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Bioactivation of monocrotaline by P-450 3A in rat liver
Y Kasahara1, K Kiyatake, K Tatsumi
1Department of Chest Medicine, School of Medicine, Chiba University, Japan.
Abstract:
Monocrotaline (MCT) is bioactivated in liver cytochrome P-450s to MCT pyrrole (MCTP), which primarily injures the lung endothelium to result in the development of pulmonary hypertension (PH) in rats. However, whether there is a relation between the degree of PH and the activity of liver cytochrome P-450 to convert MCT to MCTP remains unclear. To examine the relation between these physiological and biochemical changes, we first measured the severity of MCT-induced (20 mg/kg) PH in male, female, castrated male, and phenobarbital (PB, liver P-450s inducer)-pretreated male rats. The degree of right ventricular hypertrophy was more severe in PB-pretreated male than in control male rats. It was also more severe in male than in either female or castrated male rats, suggesting that sex-specific P-450s could be involved in the metabolic pathways of MCT in the liver. Further to explore which of the isozymes (2A2, 2C11, and 3A) of P-450s in the liver is responsible for the bioactivation of MCT, we measured the rate of MCTP production in hepatic microsomes by a modified Mattock's method. Treatment of male rats with PB and pregnenolone 16alpha-carbonitrile (PCN), which is the specific inducer of P-450 3A, increased the rate of MCTP production, suggesting that P-450 3A may contribute to the conversion to pyrrole. Therefore we measured the amount of P-450 3A protein by immunoblotting and attempted to inhibit MCT metabolism by using antibodies to P-450 3A. P-450 3A was significantly induced by PCN (6.5-fold) and PB (4.6-fold) treatment and reduced by castration (0.38-fold). The amount of P-450 3A was closely correlated with the production of MCTP, and the conversion of MCT to MCTP was strongly inhibited by antibodies against P-450 3A. These results indicated that P-450 3A was predominantly responsible for the metabolism of MCT to MCTP in rat liver and suggested a tight linkage between the degree of PH and the activity of liver P-450 3A.
Insights
Monocrotaline (MCT) is converted to MCT pyrrole (MCTP) by liver cytochrome P-450 3A, leading to pulmonary hypertension (PH). This study links P-450 3A activity to PH severity in rats, identifying it as the key enzyme in MCT metabolism.
Area of Science:
- Toxicology
- Pharmacology
- Biochemistry
Background:
- Monocrotaline (MCT) bioactivation by liver cytochrome P-450s produces MCT pyrrole (MCTP), a lung endothelium toxin causing pulmonary hypertension (PH) in rats.
- The specific P-450 isozymes involved and their correlation with PH severity remain unclear.
Purpose of the Study:
- To investigate the relationship between the degree of MCT-induced PH and the activity of specific liver cytochrome P-450 isozymes in MCT metabolism.
- To identify the primary P-450 isozyme responsible for MCT bioactivation to MCTP.
Main Methods:
- Assessment of MCT-induced PH severity via right ventricular hypertrophy in male, female, and castrated male rats, and phenobarbital (PB)-pretreated male rats.
- Measurement of MCTP production in hepatic microsomes using a modified Mattock's method.
- Quantification of P-450 3A protein levels by immunoblotting and inhibition studies using P-450 3A antibodies.
Main Results:
- Phenobarbital and PCN (P-450 3A inducer) treatments increased MCTP production and PH severity.
- Castration reduced P-450 3A levels and MCTP production.
- P-450 3A levels strongly correlated with MCTP production, and antibodies against P-450 3A inhibited MCT metabolism.
Conclusions:
- Cytochrome P-450 3A is the predominant isozyme responsible for MCT metabolism to MCTP in rat liver.
- A strong correlation exists between P-450 3A activity and the development of MCT-induced pulmonary hypertension.