Ragai K. Ibrahim

PLANT O-METHYLTRANSFERASES : MOLECULAR ANALYSIS, COMMON SIGNATURE AND CLASSIFICATION

Comparative analysis of the predicted amino acid sequences of a number of plant O-methyltransferase cDNA clones show that they share some 32–71% sequence identity, and can be grouped according to the different compounds they utilise as substrates. Five highly conserved regions are proposed as a signature for plant O-methyltransferases, two of which (regions I and IV) are believed to be involved in S-adenosyl-L-methionine and metal binding, respectively. The glycine-rich signature regions include a 36 amino acid domain which is located in the mid-terminal section of the carboxy terminus of most O-methyltransferase sequences. Cladistic analysis of the amino acid sequences suggests that plant O-methyltransferases may have arisen from common ancestral genes that were driven by different structural and/or functional requirements, and whose descendants segregated into different biochemical species. A comprehensive classification of plant O-methyltransferases is proposed following the guidelines of the Commission of Plant Gene Nomenclature.

Isoprenylated flavonoids—a survey

Abstract The structural variation of isoprenylated flavonoids, including the chalcones, flavones and flavonols, as well as their dihydro derivatives are reviewed. Emphasis is mainly directed to the modification of the side attachments arising from C 5 , C 10 or C 15 groups, and the frequency of their occurrence among the different classes of flavonoids. Some aspects related to their biosynthesis and enzymology, as well as their biological/pharmacological activities are also discussed. The natural occurrences of the various classes of isoprenylated flavonoids are tabulated as part of this review.

Phenolic Metabolism in Plants

Papers from the Thirty-first Annual Meeting of the Phytochemical Society of North Americas Symposium on Phenolic Metabolism in Plants, held June 1991, in Fort Collins, Colorado, summarize the last three decades of research and outline the current state of the field. Among the topics: compartmentati

Sulphated flavonoids: an update

Abstract The increasing knowledge of various aspects of flavonoid sulphates calls for an update of previous reviews. This article describes the recent advances in their structural variation and distribution patterns in plants. The methods used in their photochemical analysis and structural determination are outlined. Furthermore, the recently developed chemical and enzymatic methods for the synthesis of specifically sulphated flavonoids are reviewed, and two hitherto unreported, position-specific flavonol sulphotransferases are described.

Prenylated isoflavonoids—An update

Abstract This review deals with some bioorganic chemical aspects of isoflavonoids. Its highlights are rather restricted to the structural modifications and the diverse oxygenated side attachments originating from the isoprenoid substituents, 3,3-dimethylallyl (prenyl), 1,1-dimethylallyl and geranyl groups. Some aspects of naturally occurring isoflavonoids, their new classes and biological functions are also described.

Enzymology and compartmentation of polymethylated flavonol glucosides in Chrysosplenium americanum

Abstract The enzymatic synthesis of polymethylated flavonol glucosides in Chrysosplenium americanum is catalysed by a number of distinct, position-specific O -methyltransferases and O -glucosyltransferases. These enzymes seem to exist in the form of an aggregated, membrane-associated, multienzyme system. This tissue represents an ideal material for the study of these enzymatic steps, their regulation and compartmentation, as well as the intracellular localization of the final metabolites.

Aldonic Acids: A Novel Family of nod Gene Inducers of Mesorhizobium loti, Rhizobium lupini, and Sinorhizobium meliloti

Molecular signals, such as flavonoids (or nonflavonoid type), nod gene-inducers, and bacterial lipochitin oligosac-charides (LCOs) act as modulators of species specificity during early stages of infection in Rhizobium spp.-legume interactions. The fact that signaling in Lupinus albus remains to be determined prompted us to investigate the flavonoid signal responsible for nod gene induction in Rhizobium lupini. A screening method was used based on the measurement of β-galactosidase activity of R. lupini strains harboring nodC::lacZ fusions in the presence of (i) authentic lupin isoflavones, (ii) carbohydrate-like inducers, and (iii) high-pressure liquid chromatography (HPLC)-fractionated lupin seed effusates and root exudates, as putative nod gene inducers. The results indicate that both erythronic and tetronic acids (4-C sugar acids) led to low but significant increases in β-galactosidase activities, compared with the controls. In addition, lupi-wighteone, a monoprenylated isoflavone, exerts a synergistic...

Effects of various elicitors on the accumulation and secretion of isoflavonoids in white lupin

Abstract Roots of white lupin seedlings were exposed to yeast extract, chitosan, Rhizobium loti suspension and CuCl 2 ; and the isoflavonoids of their tissue extracts as well as their exudates were analysed by HPLC. Apart from specific responses characteristic of different elicitors, most treatments produced dramatic increases in the amounts of genistein and 2′-hydroxygenistein monoprenyls in root tissues, as well as major increases of all isoflavonoids in the exudates. The effect of symbiotic stress, in comparison to other stresses, is discussed in relation to the specificity of isoflavone production in white lupin.

Tricin—a potential multifunctional nutraceutical

This review throws light on the natural occurrence and distribution of tricin (5,7,4′-trihydroxy-3′,5′-dimethoxyflavone) and its conjugated forms, as more common natural plant constituents than previously known. It examines the current literature dealing with its biosynthesis, regulation, biological significance, pharmacological effects, and potential role as a chemopreventive and anticancer agent. Because of its common occurrence in cereal grain plants and the wide spectrum of its health promoting effects, a metabolic engineering strategy is proposed to produce tricin in sufficient amounts for further experimentation, and increase its accumulation in wheat grain endosperm as a nutraceutical.

Purification and properties of UDP-glucose: Coniferyl alcohol glucosyltransferase from suspension cultures of Paul's scarlet rose☆

Abstract UDP-glucose:coniferyl alcohol glucosyltransferase was isolated from 10-day-old, darkgrown cell suspension cultures of Pauls scarlet rose. The enzyme was purified 120-fold by (NH 4 ) 2 SO 4 fractionation and chromatography on DEAE-cellulose, hydroxyapatite, and Sephadex G-100. The enzyme has a pH optimum of 7.5 in Tris-HCl buffer, required an -SH group for activity, and is inhibited by ϱ-chloromercuribenzoate and EDTA. Its molecular weight is estimated to be 52,000. The enzyme is specific for the glucosylation of coniferyl alcohol ( K m 3.3 × 10 −6 M) and sinapyl alcohol ( K m 5.6 × 10 −6 M). With coniferyl alcohol as substrate the apparent K m value for UDP-glucose is 2 × 10 −6 m . The enzyme activity can be detected in a number of callus-tissue and cell-suspension cultures. The role of this enzyme is believed to be to catalyze the transfer of glucose from UDPG to coniferyl (or sinapyl) alcohol as storage intermediates in lignin biosynthesis.