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.

Identification of 2'-phosphodiesterase, which plays a role in the 2-5A system regulated by interferon.

The 2-5A system is one of the major pathways for antiviral and antitumor functions that can be induced by interferons (IFNs). The 2-5A system is modulated by 5'-triphosphorylated, 2',5'-phosphodiester-linked oligoadenylates (2-5A), which are synthesized by 2',5'-oligoadenylate synthetases (2',5'-OASs), inactivated by 5'-phosphatase and completely degraded by 2'-phosphodiesterase (2'-PDE). Generated 2-5A activates 2-5A-dependent endoribonuclease, RNase L, which induces RNA degradation in cells and finally apoptosis. Although 2',5'-OASs and RNase L have been molecularly cloned and studied well, the identification of 2'-PDE has remained elusive. Here, we describe the first identification of 2'-PDE, the third key enzyme of the 2-5A system. We found a putative 2'-PDE band on SDS-PAGE by successive six-step chromatographies from ammonium sulfate precipitates of bovine liver and identified a partial amino acid sequence of the human 2'-PDE by mass spectrometry. Based on the full-length sequence of the human 2'-PDE obtained by in silico expressed sequence tag assembly, the gene was cloned by reverse transcription-PCR. The recombinant human 2'-PDE expressed in mammalian cells certainly cleaved the 2',5'-phosphodiester bond of 2-5A trimer and 2-5A analogs. Because no sequences with high homology to this human 2'-PDE were found, the human 2'-PDE was considered to be a unique enzyme without isoform. Suppression of 2'-PDE by a small interfering RNA and a 2'-PDE inhibitor resulted in significant reduction of viral replication, whereas overexpression of 2'-PDE protected cells from IFN-induced antiproliferative activity. These observations identify 2'-PDE as a key regulator of the 2-5A system and as a potential novel target for antiviral and antitumor treatments.