Phosphate-mediated degradation of organic pollutants in water with peroxymonosulfate revisited: Radical or non-radical oxidation?

Whilst it is generally recognized that phosphate enables to promote the removal of some organic pollutants with peroxymonosulfate (PMS) oxidation, however, there is an ongoing debate as to whether free radicals are involved. By integrating different methodologies, here we provide new insights into the reaction mechanism of the binary mixture of phosphates (i.e., NaH2PO4, Na2HPO3, and NaH2PO2) with peroxymonosulfate (PMS) or hydrogen peroxide (H2O2). Enhanced degradation of organic pollutants and observation of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) adducts (i.e. DMPOOH and 5,5-dimethyl-2-oxopyrroline-1-oxyl (DMPOX)) with electron paramagnetic resonance (EPR) in most phosphates/PMS system seemly support a radical-dominant mechanism. However, fluorescence probe experiments confirm that no significant amount of hydroxyl radicals (•OH) are produced in such reaction systems. PMS in the phosphate solutions (without any organics) remains relatively stable, but is only consumed while organic substrates are present, which is distinct from a typical radical-dominant Co2+/PMS system where PMS is continuously decomposed. Through density functional theory (DFT) calculation, the energy barriers of the phosphates/PMS reaction processes are greatly decreased when non-radical mechanism dominates. Complementary evidence suggests that the reactive intermediates of PMS-phosphate complex, rather than the free radicals, are capable of oxidizing electron-rich substrates such as DMPO and organic pollutants. Taking the case of phosphate/PMS system as an example, this study demonstrates the necessity of acquisition of lines of evidence for resolving paradoxes in identifying EPR adducts.

The role of CYP2A and CYP2E1 in the metabolism of 3-methylindole in primary cultured porcine hepatocytes

The accumulation of 3-methylindole (3MI) in uncastrated male pigs (boars) is a major cause of boar taint, which negatively affects the quality of meat from the animal. Previously, CYP2E1 and CYP2A have been identified as cytochrome P450 (P450) isoforms involved in the metabolism of 3MI using porcine liver microsomes. This study further examines the role of these isoforms in the metabolism of 3MI using a primary porcine hepatocyte model by examining metabolic profiles of 3MI after incubation with P450 inhibitors. Incubation of hepatocytes with 4-methylpyrazole resulted in a selective inhibition of CYP2E1 activity as determined by p-nitrophenol hydroxylase activity and an associated significant decrease in the production of the 3MI metabolites 3-hydroxy-3-methyloxindole and 3-methyloxindole. Furthermore, inhibition of CYP2A, as assayed by coumarin 7-hydroxylase activity, using 8-methoxypsoralen and diethyldithiocarbamate was not associated with any further significant inhibition of the production of 3MI metabolites. Treatment with general P450 inhibitors resulted in further decreases in CYP2E1 activity and a more dramatic decrease in the production of 3MI metabolites, suggesting that additional P450s may be involved in the phase 1 metabolism of 3-methylindole. In conclusion, CYP2E1 activity levels are more important than CYP2A activity levels for the metabolism of 3-methylindole in isolated pig hepatocytes.

LIVER GRAFTS PRESERVED IN CELSIOR SOLUTION AS SOURCE OF HEPATOCYTES FOR DRUG METABOLISM STUDIES: COMPARISON WITH SURGICAL LIVER BIOPSIES

Suitability of human liver grafts preserved in Celsior solution (CS) for preparing metabolically competent hepatocyte cultures has been examined. To this end, basal and induced activity and mRNA levels of major hepatic cytochrome P450 (P450) enzymes have been measured. By 24 h in culture, measurable levels of the 10 P450 mRNAs studied were found in all hepatocyte preparations examined, with CYP2E1, CYP2C9, and CYP3A4 mRNAs being the most abundant. Compared with hepatocytes obtained from surgical liver resections (SLRs), lower content of each P450 mRNA was found in hepatocytes from the CS group; however, the relative distribution of individual P450 mRNAs was similar. Similar results were observed after measuring P450 activities. CYP1A2, CYP2A6, CYP2C9, CYP2C19, CYP2E1, and CYP3A4 activities in hepatocytes from CS-flushed grafts were lower than but comparable with those of cultures prepared from SLRs. No differences in the metabolite profile of testosterone were found. Treatment of hepatocytes from CS-preserved grafts with model P450 inducers shows that 2 microM methylcholanthrene only increased CYP1A1 and CYP1A2 mRNAs (>100-fold over control), 1 mM phenobarbital markedly increased CYP2A6, CYP2B6, and CYP3A4 mRNA content (>7-fold), and 50 microM rifampicin highly increased CYP3A4 mRNA levels (>10-fold), whereas minor effects (<3-fold) were observed in CYP2A6, CYP2B6, and CYP2C9 mRNAs. This induction pattern of P450s was similar, in terms of magnitude, reproducibility, and specificity, to that shown in primary hepatocytes from surgical biopsies. Overall, our results indicate that, cold-preserved in CS, liver grafts constitute a valuable source of human hepatocytes for drug metabolism studies.

