In planta anthocyanin degradation by a vacuolar class III peroxidase in Brunfelsia calycina flowers

In contrast to detailed knowledge regarding the biosynthesis of anthocyanins, the largest group of plant pigments, little is known about their in planta degradation. It has been suggested that anthocyanin degradation is enzymatically controlled and induced when beneficial to the plant. Here we investigated the enzymatic process in Brunfelsia calycina flowers, as they changed color from purple to white. We characterized the enzymatic process by which B. calycina protein extracts degrade anthocyanins. A candidate peroxidase was partially purified and characterized and its intracellular localization was determined. The transcript sequence of this peroxidase was fully identified. A basic peroxidase, BcPrx01, is responsible for the in planta degradation of anthocyanins in B. calycina flowers. BcPrx01 has the ability to degrade complex anthocyanins, it co-localizes with these pigments in the vacuoles of petals, and both the mRNA and protein levels of BcPrx01 are greatly induced parallel to the degradation of anthocyanins. Both isoelectric focusing (IEF) gel analysis and 3D structure prediction indicated that BcPrx01 is cationic. Identification of BcPrx01 is a significant breakthrough both in the understanding of anthocyanin catabolism in plants and in the field of peroxidases, where such a consistent relationship between expression levels, in planta subcellular localization and activity has seldom been demonstrated.

2-Thioether-5'-O-(1-thiotriphosphate)-adenosine derivatives: new insulin secretagogues acting through P2Y-receptors

P2-receptors (P2-Rs) represent significant targets for novel drug development. P2-Rs were identified also on pancreatic B cells and are involved in insulin secretion. The aim of our study was to synthesize and evaluate pharmacologically the novel P2Y-R ligands, 2-thioether-5'-O-phosphorothioate adenosine derivatives, as potential insulin secretagogues. An efficient synthesis of these nucleosides and a facile method for separation of the chiral products is described. The enzymatic stability of the compounds towards pig-pancreas NTPDase was evaluated. The rate of hydrolysis of 2-hexylthio-5'-O-(1-thiotriphosphate)-adenosine (2-hexylthio-ATP-alpha-S) isomers by NTPDase was 28% that of ATP. The apparent affinity of the compounds to P2Y1-R was determined by measurement of P2Y-receptor-promoted phospholipase C activity in turkey erythrocyte membranes. 2-RS-ATP-alpha-S derivatives were agonists, stimulating the production of inositol phosphates with K0.5 values in the nM range. 2-RS-AMP-S derivatives were full agonists although 2 orders of magnitude less potent. All the compounds were more potent than ATP. The effect on insulin secretion and pancreatic flow rate was evaluated on isolated and perfused rat pancreas. A high increase, up to 500%, in glucose-induced insulin secretion was due to addition of 2-hexylthio-ATP-alpha-S in the nM concentration range, which represents 100-fold enhancement of activity relative to ATP. 2-Hexylthio-AMP-S was 2.5 orders of magnitude less effective. A high chemical hydrolytic stability was observed for 2-hexylthio-ATP-alpha-S. Hydrolysis of the phosphoester bond, which was the only detectable degrading reaction under the investigation conditions (pH 7.4, 37 degrees C), was slow, with a half-life of 264 hours. Moreover, even at gastric juice conditions (pH 1.4, 37 degrees C), hydrolysis of the terminal phosphate was the only detectable reaction, with a half-life of 17.5 hours. 2-Hexylthio-ATP-alpha-S isomers are enzymatically and chemically stable. These isomers are highly potent and effective insulin secretagogues, increasing, however, pancreatic vascular resistance.