TREHALOSE AS A POSSIBLE PRECURSOR OF THE SULFATED L-GALACTAN IN THE ASCIDIAN TUNIC

Among sulfated polysaccharides, those in the tunic of ascidians are un ique: their major constituent sugar is galactose, which occurs exclusi vely in the L-enantiomeric form. Incorporation of D-[C-14]glucose into tunic slices in. vitro revealed that the cells epimerize D-glucose in to L-galactose during biosynthesis of the sulfated polysaccharides. Th e interconversion of these two sugars involves exchange of hydrogen at oms at the epimerization sites with protons of the medium. Tunic cells also synthesize trehalose, although this disaccharide is not a promin ent constituent of the tissue. Pulse-chase experiments using D-[C-14]g lucose reveal that incorporation of label into trehalose precedes the synthesis of the sulfated L-galactan. In addition, the loss of label f rom trehalose coincides with the appearance of label in the sulfated L -galactan. Based on these results, we speculate that trehalose in the ascidian tunic may be a precursor of the sulfated L-galactan.