Biosynthesis of dermatan sulfate. II. Substrate specificity of the C-5 uronosyl epimerase.

Epimerization of D-glucuronosyl residues to L-iduronosyl ones during biosynthesis of dermatan sulfate involves an abstraction of the C-5 hydrogen of the target sugar residue. After inversion, a hydrogen from the medium is reinserted into the uronosyl residue. In the present study, microsomal enzyme prepared from cultured embryonic skin fibroblasts was incubated with dermatan or chondroitin in the presence of 3H2O of high specific activity. Incubation resulted in incorporation of tritium on C-5 of uronosyl residues of the substrates. The rate of the reaction was highest for dermatan. Incubation of the products with chondroitinase ABC released essentially all the tritium. Dermatan sulfate and chondroitin sulfate were inactive as substrates, which indicates that epimerization takes place before sulfation. Analyses of the product obtained after incubation of chondroitin in 3H2O-containing medium for different incubation times showed that tritium accumulated first in L-iduronosyl residues. Later, tritium was also found in D-glucuronosyl residues. The reverse situation was observed when dermatan was used as substrate. After extended incubation times, the ratio of D-[3H]glucuronic acid to L-[3H]iduronic acid in both dermatan and chondroitin reached a value of 85/15, which may reflect the equilibrium value. Digestion of labeled chondroitin with chondroitinase AC and oxidation of labeled dermatan with periodate showed that after 96 h of incubation with the epimerase and 3H2O, most of the uronic acid residues had been involved in the reaction. Both products were composed of long blocks of D-glucuronic acid-containing disaccharides interrupted by a few L-iduronic acid-containing disaccharides arranged singly of in clusters of two to three. Reincubation of the 3H-labeled products originating from dermatan or chondroitin with the epimerase resulted in release of tritium, which was linear with time and with increasing protein concentration.