Biosynthesis of chondroitin sulfate. Chain termination

Incubation of chick embryo epiphyseal microsomal preparations with either UDP-[14C]GlcUA or UDP-[14C]-GalNAc plus exogenous chondroitin 6-sulfate resulted in the incorporation of either a single [14C]GlcUA or a [14C]GalNAc onto the nonreducing ends of the exogenous glycosaminoglycan. Degradation by chondroitinase ABC yielded the terminal products [14C]Di-OS, [14C]Di-6S, and [14C]GalNAc. Incubations of the microsomal preparations with either UDP-[14C]GlcUA or UDP-GalN[3H]Ac without exogenous chondroitin 6-sulfate resulted in the addition of a single sugar onto the nonreducing end of endogenous chondroitin sulfate. Degradation by chondroitinase ABC yielded the terminal products [14C]Di-OS, [14C]Di-6S, and GalN[3H]Ac in a molar ratio of approximately 1:1:3.5. Incubations of the microsomal preparations with both UDP-[14C]-GlcUA and UDP-GalN[3H]Ac together resulted in formation of [14C,3H]chondroitin chains added to the endogenous chondroitin sulfate. Degradation by chondroitinase ABC resulted in products with a molar ratio of [14C,3H]Di-OS to GalN[3H]Ac varying from approximately 1:1.5 to 1:3. The results of these experiments indicate that chondroitin 6-sulfate terminates at its nonreducing end in a mixture of GlcUA and GalNAc (some sulfated). GalNAc is somewhat more frequent as the terminal sugar and adds more readily to endogenous acceptors.