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 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.