INCORPORATION OF AN OXYGEN FROM WATER INTO TROGLITAZONE QUINONE BY CYTOCHROME P450 AND MYELOPEROXIDASE

Stable isotope labeling strategy for curcumin metabolite study in human liver microsomes by liquid chromatography-tandem mass spectrometry

The identification of drug metabolites is very important in drug development. Nowadays, the most widely used methods are isotopes and mass spectrometry. However, the commercial isotopic labeled reagents are usually very expensive, and the rapid and convenient identification of metabolites is still difficult. In this paper, an (18)O isotope labeling strategy was developed and the isotopes were used as a tool to identify drug metabolites using mass spectrometry. Curcumin was selected as a model drug to evaluate the established method, and the (18)O labeled curcumin was successfully synthesized. The non-labeled and (18)O labeled curcumin were simultaneously metabolized in human liver microsomes (HLMs) and analyzed by liquid chromatography/mass spectrometry (LC-MS). The two groups of chromatograms obtained from metabolic reaction mixture with and without cofactors were compared and analyzed using Metabolynx software (Waters Corp., Milford, MA, USA). The mass spectra of the newly appearing chromatographic peaks in the experimental sample were further analyzed to find the metabolite candidates. Their chemical structures were confirmed by tandem mass spectrometry. Three metabolites, including two reduction products and a glucuronide conjugate, were successfully detected under their specific HLMs metabolic conditions, which were in accordance with the literature reported results. The results demonstrated that the developed isotope labeling method, together with post-acquisition data processing using Metabolynx software, could be used for fast identification of new drug metabolites.

Application of stable isotope-labeled compounds in metabolism and in metabolism-mediated toxicity studies

Stable isotope-labeled compounds have been synthesized and utilized by scientists from various areas of biomedical research during the last several decades. Compounds labeled with stable isotopes, such as deuterium and carbon-13, have been used effectively by drug metabolism scientists and toxicologists to gain better understanding of drugs' disposition and their potential role in target organ toxicities. The combination of stable isotope-labeling techniques with mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy, which allows rapid acquisition and interpretation of data, has promoted greater use of these stable isotope-labeled compounds in absorption, distribution, metabolism, and excretion (ADME) studies. Examples of the use of stable isotope-labeled compounds in elucidating structures of metabolites and delineating complex metabolic pathways are presented in this review. The application of labeled compounds in mechanistic toxicity studies will be discussed by providing an example of how strategic placement of a deuterium atom in a drug molecule mitigated specific-specific renal toxicity. Other examples from the literature demonstrating the application of stable isotope-labeled compounds in understanding metabolism-mediated toxicities are presented. Furthermore, an example of how a stable isotope-labeled compound was utilized to better understand some of the gene changes in toxicogenomic studies is discussed. The interpretation of large sets of data produced from toxicogenomics studies can be a challenge. One approach that could be used to simplify interpretation of the data, especially from studies designed to link gene changes with the formation of reactive metabolites thought to be responsible for toxicities, is through the use of stable isotope-labeled compounds. This is a relatively unexplored territory and needs to be further investigated. The employment of analytical techniques, especially mass spectrometry and NMR, used in conjunction with stable isotope-labeled compounds to establish and understand mechanistic link between reactive metabolite formation, genomic, and proteomic changes and onset of toxicity is proposed. The use of stable isotope-labeled compounds in early human ADME studies as a way of identifying and possibly quantifying all drug-related components present in systemic circulation is suggested.

CYP201A2, a cytochrome P450 from Rhodopseudomonas palustris, plays a key role in the biodegradation of tributyl phosphate

Tributyl phosphate (TBP) is a toxic organophosphorous compound widely used in nuclear fuel processing and chemical industries. Rhodopseudomonas palustris, one of the most metabolically versatile photosynthetic bacteria, is shown here to degrade TBP efficiently under photosynthetic conditions. This study shows that this O(2)- and NADPH/FMNH(2)-dependent process was also catalyzed when TBP was incubated with membrane-associated proteins extracted from this strain. The effects of several regulators of cytochrome P450 activity on the TBP consumption suggest a key role for a cytochrome P450 in this process. Disruption of the rpa0241 gene encoding a putative cytochrome P450 led to a 60% decrease of the TBP catabolism, whereas reintroducing the gene in the mutant restored the wild-type phenotype. The rpa0241 gene was expressed and purified in Escherichia coli. Characterization by UV-visible spectroscopy of the purified recombinant membrane-bound protein (CYP201A2) encoded by the rpa0241 gene revealed typical spectral characteristics of cytochrome P450 with a large spin state change of the heme iron associated with binding of TBP (K (d) approximately 65 microM). It is proposed that CYP201A2 catalyzes the initial step of the biodegradation process of TBP.

Drug metabolite profiling and elucidation of drug-induced hepatotoxicity

Drug metabolism studies, together with pathologic and histologic evaluation, provide critical data sets to help understand mechanisms underlying drug-related hepatotoxicity. A common practice is to trace morphologic changes resulting from liver injury back to perturbation of biochemical processes and to identify drug metabolites that affect those processes as possible culprits. This strategy can be illustrated in efforts of elucidating the cause of acetaminophen-, troglitazone- and valproic acid-induced hepatic necrosis, microvesicular steatosis and cholestasis with the aid of information from qualitative and quantitative analysis of metabolites. From a pharmaceutical research perspective, metabolite profiling represents an important function because a structure-activity relationship is essential to rational drug design. In addition, drugs are known to induce idiosyncratic hepatotoxicity, which usually escapes the detection by preclinical safety assessment and clinical trials. This issue is addressed, at present, by eliminating those molecules that are prone to metabolic bioactivation, based on the concept that formation of electrophilic metabolites triggers covalent protein modification and subsequent organ toxicity. Although pragmatic, such an approach has its limitations as a linear correlation does not exist between toxicity and the extent of bioactivation. It may be possible in the future that the advance of proteomics, metabonomics and genomics would pave the way leading to personalized medication in which beneficial effect of a drug is maximized, whereas toxicity risk is minimized.