Carotenoid Isomerase Research Articles

Identification of All-trans-Retinol:All-trans-13,14-dihydroretinol Saturase

Retinoids carry out essential functions in vertebrate development and vision. Many of the retinoid processing enzymes remain to be identified at the molecular level. To expand the knowledge of retinoid biochemistry in vertebrates, we studied the enzymes involved in plant metabolism of carotenoids, a related group of compounds. We identified a family of vertebrate enzymes that share significant similarity and a putative phytoene desaturase domain with a recently described plant carotenoid isomerase (CRTISO), which isomerizes prolycopene to all-trans-lycopene. Comparison of heterologously expressed mouse and plant enzymes indicates that unlike plant CRTISO, the CRTISO-related mouse enzyme is inactive toward prolycopene. Instead, the CRTISO-related mouse enzyme is a retinol saturase carrying out the saturation of the 13-14 double bond of all-trans-retinol to produce all-trans-13,14-dihydroretinol. The product of mouse retinol saturase (RetSat) has a shifted UV absorbance maximum, lambda(max) = 290 nm, compared with the parent compound, all-trans-retinol (lambda(max) = 325 nm), and its MS analysis (m/z = 288) indicates saturation of a double bond. The product was further identified as all-trans-13,14-dihydroretinol, since its characteristics were identical to those of a synthetic standard. Mouse RetSat is membrane-associated and expressed in many tissues, with the highest levels in liver, kidney, and intestine. All-trans-13,14-dihydroretinol was also detected in several tissues of animals maintained on a normal diet. Thus, saturation of all-trans-retinol to all-trans-13,14-dihydroretinol by RetSat produces a new metabolite of yet unknown biological function.

Accumulation of carotenoids and expression of carotenoid biosynthetic genes during maturation in citrus fruit.

The relationship between carotenoid accumulation and the expression of carotenoid biosynthetic genes during fruit maturation was investigated in three citrus varieties, Satsuma mandarin (Citrus unshiu Marc.), Valencia orange (Citrus sinensis Osbeck), and Lisbon lemon (Citrus limon Burm.f.). We cloned the cDNAs for phytoene synthase (CitPSY), phytoene desaturase (CitPDS), zeta-carotene (car) desaturase (CitZDS), carotenoid isomerase (CitCRTISO), lycopene beta-cyclase (CitLCYb), beta-ring hydroxylase (CitHYb), zeaxanthin (zea) epoxidase (CitZEP), and lycopene epsilon-cyclase (CitLCYe) from Satsuma mandarin, which shared high identities in nucleotide sequences with Valencia orange, Lisbon lemon, and other plant species. With the transition of peel color from green to orange, the change from beta,epsilon-carotenoid (alpha-car and lutein) accumulation to beta,beta-carotenoid (beta-car, beta-cryptoxanthin, zea, and violaxanthin) accumulation was observed in the flavedos of Satsuma mandarin and Valencia orange, accompanying the disappearance of CitLCYe transcripts and the increase in CitLCYb transcripts. Even in green fruit, high levels of beta,epsilon-carotenoids and CitLCYe transcripts were not observed in the juice sacs. As fruit maturation progressed in Satsuma mandarin and Valencia orange, a simultaneous increase in the expression of genes (CitPSY, CitPDS, CitZDS, CitLCYb, CitHYb, and CitZEP) led to massive beta,beta-xanthophyll (beta-cryptoxanthin, zea, and violaxanthin) accumulation in both the flavedo and juice sacs. The gene expression of CitCRTISO was kept low or decreased in the flavedo during massive beta,beta-xanthophyll accumulation. In the flavedo of Lisbon lemon and Satsuma mandarin, massive accumulation of phytoene was observed with a decrease in the transcript level for CitPDS. Thus, the carotenoid accumulation during citrus fruit maturation was highly regulated by the coordination of the expression among carotenoid biosynthetic genes. In this paper, the mechanism leading to diversity in beta,beta-xanthophyll compositions between Satsuma mandarin and Valencia orange was also discussed on the basis of the substrate specificity of beta-ring hydroxylase and the balance of expression between upstream synthesis genes (CitPSY, CitPDS, CitZDS, and CitLCYb) and downstream synthesis genes (CitHYb and CitZEP).

Chlorophyll biosynthesis in higher plants. Regulatory aspects of 5-aminolevulinate formation

Chlorophyll, heme, phytochrome and siroheme biosynthesis in higher plants is regulated by the supply of their precursor 5-aminolevulinate (ALA), which is derived from glutamate. Glutamate conversion to ALA occurs only in the plastid and requires chloroplast coded tRNAGlu and nuclear coded glutamyl-tRNA synthetase, glutamyl-tRNA reductase and glutamate 1 -semialdehyde aminotransferase. The chromosomal location inArabidopsis thaliana, rice and barley of these factors are presented with UniGene data for cDNA tissue locations. Similarly also for other nuclear gene products affecting ALA formation: sigma factor sigB; theA. thaliana Flu and monocot (Tigrina-d) homologs; ξ-carotene desaturase, lycopene synthase and carotenoid isomerase. Cytokinin is a positive regulator of tRNAGlu amounts, which correlate with ALA formation. SigB is required for tRNAGlu transcription. Phytochrome A, cryptochrome and Mg-protoporphyrin repress transcription of glutamyl-tRNA reductase in the dark, which is de-repressed by red and blue light Post-transcriptional control is little understood but may inPoaceae involve messages stabilised by 5′UTR stem-loops. The reductase is inhibited by heme. The Mg-branch has its own inhibitor,A. thaliana Flu, encoding a membrane protein, recently found to be identical to barleyTigrina-d. It interacts with glutamyl-tRNA reductase through TPR domains mediating Mg-protoporphyrin inhibition. Carotenoid deficient regulatorytigrina or their phenocopies point to membranes as a regulatory site or to abscisic acid as a negative regulator.

Carotenoid isomerase: a tale of light and isomers

Carotenoids are terpenoid pigments containing systems of conjugated double bonds, each of which can exist in a cis or trans configuration. The existence of a carotenoid isomerase (CrtISO) mediating cis-to- trans isomerization has been suspected ever since the description of the tomato tangerine mutant in the early 1940s, whose fruits accumulate a poly- cis-isomer of lycopene. Recently, CrtISO has been cloned in Synechocystis and in plants, and has been shown to be related to bacterial carotene desaturases.

Identification of the elusive carotenoid isomerase enzyme redefines carotenoid biosynthesis and reveals a role for carotenoids in the membrane dynamics of plastid development

Carotenoids are essential photoprotective and antioxidant pigments synthesized by all photosynthetic organisms. Most carotenoid biosynthetic enzymes were thought to have evolved independently in bacteria and plants. In bacteria, a single enzyme catalyzes four desaturations leading from colorless phytoene to red lycopene, while in plants two desaturases, unrelated to the bacterial enzyme, are required. We have demonstrated that carotenoid desaturation in plants requires a third and different enzyme, the carotenoid isomerase (CrtS). Mutations disrupting CrtS in Arabidopsis cause accumulation of poly-cis carotenoids in the etioplast, the progenitor plastid of dark-grown plants and reveal a requirement for all-trans carotenoids in formation of the prolamellar body (PLB), the lattice of membranes that defines an etioplast. The absence of PLBs in CrtS mutants demonstrates a function for this unique structure in facilitating chloroplast development during the first critical days of seedling germination and photomorphogenesis.