V. Leighton et al., SUBSTRATE-SPECIFICITY OF A GLUCOSYLTRANSFERASE AND AN N-HYDROXYLASE INVOLVED IN THE BIOSYNTHESIS OF CYCLIC HYDROXAMIC ACIDS IN GRAMINEAE, Phytochemistry, 36(4), 1994, pp. 887-892
Microsomal preparations from maize seedlings exhibited N-hydroxylase a
ctivity with 2-hydroxy-1,4-benzoxazin-3-one (HBOA) as substrate, but n
ot with its 7-methoxy analogue (HMBOA), or their corresponding 2-O-bet
a-D-glucosides. Extracts of the hydroxamic acid (Hx)-accumulating spec
ies rye, wheat and Hordeum lechleri, showed UDP-glucose:Hx-glucosyltra
nsferase activity. The hydroxamic acid, 2,4-dihydroxy-1,4-benzoxazin-3
-one (DIBOA), and its 7-methoxy analogue, DIMBOA, were accepted as sub
strates, but not HBOA or HMBOA. The Hx-glucosyltransferase in the prot
ein precipitate obtained between 30 and 60% ammonium sulphate saturati
on from either rye, wheat or H. lechleri had a higher V-max value and
lower K-m value with DIMBOA as substrate. The Hx-glucosyltransferase f
rom rye, which occurred in both roots and shoots throughout plant deve
lopment, was purified 35-fold and characterized. The M(r) of the enzym
e was 43 000 and the isoelectric point 4.4. The K-m values for DIBOA a
nd DIMBOA in the partly purified fraction were 73 and 82 mu M, respect
ively, and the V-max for DIMBOA twice that for DIBOA. The results indi
cate that the glucosides of HBOA and HMBOA are not intermediates in th
e pathways to Hx-glucosides, and that the Hx-glucosyltransferases from
species with different patterns of Hx-accumulation, are similar.