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Kinetic characterization of caffeoyl-coenzyme a-specific 3-o-methyltransferase from elicited parsley cell suspensions.

The activity of caffeoyl-coenzyme A (CoA) 3-O-methyltransferase, an enzyme widely distributed in plants and involved in cell wall reinforcement in a disease resistance response, appears to be subject to a complex type of regulation in vivo. In cultured parsley (Petroselinum crispum) cells treated with an elicitor from Phytophthora megasperma f.sp. glycinea, the enzyme activity is rapidly induced by a transient increase in the rate of de novo transcription. Parsley caffeoyl-CoA-specific methyltransferase differs in several aspects from other plant O-methyltransferases but shows limited homology to bacterial adenine-specific DNA methyltransferases. Kinetic analysis revealed an Ordered Bi Bi mechanism for catalysis, with caffeoyl-CoA bound prior to S-adenosyl-l-methionine and feruloyl-CoA released last from the enzyme. The small inhibitory constant determined in vitro for feruloyl-CoA suggests that, in vivo, the enzyme activity is also under tight control by the steady-state product concentration in addition to the rate of transcription that becomes affected upon elicitor challenge.