Substrate specificity of a glucosyltransferase and an N-hydroxylase involved in the biosynthesis of cyclic hydroxamic acids in gramineae

Abstract

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-β-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 puri