The impact of protein size and charge on its retention in articular cartilage

The interaction of several globular proteins with intact murine hyaline articular cartilage was studied in vitro. Proteins with molecular weights from 12 to 440 kDa and isoelectric points (pI) from 4.5 to 10 were tested for the ability to penetrate and persist in cartilage. Native proteins were modified for a range of pI. Using radiolabeled proteins we showed that retention of proteins in cartilage is a function of their pI. At pI 8.5-9 all proteins showed a sharp increase in cartilage when incubated at physiologic pH. The molecular weight of a protein and its charge is a determining factor for penetration of cartilage. By autoradiography highly cationic proteins up to 150 kDa (IgG) readily penetrated cartilage. Immunofluorescence confirmed these findings. Cationic catalase (240 kDa) showed superficial penetration, but penetration of cationic ferritin (440 kDa) was not demonstrated, suggesting that 240 to 440 kDa represents the upper range for penetration. Small anionic proteins (cytochrome-c; pI less than 4.5; 12 kDa) penetrate in small quantities but do not persist, whereas larger anionic proteins (IgG; pI less than 4.5; 150 kDa) cannot penetrate at all. Our data help define the properties of proteins that are able to interact with cartilage matrix and chondrocytes.