Chondroitin 4-sulfotransferase, which transfers sulfate from 3-phosphoadenosine 5-phosphosulfate to position 4 of N-acetylgalactosamine in chondroitin, was purified 1900-fold to apparent homogeneity with 6.1% yield from the serum-free culture medium of rat chondrosarcoma cells by affinity chromatography on heparin-Sepharose CL-6B, Matrex gel red A-agarose, 3,5-ADP-agarose, and the second heparin-Sepharose CL-6B. SDS-polyacrylamide gel electrophoresis of the purified enzyme showed two protein bands. Molecular masses of these protein were 60 and 64 kDa under reducing conditions and 50 and 54 kDa under nonreducing conditions. Both the protein bands coeluted with chondroitin 4-sulfotransferase activity from Toyopearl HW-55 around the position of 50 kDa, indicating that the active form of chondroitin 4-sulfotransferase is a monomer. Dithiothreitol activated the purified chondroitin 4-sulfotransferase. The purified enzyme transferred sulfate to chondroitin and desulfated dermatan sulfate. Chondroitin sulfate A and chondroitin sulfate C were poor acceptors. Chondroitin sulfate E from squid cartilage, dermatan sulfate, heparan sulfate, and completely desulfated N-resulfated heparin hardly served as acceptors of the sulfotransferase. The transfer of sulfate to the desulfated dermatan sulfate occurred preferentially at position 4 of the N-acetylgalactosamine residues flanked with glucuronic acid residues on both reducing and nonreducing sides.Chondroitin sulfate proteoglycan is found in various tissues and thought to play important roles in various cellular interactions involving cell adhesion (1, 2), regulation of neurite outgrowth (3-6), migration of neural crest cells (7), binding of phospholipase A 2 (8), and an adherence receptor for Plasmodium falciparum-infected erythrocytes (9). In most of mammalian and avian chondroitin sulfate proteoglycans, the glycosaminoglycan chains bear sulfate at position 4 or 6 of N-acetylgalactosamine residues. The ratio of chondroitin 4-sulfate/chondroitin 6-sulfate has been reported to vary with the development of animals (10 -12), malignant change (13), and leukocyte differentiation (14). Sulfation of positions 4 and 6 of GalNAc residue was shown to be catalyzed by different sulfotransferases (15); chondroitin 4-sulfotransferase (C4ST) 1 and chondroitin 6-sulfotransferase (C6ST). Characterization of the purified sulfotransferases and molecular cloning of these cDNAs are basically important to reveal the functional roles of these chondroitin sulfate isomers. We have purified C6ST from the culture medium of chick chondrocytes (16) and cloned the cDNA (17). Unexpectedly, the purified C6ST was found to catalyze not only chondroitin but also keratan sulfate (18) and sialyl N-acetyllactosamine oligosaccharides (19). We previously observed that C4ST was also secreted to the culture medium of chick chondrocytes (20), but the purification of C4ST from the culture medium of chick chondrocytes has been hampered by the presence of an excess amount of C6ST. Unlike chick chondrocytes, rat chondrosa...