Proteoglycans consist of one or more glycosaminoglycan side chains covalently bound to a core protein (1, 2). The heparan sulfate proteoglycans (HSPGs) 1 constitute a major class of proteoglycans that are found in the extracellular matrix, especially in basement membranes, and at the cell surface, associated with the cell membrane (3-5). Many biological activities of HSPGs are due to interactions between the heparan sulfate (HS) polysaccharide side chains and a variety of proteins, which include extracellular matrix molecules, enzymes, enzyme inhibitors, growth factors and other cytokines (2, 5-7). These interactions can be specific, dependent on defined sulfation patterns within given sequences of sugar residues, as described for antithrombin (8), basic fibroblast growth factor (9, 10), and hepatocyte growth factor (11); others appear to be mainly based on relatively nonspecific electrostatic interactions, and involve proteins such as lipoprotein lipase (12), platelet factor 4 (13) and mast cell protease I (14) (for a general discussion, see Ref. 15).The biosynthesis of HS involves the formation of a nonsulfated (GlcA1,4-GlcNAc␣1,4) n precursor polysaccharide, which subsequently undergoes a series of polymer-modification reactions. These reactions start with N-deacetylation/N-sulfation of GlcNAc residues, which is followed by C-5 epimerization of GlcA to iduronic acid (IdoA) units, and finally by O-sulfation at various positions (5). The GlcA C-5 epimerization and O-sulfation reactions occur in the close vicinity of N-sulfate groups, pointing to a key role for the glucosaminyl N-deacetylase/ N-sulfotransferase enzyme in determining the overall extent of modification of the HS chain. Structural analysis of HS preparations have revealed that the modifications tend to colocalize in block sequences, separated by relatively unmodified domains (16 -19). The extent of biosynthetic modification, affecting the number, length, and substitution patterns of the modified domains as well as their position along the HS chain, may differ among cell types (20), alter during proliferation (21), and change as a result of cell transformation (22,23). Structural analysis of HS is complicated by the fact that even highly purified and uniform preparations consist of mixtures of polysaccharide chains that have reached different levels of modification. Monoclonal antibodies (mAbs) that specifically recognize well defined epitopes in HS could be major tools in such analysis. We recently described the production of such an an-