FLUORESCENCE-BASED ASSAYS FOR SCREENING NINE CYTOCHROME P450 (P450) ACTIVITIES IN INTACT CELLS EXPRESSING INDIVIDUAL HUMAN P450 ENZYMES

In this study we describe a battery of fluorescence assays for rapid measurement in intact cells of the activity of nine cytochromes P450 (P450s) involved in drug metabolism. The assays are based on the direct incubation of monolayers of cells expressing individual P450 enzymes with a fluorogenic substrate followed by fluorimetric quantification of the product formed and released into incubation medium. For each individual P450 activity, different fluorescence probes were examined, and the one showing the best properties (highest metabolic rates, lowest background fluorescence) was selected: 3-cyano-7-ethoxycoumarin for CYP1A2 and CYP2C19, coumarin for CYP2A6, 7-ethoxy-4-trifluoromethylcoumarin for CYP2B6, dibenzylfluorescein for CYP2C8, 7-methoxy-4-trifluoromethylcoumarin (MFC) for CYP2C9 and CYP2E1, 3-[2-(N,N-diethyl-N-methylammonium)ethyl]-7-methoxy-4-methylcoumarin for CYP2D6, and 7-benzyloxy-4-trifluoromethylcoumarin for CYP3A4. Fluorescence-based assays are highly sensitive and allow the simultaneous measurement of a large number of samples using plate readers, thus enhancing sample throughput. Major advantages over high-throughput assays in subcellular fractions are that, as living cells are used, manual handling and enzyme damage are minimized, the endoplasmic reticulum of the cells remains intact, exogenous cofactors or NADPH-regenerating systems are not required, and transport processes are maintained. These assays can be applied to preliminary screening of inhibitory effects of new drugs on individual P450 enzymes. After comparison of the results obtained using the fluorescent probes in intact P450-expressing cells and those obtained using the high-performance liquid chromatography-based selective assays in the same cells, in primary human hepatocytes or in human liver microsomes, a fairly good agreement was found.

Expression and induction of a large set of drug-metabolizing enzymes by the highly differentiated human hepatoma cell line BC2

The BC2 cell line derived from the human hepatocarcinoma, HGB, undergoes a spontaneous sharp differentiation process in culture as it becomes confluent, remains stably differentiated for several weeks, and may return to proliferation thereafter under appropriate density conditions. The relevance of the line as an hepatic model has been evaluated. Cells synthesize a large number of plasma proteins, and rates of glycogen and urea synthesis increase with time of confluency and become sensitive to insulin, reflecting the process of differentiation. Differentiated BC2 cells express the most relevant cytochrome P-450 (CYP) isozyme activities (CYP1A1/2, 2A6, 2B6, 2C9, 2E1, and 3A4) and conjugating enzymes (glutathione S-transferase and UDP-glucuronyltransferase) and also respond to model inducers. Methylcholanthrene induced an increase in CYP1A1/2 enzyme activity (eightfold), phenobarbital induced CYP2B6 activity (1.7-fold), and dexamethasone induced CYP3A4 activity (fivefold). In parallel, expression of the most relevant liver-enriched transcription factors, HNF-4, HNF-1, C/EBP-alpha and C/EBP-beta mRNAs, was significantly increased in differentiated cultures. This increase was largest in HNF-1 and HNF-4, which supports the idea that a redifferentiation process towards the hepatic phenotype takes place. BC2 is an hepatic cell line that is able to express most hepatic functions, especially the drug-biotransformation function, far more efficiently than any previously described human hepatoma cell line.