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Biosynthesis of chondroitin sulfate. Independent addition of glucuronic acid and N-acetylgalactosamine to oligosaccharides.
Pentasaccharide 6-sulfate and hexasaccharide 6-sulfate were prepared from chondroitin 6-sulfate. Each oligosaccharide was incubated with a chick cartilage microsomal enzyme preparation and UDP [14C] glucuronic acid and/or UDP-N-[3H] acetylgalactosamine. As previously reported by other investigators, a single sugar was added from UDP-[14C] glucuronic acid to the nonreducing end of pentasaccharide 6-sulfate and from UDP-N-[3H] acetylgalactosamine to the nonreducing end of hexasaccharide 6-sulfate. The labeled oligosaccharides were characterized by gel chromatography and degradation by chondroitinase ABC followed by identification of products. The oligosaccharides in concentrations above their Km inhibited chondroitin synthesis on endogenous primers, reinforcing the assumption that the enzymes involved in the additions to exogenous oligosaccharides are the same as those involved in chondroitin polymerization. When either the pentasaccharide 6-sulfate or hexasaccharide 6-sulfate was incubated in reaction mixtures containing both of the sugar nucleotides there was generally growth of oligosaccharide by two or three sugars. With longer incubation under conditions of limiting oligosaccharide concentration, as many as 14 to 16 sugars could be added but no further chondroitin polymerization took place. Addition of each sugar was shown to depend upon the concentration of appropriate acceptor but was otherwise independent of the addition of the alternate sugar. No paired addition of sugars was noted. It was concluded that two specific enzymes are involved in alternate additions of sugars to the oligosaccharides and that the two enzymes have no apparent interaction with one another. It is suggested that the rapid polymerization to form large chondroitin chains which previously has been shown to take place on endogenous primers is facilitated by interaction of the two enzymes with a component of the endogenous primer. This component is not present in the exogenous oligosaccharides since they do not serve in the same fashion as primers for polymerization.
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