Lutein biosynthesis

Replacement of the hydrogen at the 3 or 3 position of the carotene ring with a hydroxyl is the next step in both branches of the pathway. Hydroxylation of the rings of the carotenes leads to biosynthesis of the xanthophylls, including the well-known lutein and zeaxanthin food pigments. Lutein is formed by hydroxylation of a-carotene zeaxanthin is formed by hydroxylation of P-carotene. 

Absorption and Raman analysis of LHCII complexes from xanthophyll biosynthesis mutants and plants containing unusual carotenoids (e.g., lactucoxanthin and lutein-epoxide) should also be interesting, since the role of these pigments and their binding properties are unknown. Understanding the specificity of binding can help to understand the reasons for xanthophyll variety in photosynthetic antennae and aid in the discovery of yet unknown functions for these molecules. 

To better understand these syndromes we must first review TSH biosynthesis and secretion. TSH is synthesized in the anterior pituitary as separate a- and 8-subunit precursors. The a subunits from luteinizing hormone (LH), foUicle-stimulating hormone (FSH), human chorionic gonadotropin (hCG), and TSH are similar, whereas the P subunits are unique and confer immunologic and biologic speci-... 

The biosynthetic results of Goodwin and co-workers using [2- C,31 ,5K- H]-mevalonic acid show that hydroxylation at C-3 results in the loss of tritium when zeaxanthin (6) or jS-cryptoxanthin is formed. Since the absolute stereochemistry of the tritium atom at C-3 is known before hydroxylation this result confirms that there is retention of configuration. Assuming retention on hydroxylation to give lutein (7) the loss of tritium from both positions in its biosynthesis suggests the absolute stereochemistry indicated at C-3 and C-3. ... 

Pathways. Studies of carotenoid transformations that take place when a mutant strain, PGl, of the green alga Scenedesmus obliquus is transferred from dark to light conditions have indicated that the transformations 15-cw-phytoene (180) 15-c/5-phytofluene (181) - 15-cis- -carotene (182) -> trans-C-caro-tene (183) (Scheme 7) take place in the biosynthesis of the normal cyclic carotenoids. The results were also in agreement with the formation of the xanthophylls lutein (16) and zeaxanthin (174) from the corresponding carotenes. 

Prior to 1995, only one locus affecting Xanthophyll biosynthesis in photosynthetic tissues of Arabidopsis had been identified, the ABA i locus, the mutation of which disrupts zeaxanthin deepoxidase, one of two xanthophyll cycle enzymes (Koomneef et al, 1982 Rock and Zeevaart, 1991 Rock et al., 1992). As a step toward advancing understanding of xanthophyll biosynthesis, incorporation, and function in plants, the author s laboratory has screened for and identified mutations defining two additional loci required for xanthophyll biosynthesis in Arabidopsis, LUTl and LUT2 LUT= LUTein deficient). Mutations at either locus result in defects in the synthesis of lutein, the most predominant xanthophyll in plants. Singly and in combination with the aba mutation, these lut mutations have allowed the genetic construction of five distinct mutant lines which differ dramatically in their carotenoid composition relative to wild-type Arabidopsis. In the remainder of this chapter I will first briefly discuss the aba mutation followed by a... 

The aba mutant of Arabidopsis is impaired in epoxy carotenoid biosynthesis and thus, makes zeaxanthin but not violaxanthin, antheraxanthin, and neoxanthin (Rock and Zeevaart, 1991 Rock et al, 1992). The pigment stoichiometries indicate a 1 1 replacement of neoxanthin and violaxanthin by zeaxanthin. The lutein content decreases compared to wildtype Arabidopsis, indicating a reduced Chi a/b antenna. Also comparing the composition of the antenna changes, there is less ofthe major LHCII and more of the minor Chi a/b complexes (Hurry et al, 1997). 

FIGURE 17.4 Growth change of salt-treated romaine lettuce (top), carotenoid biosynthesis pathway (left), and the contents of lutein and (3-carotene in salt-treated romaine lettuce (right). (From [43].)... 

Luteinizing hormone binds to its receptor on the surface of the Leydig cells to initiate testosterone biosynthesis. As in other endocrine cells, the binding of gonadotropin activates the Gs signal transduction pathway, increasing intracellular cyclic adenosine monophosphate (cAMP) levels via activation of adenylate cyclase. One of the processes influenced by elevated cAMP levels is the... 

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