Biosynthesis of dermatan sulphate. Defructosylated Escherichia coli K4 capsular polysaccharide as a substrate for the d-glucuronyl C-5 epimerase, and an indication of a two-base reaction mechanism Available to Purchase

The capsular polysaccharide from Escherichia coli K4 consists of a chondroitin {[GlcA(β1→3)GalNAc(β1→4)]_n} backbone, to which β-fructofuranose units are linked to C-3 of D-glucuronic acid (GlcA) residues. Removal of the fructose units by mild acid hydrolysis provided a substrate for the GlcA C-5 epimerase, which is involved in the generation of L-iduronic acid (IdoA) units during dermatan sulphate biosynthesis. Incubation of this substrate with solubilized fibroblast microsomal enzyme in the presence of ³H₂O resulted in the incorporation of tritium at C-5 of hexuronyl units. A K_m of 67×10^-6 M hexuronic acid (equivalent to disaccharide units) was determined, which is similar to that (80×10^-6 M) obtained for dermatan (desulphated dermatan sulphate). V_max. was about 4 times higher with dermatan than with the K4 substrate. A defructosylated K4 polysaccharide isolated after incubation of bacteria with D-[5-³H]glucose released ³H₂O on reaction with the epimerase, and thus could be used to assay the enzyme. Incubation of a K4 substrate with solubilized microsomal epimerase for 6 h in the presence of ³H₂O resulted in the formation of about 5% IdoA and approximately equal amounts of ³H in GlcA and IdoA. A corresponding incubation of dermatan yielded approx. 22% GlcA, which contained virtually all the ³H label. These results are tentatively explained in terms of a two-base reaction mechanism, involving a monoprotic L-ido-specific base and a polyprotic D-gluco-specific base. Most of the IdoA residues generated by the enzyme occurred singly, although some formation of two or three consecutive IdoA-containing disaccharide units was observed.