The heterogeneity of cartilage proteoglycans. Isolation of different types of proteoglycans from bovine articular cartilage.

Proteoglycans, proteins, and glycoproteins were extracted from femoral condyle (FC) and metacarpalphalangeal (MP) articular cartilages and fractionated by sequential density gradient sedimentation under dissociative conditions. The major high density proteoglycan (PG-I) accounted for 74% of the total proteoglycan extracted from the MP cartilage and 79% of that from the femoral condyle. These proteoglycans were polydisperse, had a weight average molecular weight of 1.28 X 10” (MP) and 1.31 X lo6 (FC), and contained 16% (w/w) as amino acids. Fractionation by gel chromatography showed that the polydispersity was due to variations in the length of the protein core and the number of chondroitin sulfate side chains. The PG-I proteoglycans with different molecular weights contained relatively constant amounts of keratan sulfate, but the analyses did not indicate that they contained a hyaluronic acid-binding region with the properties described previously (Heinegard, D., and Hascall, V. C. (1974) J. Biol. Chem. 249, 2450-2456; Heinegard, D. (1977) J. Biol. Chem. 252, 1980-1989). Infrared analysis showed that the PG-I glycosaminoglycans were primarily 6-sulfated. A second type of proteoglycan (PG-III) was isolated from the low buoyant density gradient fraction. PG-III accounted for about 11% of the total proteoglycans extracted from the MP cartilage. The molecular weight of PG-III isolated by gel filtration and ion exchange chromatography was 0.24 x 106. The amino acid composition and the protein (40%), glucosamine (2.8%), and galactosamine contents (10.1%) indicated that it was not derived from PG-I proteoglycans by degradation. Infrared analysis showed that the PG-III glycosaminoglycans were primarily 4-sulfated. Based on the sulfate content of the low density gradient fractions, PG-III accounted for 2% of the total proteoglycans extracted from the FC cartilage. About 11% of the total proteoglycans in both the FC and MP cartilages occurred as a medium density component (PG-II) with a protein content of 24% and a different amino acid composition than PG-I. Following reduction and alkylation, PG-II was apparently cleaved to yield a proteoglycan fraction with a similar composition to PG-I and a low molecular weight peptidecontaining fraction, indicating that PG-II contained two peptide components. The proteins and glycoproteins accounted for 30% of the constituents extracted from the MP cartilage compared to 14% for the FC cartilage. Based on the properties of PG-I, -11, and -111,