Purification and characterization of S-adenosyl-L-Methionine: Desoxyhemigossypol-6-O-methyltransferase from cotton plants. An enzyme capable of methylating the defense terpenoids of cotton

Cotton contains a unique group of terpenoids including desoxyhemigossypol, hemigossypol, gossypol, hemigossypolone, and the heliocides that are part o f the plant's defense system against pathogenic fungi and insects. Desoxyhe migossypol is a key intermediate in the biosynthesis of these compounds. We have isolated, purified, and characterized from cotton stele tissue infect ed with Verticilium dahliae a methyltransferase (S-adenosyl-L-Met: desoxyhe migossypol-6-O-methyltransferase) that specifically methylates the 6-positi on of desoxyhemigossypol to form desoxyhemigossypol-6-methyl ether with a K -m value of 4.5 mu M for desoxyhemigossypol and a K-cat/K-m of 5.08 x 10(4) s(-1) (mol/L)(-1). The molecular mass of the native enzyme is 81.4 kD and is dissociated into two subunits of 41.2 kD on sodium dodecyl sulfate-polya crylamide gel electrophoresis gels. The enzymatic reaction does not require Mg+2 and is inhibited 98% with 10 mM p-chloromercuribenzoate. Desoxyhemigo ssypol-6-methyl ether leads to the biosynthesis of methylated hemigossypol, gossypol, hemigossypolone, and the heliocides, which lowers their effectiv eness as phytoalexins and insecticides.