Binding of low density lipoprotein (LDL) to proteoglycans and modification of LDL are key processes in atherogenesis. Recently, it has been demonstrated that during atherogenesis the extracellular pH of atherosclerotic lesions decreases. We have examined the effect of the decreased pH on the binding of LDL to human aortic proteoglycans. The binding of native, oxidized, proteolyzed (␣-chymotrypsin-treated), or lipolyzed (sphingomyelinase-or phospholipase A 2 -treated) LDL particles to proteoglycans were measured in microtiter well assays at pH 5.5-7.5. We found that the lower the pH, the higher the amount of binding of LDL to proteoglycans. At the lowest pH tested (pH 5.5), the amounts of proteoglycan-bound native, proteolyzed, sphingomyelinase-, and phospholipase A 2 -treated LDL were 20-, 23-, 30-, and 37-fold higher, respectively, than at pH 7.5. Interestingly, although oxidized LDL failed to bind to proteoglycans at neutral pH, there was significant binding at acidic pH. Binding of native and modified LDL to proteoglycans at pH 5.5 was blocked by 1 M NaCl, indicating that at neutral pH LDL binds to proteoglycans via ionic interactions. Inhibition of this binding by acetylation and cyclohexanedione treatment of LDL showed that the positively charged amino acids of apolipoprotein B-100, lysine, and arginine, respectively, mediated the ionic interaction. Taken together, our results suggest that in areas of atherosclerotic arterial intima where the extracellular pH decreases, retention of LDL by proteoglycans is enhanced, leading to extracellular accumulation of LDL and progression of the disease.Accumulation of cholesterol in the inner layer of the arterial wall, the intima, leads to the development of atherosclerotic lesions, the lesional cholesterol being mostly derived from circulating low density lipoprotein (LDL) 2 particles (1). Once LDL particles have entered the subendothelial space they are entrapped by the extracellular matrix, particularly by intimal proteoglycans (2-4), which form an organized, negatively charged, tight network (5, 6). The retention of LDL particles by proteoglycans is considered to have key role in the development of atherosclerosis (7,8). A role for LDL retention in experimental atherosclerosis has been directly demonstrated in mice expressing proteoglycan bindingdefective human apoB-100. In these transgenic mice, formation of early atherosclerotic plaques is significantly decreased (9, 10), but this modification does not prevent the development of atherosclerosis at later stages (10).Numerous hydrolytic enzymes and pro-oxidative agents are found in the arterial intima, especially in atherosclerotic lesions, and LDL particles that enter the arterial intima are susceptible to proteolytic, lipolytic, and oxidative modifications, which can lead to aggregation and fusion of the modified LDL particles (11). Indeed, intimal LDL particles show signs of proteolysis, phospholipolysis, and oxidation, and intimal lipid droplets have features that suggest they are formed from modified LDL part...