6-Hydroxymellein (6HM) synthase, a biosynthetic enzyme of carrot phyto
alexin 6-methoxymellein (6MM), is induced in root slices on treatment
with elicitor. 6HM is synthesized via the polyketide pathway, and K-m
values of the synthase for its substrates, acetyl-CoA and malonyl-CoA,
were 15 and 6 mu M, respectively. The K-m values of the transacylases
included in fatty acid synthesis which share these acyl-CoAs as commo
n substrates were 4.7 and 3.2 mM. Another biosynthetic enzyme of 6MM p
roduction, 6HM O-methyltransferase, also showed a very low K-m value f
or S-adenosyl-L-methionine (0.4 mu M). These results suggest that 6MM
synthetic enzymes have high affinities for acyl-CoAs and S-adenosyl-L-
methionine and preferentially utilize these substrates as compared wit
h other constitutive enzymes. The reaction products of 6HM synthase sh
owed only slight inhibitory activities for 6HM synthase, while 6HM O-m
ethyltransferase activity was significantly inhibited in the presence
of either 6MM or S-adenosyl-L-homocysteine (K-i, 37 and 27 mu M, respe
ctively). It appears, therefore that the product inhibition of 6HM O-m
ethyltransferase is an important mechanism for regulating the concentr
ation of 6MM in carrot cells.