F. Kurosaki, PARTIAL-PURIFICATION AND CHARACTERIZATION OF 6-HYDROXYMELLEIN-O-METHYLTRANSFERASE FROM ELICITOR-TREATED CARROT CELLS, Phytochemistry, 41(4), 1996, pp. 1023-1027
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.