Effect of phenobarbital and beta-naphthoflavone on activities of different rat esterases after paraoxon exposure

Variation in paraoxonase-1 activity and atherosclerosis

PURPOSE OF REVIEW

Paraoxonase-1 (PON1) is an HDL-associated protein of 354 amino acids with a molecular mass of 43 000 Da. It is synthesized in the liver, and in serum it is almost exclusively associated with HDL. PON1 has been reported to be an important contributor to the antioxidant and anti-inflammatory activities of HDL. PON1 impedes oxidative modification of LDL. PON1 serum activity is related to systemic lipid peroxidation stress and prospective cardiovascular risk. In this review, we discuss the relationship between PON1 activity and atherosclerotic diseases and various factors modulating PON1 activity including genes, age, lifestyle factors and medical conditions. Finally, evidence that pharmacological agents may affect PON1 activity is summarized.

RECENT FINDINGS

There is increasing evidence from both animal and human studies linking low PON1 activity to an increased likelihood of cardiovascular diseases. Two prospective studies reported a significantly lower incidence of major cardiovascular events in participants with the highest systemic PON1 activity, compared with those with the lowest activity.

SUMMARY

PON1 is a potentially antiatherogenic HDL-associated enzyme that protects LDL from oxidative modification. Enhancing PON1 activity could be an important target for future pharmacological agents aimed at decreasing cardiovascular risk.

Involvement of phenobarbital and SKF 525A in the hepatotoxicity of antifungal drugs itraconazole and fluconazole in rats

Itraconazole and fluconazole are potent wide spectrum antifungal drugs. Both of these drugs induce hepatotoxicity clinically. The mechanism underlying the hepatotoxicity is unknown. The purpose of this study was to investigate the role of phenobarbital (PB), an inducer of cytochrome P450 (CYP), and SKF 525A, an inhibitor of CYP, in the mechanism of hepatotoxicity induced by these two drugs in vivo. Rats were pretreated with PB (75 mg/kg for 4 days) prior to itraconazole or fluconazole dosing (20 and 200 mg/kg for 4 days). In the inhibition study, for 4 consecutive days, rats were pretreated with SKF 525A (50 mg/kg) or saline followed by itraconazole or fluconazole (20 and 200 mg/kg) Dose-dependent increases in plasma alanine aminotransferase (ALT), gamma-glutamyl transferase (gamma-GT), and alkaline phosphatase (ALP) activities and in liver weight were detected in rats receiving itraconazole treatment. Interestingly, pretreatment with PB prior to itraconazole reduced the ALT and gamma-GT activities and the liver weight of rats. No changes were observed in rats treated with fluconazole. Pretreatment with SKF 525A induced more severe hepatotoxicity for both itraconazole and fluconazole. CYP 3A activity was inhibited dose-dependently by itraconazole treatment. Itraconazole had no effects on the activity of CYP 1A and 2E. Fluconazole potently inhibited all three isoenzymes of CYP. PB plays a role in hepatoprotection to itraconazole-induced but not fluconazole-induced hepatotoxicity. SKF 525A enhanced the hepatotoxicity of both antifungal drugs in vivo. Therefore, it can be concluded that inhibition of CYP may play a key role in the mechanism of hepatotoxicity induced by itraconazole and fluconazole.

Role of metabolism in parathion-induced hepatotoxicity and immunotoxicity

The objective of this study was to investigate whether metabolic activation of parathion by cytochrome P-450s (CYPs) was responsible for pesticide-induced hepatotoxicity and immunotoxicity. Initially, to investigate parathion metabolism in vitro, the production of paraoxon and p-nitrophenol, major metabolites of parathion, was determined by high-performance liquid chromatography (HPLC). Subsequently, metabolic fate and CYP enzymes involved in the metabolism of parathion were partially monitored in rat liver microsomes in the presence of the NADPH-generating system. Among others, phenobarbital (PB)-induced microsomes produced the metabolites paraoxon and p-nitrophenol to the greatest extent, indicating the involvement of CYP 2B in parathion metabolism. When female BALB/c mice were treated orally with 1, 4, or 16 mg/kg of parathion in corn oil once, parathion suppressed the antibody response to sheep red blood cells. To further investigate a possible role of metabolic activation by CYP enzymes in parathion-induced toxicity, female BALB/c mice were pretreated intraperitoneally with 40 mg/kg PB for 3 d, followed by a single oral treatment with 16 mg/kg parathion. PB pretreatment produced a decrease in hepatic glutathione content and increases in hepatotoxic paramenters in parathion-treated mice with no changes in the antibody response. In addition, greater p-nitrophenol amounts were produced when mice were pretreated with PB, compared to treatment with parathion alone. These results indicate that parathion-induced hepatotoxicity might be differentiated from immunotoxicity in mice.

Modulation of paraoxonase (PON1) activity

Paraoxonase 1 (PON1) is a serum enzyme closely associated with high density lipoprotein (HDL). PON1 hydrolyzes several organophosphorus compounds used as insecticides, as well as nerve agents; it metabolizes toxic oxidized lipids associated with both low density lipoprotein (LDL) and HDL; and it can hydrolyze a number of lactone-containing pharmaceutical compounds, inactivating some, while activating others. Serum PON1 activity in a given population can vary by 40-fold. Though most of this variation can be explained by polymorphisms in the coding region (Q192R) and the 5' regulatory region (T-108C), modulation of PON1 by a variety of other factors should be taken into account, including other polymorphisms recently discovered but not yet characterized. This paper examines the major factors (environmental chemicals, drugs, smoking, alcohol, diet, age, disease conditions) that have been shown to modulate PON1 activity in either direction. As PON1 plays a protective role in organophosphate toxicity, and, because of its antioxidant capacity, in cardiovascular disease, a better understanding of how PON1 can be modulated by environmental factors has potential toxicological and clinical consequences.

Assessment of the genotoxic and cytotoxic potential of an anti-epileptic drug, phenobarbital, in mice: a time course study

To examine if chronic oral administration of phenobarbital (PB), a widely used anti-epileptic drug, has any genotoxic and cytotoxic potential in mice, a mammalian model, cytogenetic assays through several endpoints such as chromosome aberrations, induction of micronuclei, mitotic index of bone marrow cells, sperm-head anomaly in testis and enzymatic assays of several toxicity marker enzymes have been conducted by use of standard techniques. Mice of both treated (chronically receiving an oral dose of PB at 1.2 mg/kg bw) and control (without receiving PB) groups were sacrificed at 7, 15, 30, 60, 90 and 120 days for the study with all the above-mentioned protocols. Further, total protein profiles in liver of both control and treated mice were analyzed through the SDS-PAGE technique at day 60. The results of all these studies, when compared with controls, showed that PB has both genotoxic and cytotoxic potential in apparently increasing intensity at longer periods of chronic feeding in mice, which would warrant due consideration in its long-term use on human subjects.

Functional genomic of the paraoxonase (PON1) polymorphisms: effects on pesticide sensitivity, cardiovascular disease, and drug metabolism

This review focuses on the functional genomics of the human paraoxonase (PON1) polymorphisms. Levels and genetic variability of the PON1 position 192 isoforms (Gln/Arg) influence sensitivity to specific insecticides or nerve agents and risk for cardiovascular disease. A more recent area of investigation, the role of PON1 in drug metabolism, is also discussed. We emphasize the importance of considering both PON1 isoforms and PON1 levels in disease/sensitivity association studies.