SUBSTRATE-SPECIFICITY OF A GLUCOSYLTRANSFERASE AND AN N-HYDROXYLASE INVOLVED IN THE BIOSYNTHESIS OF CYCLIC HYDROXAMIC ACIDS IN GRAMINEAE

Microsomal preparations from maize seedlings exhibited N-hydroxylase activity with 2-hydroxy-1,4-benzoxazin-3-one (HBOA) as substrate, but not with its 7-methoxy analogue (HMBOA), or their corresponding 2-O-beta-D-glucosides. Extracts of the hydroxamic acid (Hx)-accumulating species rye, wheat and Hordeum lechleri, showed UDP-glucose:Hx-glucosyltransferase activity. The hydroxamic acid, 2,4-dihydroxy-1,4-benzoxazin-3-one (DIBOA), and its 7-methoxy analogue, DIMBOA, were accepted as substrates, but not HBOA or HMBOA. The Hx-glucosyltransferase in the protein precipitate obtained between 30 and 60% ammonium sulphate saturation 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 from rye, which occurred in both roots and shoots throughout plant development, was purified 35-fold and characterized. The M(r) of the enzyme was 43 000 and the isoelectric point 4.4. The K-m values for DIBOA and DIMBOA in the partly purified fraction were 73 and 82 mu M, respectively, and the V-max for DIMBOA twice that for DIBOA. The results indicate that the glucosides of HBOA and HMBOA are not intermediates in the pathways to Hx-glucosides, and that the Hx-glucosyltransferases from species with different patterns of Hx-accumulation, are similar.