Substrate specificity of a glucosyltransferase and an N-hydroxylase involved in the biosynthesis of cyclic hydroxamic acids in Gramineae
1994
Leighton, V. | Niemeyer, H.M. | Jonsson, L.M.V.
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 Vmax value and lower Km 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 Mr of the enzyme was 43 000 and the isoelectric point 4.4. The Km values for DIBOA and DIMBOA in the partly purified fraction were 73 and 82 micromolar, respectively, and the Vmax 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.
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