Paraoxonase 1 as a major bioactivating hydrolase for olmesartan medoxomil in human blood circulation: molecular identification and contribution to plasma metabolism.

Olmesartan medoxomil (OM) is a prodrug-type angiotensin II type 1 receptor antagonist. The OM-hydrolyzing enzyme responsible for prodrug bioactivation was purified from human plasma through successive column chromatography and was molecularly identified through N-terminal amino acid sequencing, which resulted in a sequence of 20 amino acids identical to that of human paraoxonase 1 (PON1). Two recombinant allozymes of human PON1 (PON1(192QQ) and PON1(192RR)) were constructed and were clearly demonstrated to hydrolyze OM; hydrolysis by the latter allozyme was slightly faster than that by the former. In addition, we evaluated the contribution of PON1 to OM bioactivation in human plasma. Enzyme kinetic studies demonstrated that OM was hydrolyzed more effectively by the recombinant PON1 proteins than by purified albumin. The OM-hydrolyzing activities of the recombinant PON1 proteins and diluted plasma were greatly reduced in the absence of calcium ions. Immunoprecipitation with anti-PON1 IgG completely abolished the OM-hydrolyzing activity in human plasma, whereas the activity was partially inhibited with anti-albumin IgG. The distribution pattern of the OM-hydrolyzing activity in human serum lipoprotein fractions and lipoprotein-deficient serum was examined and showed that most of the OM-hydrolyzing activity was located in the high-density lipoprotein fraction, with which PON1 is closely associated. In conclusion, we identified PON1 as the OM-bioactivating hydrolase in human plasma on a molecular basis and demonstrated that PON1, but not albumin, plays a major role in OM bioactivation in human plasma.