Biosyntheses of galactosyl lipids and polysaccharide in Streptococcus mutans

Abstract

The syntheses of galactosylphospholipids and a galactose-containing polymer were observed when radio-labeled UDP-galactose was incubated with the membrane enzymes prepared from a strain of Streptococcus mutans, FA-1. The lipids were resolved into two components, lipids-1 and -2, by thin-layer and DEAE-cellulose column chromatographies. In the latter chromatography, lipid-1 was eluted by 0.0075 M and lipid-2 by 0.18 M ammonium acetate. The syntheses of lipids-1 and -2 were strongly inhibited by UDP and UMP, respectively. Both lipids-1 and -2 were degraded by mild acid, but were stable to mild alkaline hydrolysis. These results, together with their mobilities on thin-layer chromatography, suggest that lipid-1 is a galactosylphosphorylundecaprenol, and lipid-2 is a galactosylpyrophosphorylundecaprenol. When UDP-galactose was incubated with radiolabeled undecaprenol and ATP in the presence of membrane enzymes, lipids with thin-layer chromatographic mobilities of lipid-1 and lipid-2 were observed. The phosphate-to-galactose ratios in lipid-1 and lipid-2 were determined to be 1:1 and 2:1, respectively. These results indicated that lipid-1 and lipid-2 formed are galactosylmonophosphorylundecaprenol and galactosylpyrophosphorylundecaprenol, respectively. The polymer formed was eluted from the DEAE-cellulose column with a low concentration of salts (less than 0.1 M), suggesting that it is probably a polysaccharide, but not a lipoteichoic acid or teichoic acid-type polymer. In order to identify the sugars present in the polymer synthesized, the polymer purified by Sephadex G-50 and DEAE-cellulose column chromatographies was subjected to acid hydrolysis followed by NaB3H4 reduction and paper chromatographic analysis. [3H]Galactitol and a small amount of [3H]galactosaminitol were detected. This result suggests that the polymer is a nascent polysaccharide containing mainly galactose and a small amount of galactosamine, which probably derived from N-acetylgalactosamine during acid hydrolysis of the polymer.