Acyl glucuronides are electrophilic metabolites that are readily hydrolyzed, undergo intramolecular rearrangement, and bind covalently to endogenous proteins. Gemfibrozil is a fibrate lipid-lowering agent that is extensively metabolized to an acyl glucuronide conjugate in humans. The aims of this study were to examine the interactions of 1-O-gemfibrozil-beta-D-glucuronide with human serum albumin. The degradation of 1-O-gemfibrozil-beta-D-glucuronide (approximately 200 microM) was examined in vitro during incubations at 37 degrees C with phosphate buffer (pH 7.4 or 9.0), solutions of human serum albumin (pH 7.4), or fresh human plasma (pH 7.4). The effects of diazepam, oxyphenbutazone, and gemfibrozil on the degradation of 1-O-gemfibrozil-beta-D-glucuronide, and its reversible binding to albumin were also studied. A pilot in vivo study was performed on two patient volunteers administered 1 g/day p.o. gemfibrozil. 1-O-Gemfibrozil-beta-D-glucuronide was unstable, with degradation half-lives in buffer of 4.1 hr and 44 hr at pH 9.0 and 7.4, respectively; and 8.5 hr and 5.5 hr in pH 7.4 solutions of human serum albumin or fresh plasma, respectively. Degradation was dependent on pH and the presence of albumin, which seemed to accelerate the intramolecular rearrangement and hydrolysis of the conjugate. 1-O-Gemfibrozil-beta-D-glucuronide was highly reversibly bound to albumin, with a mean unbound fraction of 0.028, and its degradation seemed to be related to the degree of reversible binding. Hydrolysis and covalent binding were associated with the site II binding domain on albumin, because only diazepam inhibited these reactions. However, intramolecular rearrangement was increased when binding to the site I domain was inhibited. Covalent binding was also detected in vivo to human plasma proteins. The half-life of the gemfibrozil-protein adducts was 2.5-3 days. Albumin plays an important role in the disposition of acyl glucuronides by acting as: i) a transporter protein; ii) a potential catalyst for their degradation and, therefore, clearance; and iii) a target for covalent adduct formation.