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 polysacchari de called galactan which is utilized by the developing embryos. With [ 6-H-3]-uridine 5'diphosphogalactose the incorporation of labelled D-ga lactose into the polysaccharide can be traced in freshly removed gland s 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 a ctivity and can be used for comparison of the incorporation rates into different native galactans or in various oligosaccharides. A highly e fficient beta-(1 --> 6) galactosyltransferase was detected when methyl (beta-D-galactopyranosyl)-beta-D-galactopyranoside was offered as acc eptor. The substitution at the penultimate residue resulted in a branc hed trisaccharide as demonstrated by H-1-NMR-spectroscopy and permethy lation analysis of the reaction product. Comparable results were obtai ned with various oligosaccharides containing an internal galactose uni t glycosidically linked beta 1 --> 3. Attempts to separate and purify the various enzymes involved resulted in the isolation of a fraction w hich 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 galactopyrano syl)-D-galactopyranose, but without any for -(beta-D-galactopyranosyl) -beta-D-galactopyranose. It is thus concluded that at least three diff erent enzymes are involved in the biosynthesis of this snail galactan.