Acyl glucuronides are ubiquitous metabolites formed from acidic xenobiotics and endogenous compounds, such as bilirubin. Previous studies indicated that the covalent binding of acyl glucuronides to proteins occurs via an imine intermediate in a manner analogous to the glycation of proteins via reducing sugars. When glucuronic acid was incubated in solution with albumin, it formed 10 and 4 times more fluorescent, Maillard reaction products with albumin after 25 days than did glucose or fructose, respectively. However, radiolabeled glucuronic acid exhibited less covalent binding to albumin than either glucose or fructose. Circular dichroism measurements indicate that glucuronic acid is about 0.02% open chain form with exposure of the reactive aldehyde, whereas fructose and glucose have 2 and 0.0026% present in solution as the open chain; thus, differences in reactivity of the reducing sugars were not correlated with exposure of the free aldehyde. Methyl glucuronate formed little fluorescent product with albumin, suggesting that the C-6 carboxylate of glucuronic acid may facilitate the reactions after covalent binding that lead to the formation of fluorescent products. When acyl glucuronide metabolites of two previously marketed acidic drugs, zomepirac and suprofen, were incubated with albumin at a concentration of 2.5 mM, more fluorescent product was formed than by 500 mM glucose. Reversible binding of the acyl glucuronides to albumin was 60-90%, but almost zero for the free reducing sugars, which indicates that reversible binding may explain the enhanced reactivity of the acyl glucuronides in forming fluorescent products with albumin. These results indicate that acyl glucuronides are reactive metabolites that may cause significant glycation of proteins with glucuronic acid in vivo.