Acylhydrolases from parsley (Petroselinum hortense). Relative distribution and properties of four esterases hydrolyzing malonic acid hemiesters of flavonoid glucosides.

In parsley, malonylated flavonoid glycosides are formed in response to ultraviolet irradiation and accumulate in the vacuoles. Involvement of malonyltransferases, which catalyze the transfer of malonic acid from malonyl-coenzyme A to either flavone/flavonol 7-O-glucosides or flavonol 3-O-glucosides, has been described previously. These enzymes are present in very young leaf buds, and their activities decrease rapidly when leaves begin to unfold, while at the same time esterase activity is developing. The latter enzyme activity continues to increase with tissue age. Four esterases, distinguished by pI's of 3.8, 3.9, 4.0, and 4.05, were purified to apparent homogeneity from parsley leaves and shown to hydrolyze malonic acid hemiesters of flavonoid glucosides. These esterases are unspecific and are best described as one acetyl- and three arylesterases on the basis of inhibition studies by 4-chloromercuribenzoic acid and diisopropyl fluorophosphate. Esterases and malonic acid hemiesters appear to be separated from each other within the parsley leaf cell, and only on disruption of the cells do the respective substrates become available to the enzymes. Involvement of esterases in formation of wound periderm in parsley plants is suggested.