Electrical charge of hyaline articular cartilage: its role in the retention of anionic and cationic proteins.

The mouse patella with surrounding tissue has been used to study the influence of electrical charge of a high-molecular-weight protein on its retention at joint structures. Three proteins were used: native anionic bovine serum albumin (BSA), and charge modified BSA rendered cationic (pI 8.5) either by methylation (mBSA) or amidation (aBSA). Following a 2-hr incubation the two cationic (125I-labeled) proteins were strongly retained, both in the intact patellar cartilage and the surrounding tissue. Autoradiography revealed deep penetration of the proteins to the calcified zone of the articular cartilage, and a high labeling density of the cartilage, which is probably related to its high negative fixed charge density. Evidence for the electrostatic character of the binding emerged from studies at high pH (9.5) and ionic strength (0.5 M). The binding of aBSA, but not mBSA, could be prevented by pretreatment with the polycation protamine (pI 10). Posttreatment to remove retained aBSA was less effective. As expected, native BSA was not retained, unless the tissue was preincubated with antibodies. The antibody-mediated retention was more pronounced for the tissue compared with the dense patellar cartilage. Diffusion experiments revealed that enough BSA and IgG penetrate the surrounding tissue to permit substantial in situ immune complex (IC) formation, but the penetration of intact patellar cartilage was very low. Forty times more aBSA compared with BSA was taken up by intact cartilage; after trypsin treatment, which lowers the negative fixed charge density of the cartilage, the difference was reduced to a factor 4. Our data indicate that the fixed charge density of a tissue determines the uptake and retention of charged proteins, and may be a dominant principle in the occurrence of in situ IC formation within a tissue by allowing or preventing penetration of charged IC components.