Plant Physiol, November 1999, Vol. 121, pp. 1017-1024

Purification and Characterization of S-Adenosyl-L-Methionine: Desoxyhemigossypol-6-O-Methyltransferase from Cotton Plants. An Enzyme Capable of Methylating the Defense Terpenoids of Cotton¹

Department of Biochemistry and Biophysics, Texas A&M University, College Station, Texas 77843-2128 (C.R.B.); and United States Department of Agriculture, Agricultural Research Service, Southern Crops Research Laboratory, College Station, Texas 77843 (J.L., R.D.S., A.A.B.)

Cotton contains a unique group of terpenoids including desoxyhemigossypol, hemigossypol, gossypol, hemigossypolone, and the heliocides that are part of the plant's defense system against pathogenic fungi and insects. Desoxyhemigossypol is a key intermediate in the biosynthesis of these compounds. We have isolated, purified, and characterized from cotton stele tissue infected with Verticillium dahliae a methyltransferase (S-adenosyl-L-Met: desoxyhemigossypol-6-O-methyltransferase) that specifically methylates the 6-position of desoxyhemigossypol to form desoxyhemigossypol-6-methyl ether with a K_m value of 4.5 µM for desoxyhemigossypol and a K_cat/K_m of 5.08 × 10⁴ s¹ (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-polyacrylamide gel electrophoresis gels. The enzymatic reaction does not require Mg⁺² and is inhibited 98% with 10 mM p-chloromercuribenzoate. Desoxyhemigossypol-6-methyl ether leads to the biosynthesis of methylated hemigossypol, gossypol, hemigossypolone, and the heliocides, which lowers their effectiveness as phytoalexins and insecticides.