Salvianolic acids
Salvianolic acids are a group of polyphenolic acids consisting of several combinations of caffeic acid and danshensu through ester and enol bonds. Salvianolic acids are water-soluble components produced by many species of the genus Salvia, mainly extracted from Salvia miltiorrhiza.
More than 10 distinct salvianolic acids, identified by letters from A to J, have been recognized. Notably, Sal-A and Sal-B are the most abundant forms. The combination of danshensu and a caffeic acid derivative or caffeic acid dimer produces several kinds of skeletons. Sal-A is formed by a molecule of danshensu and a dimer of caffeic acid. Sal-B, on the other hand, is constituted by three molecules of danshensu and one molecule of caffeic acid. Sal-C results from the combination of two molecules of danshensu, while Sal-D is characterized as a dimer of caffeic acid.
The formation of salvianolic acids is closely linked to that of rosmarinic acid. They are primarily synthesized through the phenylpropanoid and tyrosine-derived branches of the phenolic acid biosynthetic pathway. Rosmarinic acid undergoes a not well-understood oxidative reaction to convert into Salvianolic acid B. This reaction involves laccases, which are widely spread in many genera such as Arabidopsis, poplars, sorghum, etc., but appears to be highly expressed in Salvia miltiorrhiza and Salvia bowleyana.