Oxygen-centred free radicals can efficiently degrade the polypeptide of proteoglycans in whole cartilage

Bovine nasal cartilage slices, biosynthetically labelled in their proteoglycan with³⁵SO₄, were used as substrate for the attack of free radicals generated on exposure to a Co⁶⁰ source (which allows study of single radical species), and by chemical and enzymatic means. Systems generating hydroxyl (OH^•) and superoxide (0₂^•-) radicals degraded the proteoglycan efficiently, while the hydroperoxy radical (HO₂^•) was less efficient; addition of appropriate radical scavengers inhibited degradation. The radioactive products were heterogeneous in molecular size, but with doses up to 3600 Gy were the same size range as intact chondroitin sulphate. They contained free amino groups, and more were liberated by aminopeptidase M digestion, implying that at least a small peptide was present. Thus a major site of radical attack may be the polypeptide chain. We suggest that free-radical fragmentation of polypeptides may be important both in extracellular catabolism and in intracellular proteolysis.