncolored (trlene) carotene precursor, could be observed. 

Cell-free Systems. More direct evidence can be obtained by investigating cell-free carotene biosynthesis. From Phycomyces blakesleeanus, a fungus in which P-carotene formation is inhibited by norflurazon... 

It is the prosthetic group in many carotenoproteins, mainly in invertebrates, which vary from coloitrless to bright blue. It reduces blood glucose and improves some parameters in diabetic metabohsm. It ameliorates blood flow and vascular tone in hypertension model and regulates connexin 43 in in vitro studies, and may be a cancer chemopreventive dmg. It is obtained on a large scale from the heterobasidiomycetous yeast Phaffia rhodozyma and plays an important role in carotene biosynthesis [Schroeder Johnson J Biol Chem 270 18374 1995],... 

Table 92.2 Mutant genes that alter the normal carotene biosynthesis in tomato fmits [30, 34-36, 40, 42, 44, 45, 53-57] ...

Because rice is part of the staple diet in much of the world, researchers added two P-carotene biosynthesis genes in making a variety of Ory%a saliva rice, producing rice with a greatly enhanced P-carotene content that would help combat Vitamin A deficiency, a worldwide problem. 

In the course of our work on carotene biosynthesis we noticed that certain amounts of the radioactively labeled precursor IPP were incorporated into volatile compounds. These compounds turned out to be monoterpene hydrocarbons which also occur in the volatile pattern of daffodil flowers. These findings enabled us to makethe first reporton chromoplasts being the site of monoterpene biosynthesis (Mettal et al. 1988). Another plastid type, i.e. leucoplasts from Citrofortunella mitis fruit exocarps, has been shown to sy.nthesize monoterpene hydrocarbons from IPP (Gleizes et al. 1983). 

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