Biglycan and decorin are small dermatan sulfate-containing proteoglycans in the extracellular matrix of the artery wall. The dermatan sulfate chains are known to stimulate thrombin inhibition by heparin cofactor II (HCII), a plasma proteinase inhibitor that has been detected within the artery wall. The purpose of this study was to analyze the HCII-stimulatory activity of biglycan and decorin isolated from normal human aorta and atherosclerotic lesions type II through VI and to correlate activity with dermatan sulfate chain composition and structure. Biglycan and decorin from plaque exhibited a 24 -75% and 38 -79% loss of activity, respectively, in thrombin-HCII inhibition assays relative to proteoglycan from normal aorta. A significant negative linear relationship was observed between lesion severity and HCII stimulatory activity (r ؍ 0.79, biglycan; r ؍ 0.63, decorin; p < 0.05). Biglycan, but not decorin, from atherosclerotic plaque contained significantly reduced amounts of iduronic acid and disulfated disaccharides ⌬Di-2,4S and ⌬Di-4,6S relative to proteoglycan from normal artery. Affinity coelectrophoresis analysis of a subset of samples demonstrated that increased interaction of proteoglycan with HCII in agarose gels paralleled increased activity in thrombin-HCII inhibition assays. In conclusion, both biglycan and decorin from atherosclerotic plaque possessed reduced activity with HCII, but only biglycan demonstrated a correlation between activity and specific glycosaminoglycan structural features. Loss of the ability of biglycan and decorin in atherosclerotic lesions to regulate thrombin activity through HCII may be critical in the progression of the disease.Biglycan and decorin are small leucine-rich dermatan sulfate (DS) 1 -containing proteoglycans (PGs) found in the extracellular matrix of connective tissues such as skin, bone, and cartilage. Biglycan and decorin have also been detected in the artery wall (1). They are composed of distinct core proteins linked to one (decorin) or two (biglycan) DS chains (2) that consist of alternating hexuronic acid and N-acetylgalactosamine residues. The DS chains are heterogeneous in the extent of posttranslational modifications such as the conversion of glucuronic acid to its epimer iduronic acid and sulfation at the C-4 and C-6 positions of N-acetylgalactosamine and the C-2 position of iduronic acid (3). The predominant disaccharide in mammalian DS is hexuronic acid-N-acetylgalactosamine-4-sulfate, but small amounts of "oversulfated" disaccharides containing two or three sulfates are also usually detectable. Numerous structural studies have been carried out on DS from mucosa, skin, and cartilage. The chain composition appears to be distinct for specific tissues and species (2, 4). However, relatively little is known about the structure of human arterial DSPG in health or disease. DS content is elevated in atherosclerotic plaque compared with normal aorta (5), and DSPGs produced by cultured aortic smooth muscle cells exhibit altered sulfation patterns after trea...