Biosynthesis of the storage polysaccharide from the snail Biomphalaria glabrata, identification and specificity of a branching beta 1-->6 galactosyltransferase.

Adult snails synthesize in their albumen glands a storage polysaccharide called galactan which is utilized by the developing embryos. With [6-3H]-uridine 5'diphosphogalactose the incorporation of labelled D-galactose into the polysaccharide can be traced in freshly removed glands maintained in a bathing buffer. After centrifugation of homogenized glands, galactosyltransferase activity is only found in the insoluble fraction. Chaps extracts of this material retain almost all of their activity and can be used for comparison of the incorporation rates into different native galactans or in various oligosaccharides. A highly efficient beta-(1-->6) galactosyltransferase was detected when methyl 3-O-(beta-D-galactopyranosyl)-beta-D-galactopyranoside was offered as acceptor. The substitution at the penultimate residue resulted in a branched trisaccharide as demonstrated by 1H-NMR-spectroscopy and permethylation analysis of the reaction product. Comparable results were obtained with various oligosaccharides containing an internal galactose unit glycosidically linked beta 1-->3. Attempts to separate and purify the various enzymes involved resulted in the isolation of a fraction which is able to transfer D-Gal exclusively to native galactan, but not to oligosaccharides. A further fraction was obtained from a different resin with activity for native galactan and 6-O-(beta-D-galactopyranosyl)-D-galactopyranose, but without any for methyl-3-O-(beta-D-galactopyranosyl)-beta-D-galactopyranose. It is thus concluded that at least three different enzymes are involved in the biosynthesis of this snail galactan.