Molecular weight distribution profile of cartilage proteoglycan in aqueous guanidinium hydrochloride before and after carboxyl group modification.

The molecular weight distribution profile of a proteoglycan preparation (s^o_20.w = 23.1 S), isolated from bovine nasal cartilage in the presence of protease inhibitors, was studied by equilibrium sedimentation in 4 m guanidinium hydrochloride. Apparent reduced molecular weights ranged from 0.8 to 2.2 × 10⁶ and their concentration dependence appeared to be compatible with the presence of a heterogeneous population of self-associating macromolecules. Carbodiimide-induced modification of about 20% of the total carboxyl groups of the complex resulted in a shift of the molecular weight distribution profile, the new values ranging from 6 to 9 × 10⁵, with a marked predominance of the larger species. Exposure of the proteoglycan to carbodiimide or methylamine alone produced only a small shift of the apparent molecular weights. Moreover, chondroitin 4-sulfate molecules subjected to the same carbodiimide-promoted modification showed no significant change in their average molecular weights. It is therefore considered that the changes observed after carboxyl group modification cannot be attributed to cleavage of either the protein or the carbohydrate moiety of the proteoglycan complex. Rather, the evidence suggests that these functions are essential to the stabilization of the oligomeric species, which constitute a large proportion of the preparation even in 4 m guanidinium hydrochloride.