PARTIAL-PURIFICATION AND CHARACTERIZATION OF 6-HYDROXYMELLEIN-O-METHYLTRANSFERASE FROM ELICITOR-TREATED CARROT CELLS

6-Hydroxymellein-O-methyltransferase (6HMOMT), an inducible OMT involv ed in the biosynthesis of the carrot phytoalexin 6-methoxymellein (6MM ), was partially purified with a 105-fold increase in the specific act ivity of the enzyme. The OMT showed a maximal activity within the pH r ange of 7.4-8.0, and its M(r) was estimated to be 76 000 while its pi was 5.7. The activity of 6HMOMT was appreciably inhibited in the prese nce of several divalent cations such as CU2+, Co2+, Fe2+, Mn2+, and su lphydryl reagents. It was also inhibited by the addition of its reacti on products, 6MM and S-adenosyl-L-homocysteine. 6MM was found to inhib it the enzymatic reaction with respect to the two co-substrates of 6HM OMT, 6HM and S-adenosyl-L-methionine, in a competitive manner. Competi tive inhibition of the OMT was also observed for another product of th e enzyme, S-adenosyl-L-homocysteine, as the function of these two subs trates. These results suggest that the catalytic reaction of 6HMOMT pr oceeds by a sequential bireactant mechanism in which both the entry of the co-substrates into and the release of the co-products from the en zyme take place in random order.