Polydispersity of proteoglycans synthesized by chondrocytes from the Swarm rat chondrosarcoma.

The population of proteoglycan monomers in aggregates was purified from chondrocyte cultures after labeling with either [3H]serine and [35S]methionine or [3H]serine and [35S]sulfate. Digestion of the monomers labeled with [3H]serine and [35S]methionine with trypsin indicated that serine was enriched (approximately 80%) in the chondroitin sulfate attachment region of the core protein, while methionine was enriched (approximately 60%) in the hyaluronic acid-binding region. Sepharose CL-2B chromatography and velocity gradients were used to isolate monomer subfractions which differed in molecular size. The 3H/35S ratios for the subfractions from monomers labeled with [3H]serine and [35S]methionine were nearly constant, indicating that the core protein lengths were constant regardless of the size of the monomer. Conversely, the 3H/35S ratios for subfractions of monomers labeled with [3H]serine and [35S]sulfate increased significantly with decreasing sizes. Chondroitin sulfate chains in subfractions were released by alkaline borohydride treatment. The mean molecular weights of the chondroitin sulfate chains decreased from approximately 18,500 in the largest subclass of monomers to approximately 12,500 in the smallest. [3H]Serine-labeled monomer subfractions were digested with papain to determine the proportion of serine residues substituted with chondroitin sulfate. The distribution of label was constant for greater than 90% of the monomers from the largest to the smallest. The results indicate that, for at least 90% of the newly synthesized monomers that are able to aggregate, variation in chondroitin sulfate chain size is the only contributing factor to polydispersity in their molecular size.