PubMed · 14948497
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H KREITMAIR. 1952. [Adonis vernalis].. https://pubmed.ncbi.nlm.nih.gov/14948497/
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Carotenoids are plant secondary metabolites that comprise two main groups: carotenes and xanthophylls. The latter group includes zeaxanthin which is synthesized by beta-carotene hydroxylase catalyzing the hydroxylation of the beta-rings of beta-carotene molecules. To develop tools to alter carotenoid biosynthesis in plants, we isolated a cDNA clone encoding a candidate beta-carotene hydroxylase, CrtH1, from the flower petals of Adonis aestivalis. CrtH1 protein has homology to beta-carotene hydroxylases from other organisms, and possesses the four histidine motifs conserved in this family of enzymes. Sequence analysis predicted the presence of a putative plastid transit peptide at the amino terminus and four transmembrane helical regions. Southern-blot analysis showed CrtH1 to be encoded by a multicopy gene family with at least three members in A. aestivalis. Analysis of CrtH1 transcript abundance by Northern blotting indicates it is highly expressed in flower petals, roots and stems, with relatively low expression in leaves and developing seeds. CrtH1 was able to catalyze the formation of zeaxanthin and its intermediate precursor beta-cryptoxanthin from beta-carotene in functional assays conducted in E. coli. Expression of CrtH1 in Arabidopsis thaliana wild type and a mutant deficient for endogenous beta-carotene hydroxylases enhanced the biosynthesis of violaxanthin in the seeds.
The red ketocarotenoid astaxanthin (3,3'-dihydroxy-4,4'-diketo-beta,beta-carotene) is widely used as an additive in feed for the pigmentation of fish and crustaceans and is frequently included in human nutritional supplements as well. There is considerable interest in developing a plant-based biological production process for this valuable carotenoid. Adonis aestivalis (Ranunculaceae) is unusual among plants in synthesizing and accumulating large amounts of astaxanthin and other ketocarotenoids. The formation of astaxanthin requires only the addition of a carbonyl at the number 4 carbon of each beta-ring of zeaxanthin (3,3'-dihydroxy-beta,beta-carotene), a carotenoid typically present in the green tissues of higher plants. We screened an A. aestivalis flower library to identify cDNAs that might encode the enzyme that catalyzes the addition of the carbonyls. Two closely related cDNAs selected in this screen were found to specify polypeptides similar in sequence to plant beta-carotene 3-hydroxylases, enzymes that convert beta-carotene (beta,beta-carotene) into zeaxanthin. The Adonis enzymes, however, exhibited neither 4-ketolase nor 3-hydroxylase activity when presented with beta-carotene as the substrate in Escherichia coli. Instead, the products of the Adonis cDNAs were found to modify beta-rings in two distinctly different ways: desaturation at the 3,4 position and hydroxylation of the number 4 carbon. The 4-hydroxylated carotenoids formed in E. coli were slowly metabolized to yield compounds with ketocarotenoid-like absorption spectra. It is proposed that a 3,4-desaturation subsequent to 4-hydroxylation of the beta-ring leads to the formation of a 4-keto-beta-ring via an indirect and unexpected route: a keto-enol tautomerization.
Three horses died as a result of eating grass hay containing summer pheasant's eye (Adonis aestivalis L.), a plant containing cardenolides similar to oleander and foxglove. A 9-year-old thoroughbred gelding, a 20-year-old appaloosa gelding, and a 5-year-old quarter horse gelding initially presented with signs of colic 24-48 hours after first exposure to the hay. Gastrointestinal gaseous distension was the primary finding on clinical examination of all three horses. Two horses became moribund and were euthanatized 1 day after first showing clinical signs, and the third horse was euthanatized after 4 days of medical therapy. Endocardial hemorrhage and gaseous distension of the gastrointestinal tract were the only necropsy findings in the first two horses. On microscopic examination, both horses had scattered foci of mild, acute myocardial necrosis and neutrophilic inflammation associated with endocardial and epicardial hemorrhage. The third horse that survived for 4 days had multifocal to coalescing, irregular foci of acute, subacute, and chronic myocardial degeneration and necrosis. A. aestivalis (pheasant's eye, summer adonis) was identified in the hay. Strophanthidin, the aglycone of several cardenolides present in Adonis spp., was detected by liquid chromatography-mass spectrometry-mass spectrometry in gastrointestinal contents from all three horses. Although Adonis spp. contain cardiac glycosides, cardiac lesions have not previously been described in livestock associated with consumption of adonis, and this is the first report of adonis toxicosis in North America.