Hydroxylated cyclic carotenes

Therefore this present chapter is limited only to discussion on pulse radiolysis studies of promising compounds that exhibit potent biological activities for example, hydroxyl-flavonoids, resveratrol, curcumin, /3-carotene and cyclic nitroxides are reported here. 

Most C. contain an unsatuiated C22 chain with methyl branches and a 9-membered unit on each end, which may be cyclic as in a- and P-carotene, or acyclic as in lycopenes. When present, the terminal rings are usually 6-membered a- and P-ionone systems. The median 22 C atoms are generally constant, and oxidations to xanthophylls usually occur on the two C9 end sections. Most of the 700 known C. are xanthophylls, with 1, 2 or many hydroxyl groups, and ether, aldehyde, ketone or add substituents. Stereochemical differences arise from the ds-lram isomerism of the C s C double bonds, and from chiral centers that... 

Cyclization of lycopene proceeds only after an d -trans lycopene is formed by the action of carotene isomerase (CRTISO) in nongreen tissue. In the photosynthetic tissues, this conversion is catalyzed by light and chlorophyll (acting as a sensitizer). Oxygenation of the cyclic carotenoids yields xanthophylls. Introduction of hydroxyl groups at positions 3 and 3 in p-carotene produces zeaxanthin. Zeaxan-thin epoxidase (ZEP) and violaxanthin de-epoxidase (VDE) act in tandem to regulate the formation of violaxanthin. Violaxanthin is next converted to 9-cis-neoxanthin, ABA precursor, by neoxanthin synthase. Lutein is mainly present in photosynthetic tissues, biosynthesized from a-carotene via catalysis by p- and E-hydroxylases. 

Samples from Newospora crassa, Phycomyces blakesleeanus, Rhodospirillum rubnim, tomato fruit, and carrot root were all predominantly 15-cis (85—99.8 % of total) as was a specimen from Verticillium agaricinum (98 Cydization of Acyclic Carotenes.— Nicotine -and the herbicide CPTA [2-(p-chlorophenylthio)triethylammonium chloride] " inhibit the synthesis of cyclic compounds sudi as P-carotene (147) and cause accumulation of the acyclic lycopene (148). If cells of a Mycobacterium sp. grown in the presence of nicotine are washed free from the inhibitor and re-suspended in normal medium, then lycopene disapi ars as -carotene accumulates. The same phenomenon is observed in Blakeslea trispora, where there are indications that y-carotene (149) is an intermediate between lycopene and P-carotene, and in a Flavobacterium sp. grown in 7.5 mmol 1 nicotine and re-suspended anaerobically. in the last instance, oxygen is admitted, the reaction continues beyond P-carotene, and zeaxanthin (3,3 -dihydroxy-P-carotene) is formed almost exclusively there is no indication that hydroxy-lycopenes are involved. Thus, in the Flavobacterium, cydization is an anaerobic process and precedes the aerobic hydroxylation step, which only occurs alter cydization. These results suggested that the pathway was ... 

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