Enzymic transfer of α-L-arabinopyranosyl residues to exogenous 1,4-linked β-D-galacto-oligosaccharides using solubilized mung bean (Vigna radiata) hypocotyl microsomes and UDP-β-L-arabinopyranose

A single α-L-arabinopyranosyl (α-L-Arap) residue was shown, by a combination of chemical and spectroscopic methods, to be transferred to O-4 of the nonreducing terminal galactosyl (Gal) residue of 2-aminobenzamide (2AB)-labeled galacto-oligosaccharides when these oligosaccharides were reacted with UDP-s-L-arabinopyranose (UDP-s-L-Arap) in the presence of a Triton X-100-soluble extract of microsomal membranes isolated from mung bean (Vigna radiata, L. Wilezek) hypocotyls. Maximum-(1→4)-arabinopyranosyltransferase activity was obtained at pH 6.0–6.5 and 20°C in the presence of 25 mM Mn2+. The enzyme had an apparent K m of 45 μM for the 2AB-labeled galactoheptasaccharide and 330 μM for UDP-s-L-Arap. A series of 2AB-labeled galacto-oligosaccharides with a degree of polymerization (DP) between 6 and 10 that contained a single α-L-Arap residue linked to the former nonreducing terminal Gal residue were generated when the 2AB-labeled galactohexasaccharide (Gal6-2AB) was reacted with UDP- s-L-Ara p in the presence of UDP-β-D-Galp and the solubilized microsomal fraction. The mono-arabinosylated galacto-oligosaccharides are not acceptor substrates for the galactosyltransferase activities known to be present in mung bean microsomes. These results show that mung bean hypocotyl microsomes contain an enzyme that catalyzes the transfer of Arap to the nonreducing Gal residue of galacto-oligosaccharides and suggest that the presence of a α-L-Arap residue on the former terminal Gal residue prevents galactosylation of galacto-oligosaccharides.