Formation of labeled sugars from L-tyrosine-C14 in some higher plants

A forty-year journey in plant research: original contributions to flavonoid biochemistry

This review highlights original contributions by the author to the field of flavonoid biochemistry during his research career of more than four decades. These include elucidation of novel aspects of some of the common enzymatic reactions involved in the later steps of flavonoid biosynthesis, with emphasis on methyltransferases, glucosyltransferases, sulfotransferases, and an oxoglutarate-dependent dioxygenase, as well as cloning, and inferences about phylogenetic relationships, of the genes encoding some of these enzymes. The three-dimensional structure of a flavonol O-methyltransferase was studied through homology-based modeling, using a caffeic acid O-methyltransferase as a template, to explain their strict substrate preferences. In addition, the biological significance of enzymatic prenylation of isoflavones, as well as their role as phytoanticipins and inducers of nodulation genes, are emphasized. Finally, the potential application of knowledge about the genes encoding these enzyme reactions is discussed in terms of improving plant productivity and survival, modification of flavonoid profiles, and the search for new compounds with pharmaceutical and (or) nutraceutical value.Key words: flavonoid enzymology, metabolite localization, gene cloning, 3-D structure, phylogeny.

Biogenesis of rosmarinic acid in Mentha

The biogenesis of rosmarinic acid (alpha-O-caffeoyl-3,4-dihydroxyphenyl-lactic acid), the second most common ester of caffeic acid in the plant kingdom, was studied in Mentha arvense and Mentha piperita. Administration of (14)C-labelled compounds showed that, whereas the caffeoyl moiety was formed from phenylalanine via cinnamic acid and p-coumaric acid, the 3,4-dihydroxyphenyl-lactic acid moiety was formed from tyrosine and 3,4-dihydroxyphenylalanine. Time-course studies and the use of labelled rosmarinic acid showed that endogenous rosmarinic acid had a low turnover rate. The caffeoyl moiety did not appear to contribute to the formation of insoluble polymers, as has been suggested for chlorogenic acid in other plants.

FORMATION OF SUGARS FROM PHENYLPROPANOID COMPOUNDS IN TOBACCO LEAF DISKS

Leaf disks of Nicoliana tabacum var. Delcrest synthesize sugars from phenylpropanoid substrates. Appreciable incorporation of radioactivity into glucose, fructose, and sucrose occurs when C14-labelled L-phenylalanine, L-tyrosine, and cinnamic, p-coumaric, caffeic, and ferulic acids are supplied for 40 hours. In each case, incorporation in the light exceeds that in darkness. When L-phenylalanine or L-tyrosine are supplied, free glucose becomes labelled more rapidly than fructose or sucrose. Unlike the other substrates, L-tyrosine is not utilized in polyphenol metabolism. In terms of sugar production, however, comparable incorporation of activity occurs from each substrate.

THE BIOSYNTHESIS OF HYDRASTINE AND BERBERINE

The biosynthesis of hydrastine and of berberine was investigated by administering radioactive glucose, phenylalanine, tyrosine, and dopamine to Hydrastis canadensis L. Both hydrastine and berberine were found to be specifically derived from two molecules of tyrosine. The ratio of incorporation of the two molecules of radiotyrosine into hydrastine was not unity, and varied with time. Dopamine was also incorporated into hydrastine without randomization, but only one molecule was utilized. These results, which are consistent with the classical hypothesis and disprove the prephenic acid hypothesis of biogenesis, provide direct evidence for some of the steps in the biosynthetic pathway by which hydrastine and berberine are formed in the plant.