Induction phenobarbital

In this study, P-450-related enzyme activities (benzphetamine N-demethylase, 7-ethoxycoumarin O-deethylase) were also measured in liver homogenates (prepared 24 hours after the last treatment) from rats treated orally with MEK for 1-7 days and compared to the activity obtained with phenobarbital treatment (80 mg/kg intraperitoneally for 3 days) (Robertson et al. 1989). Total cytochrome P-450 was also measured. No consistent change was noted in benzphetamine N-demethylase activity as the result of MEK treatment, while 7-ethoxycoumarin O-deethylase was over 3 times higher than controls and comparable to phenobarbital induction. Total P-450 levels were increased to approximately 150-200% of controls with MEK and to 570% of control by phenobarbital. The authors concluded that the potentiating effects of MEK on the neurotoxicity of -hexane appear to arise, at least in part, from the activating effects of MEK on selected microsomal enzymes responsible for -hexane activation. 

Thus, induction can change the proportions of isoenzymes in a particular tissue and may increase the activity of a normally insignificant form by many times. Although phenobarbital induction increases the overall concentration of cytochromes P-450 in the liver by about threefold, specific isoenzymes may be increased up to 70-fold. Treatment with 3-methylcholanthrene can increase a specific form of the enzyme by a similar order. 

Induction involves increased synthesis of enzyme protein, which may be detected as an increase in total enzyme level as with phenobarbital induction or increase in a particular isoenzyme. Protein synthesis is increased, and this usually seems to be necessary as inhibition of protein synthesis results in inhibition of induction. The increased protein synthesis may involve increased mRNA synthesis and inhibitors of this, such as actinomycin D, block induction. For a simple diagram explaining the relationship of protein synthesis to DNA see Figure 6.38. 

Waxman DJ, Azarnoff L. Phenobarbital induction of cytochrome P450 gene expression. Biochem J 1992 281 577. 

Although phenobarbital induction has been studied for many years, the mechanism for induction has only recently been established. In bacteria a key feature of phenobarbital induction was demonstrated to involve barbiturate-mediated removal of a... 

Kakizaki, S., Y. Yamamoto, A. Ueda, R. Moore, T. Sueyoshi, and M. Negishi. Phenobarbital induction of drug/steroid-metabolizing enzymes and nuclear receptor CAR. Biochim. Biophys. Acta 1619 239-242, 2003. 

Waxman DJ, Morrissey JJ, Naik S, et al. Phenobarbital induction of cytochrome P450 high-level long-term responsiveness of primary rat hepatocyte cultures to drug induction, and glucocorticoid dependence of the phenobarbital response. Biochem J 1990 271 113-119. 

Phenobarbital represents the type of enzyme inducer with broad induction effects. After a latency period, production of cytochrome P450, cytochrome P4S0 reductase, and related enzymes is increased. In addition, hver weight, hepatic blood flow, bile flow, and production of hepatic proteins also all increase. This induction apparently increases the P450 isoenzyme mass for which debrisoquin is a substrate, because the hepatic clearance of debrisoquin is increased after phenobarbital administration. This enzyme is referred to as cytochrome P450-2D6 (Gyp 2D6). Phenobarbital induction has little effect on theophylline clearance, suggesting a different isoenzyme for theophylline metabolism. 

In experimental animals, other factors that can cause an increase in selenium levels in expired air are higher dietary levels of selenium, protein, or methionine (Ganther et al. 1966). Phenobarbital induction of microsomal enzymes has also led to increased exhalation of selenium following intravenous administration of sodium selenite (Sternberg et al. 1968). 

Waxman, D.J. and AzarofF, L. (1992) Phenobarbital induction of cytochrome P-450 gene expression. The Biochemical Journal, 281, 577-592. 

Rencurel, F., Stenhouse, A., Hawley, S. A., Friedberg, T., Hardie, D. G., Sutherland, C., and Wolf, C. R. (2005) AMP-activated protein kinase mediates phenobarbital induction of CYP2B gene expression in hepatocytes and a newly derived human hepatoma cell line. /. Biol. Chem. 280, 4367-4373. 

In vitro studies showed that rat liver microsomes activated with NADPH and molecular oxygen metabolized MMT (Hanzlik et al. 1980b). Preliminary studies with pooled liver microsomes from 5-6 normal or pheno-barbital-induced rats showed that reaction rates of metabolism were linear for the first 20 minutes. MMT and aminopyrine, a positive control compound that is metabolized exclusively by cytochrome P450, showed parallel responses to changes in incubation conditions (i.e. NADPH dependence, inhibition by carbon monoxide, induction by phenobarbital). Liver microsomes metabolized MMT with an estimated of 78 pM and a Vni of 3.12 nmol/mg protein/min. When the studies were done with liver microsomes from phenobarbital-treated rats, the remained the same, but the doubled (Hanzlik et al. 1980b). Lung microsomes were equally capable of metabolizing MMT, but phenobarbital induction did not enhance the response. 

Phenobarbital induction and tissue-specific expression of the rat CYP2B2 gene in transgenic mice. J. Biol. Chem. 268, 21722-21726. 

GH pattern. GH suppression is also a key determinant of the lower responsiveness of female rats to phenobarbital induction of CYP2B1 (refs... 

Narhi, L.O. and A.J. Fulco (1982). Phenobarbital induction of a soluble cytochrome P450 dependent fatty acid monooxygenase from Bacillus megaterium. J. Biol. Chem. 257, 2147-2150. 

Cheever KL, DeBord DG, Swearengin TF. 1991.4,4 -Methylenebis(2-chloroaniline) (MOCA) The effect of multiple oral administration, route and phenobarbital induction on macromolecular adduct formation in the rat. Fundam AppI Toxicol 16(1) 71-80. 

Ourlin JC, Handschin C, Kaufmann M, Meyer UA. A Link between cholesterol levels and phenobarbital induction of cytochromes P450. Biochem Biophys Res Commun 2002 291 378-384. 

Narhi LO, Fulco AJ (1982) Phenobarbital induction of a soluble cytochrome P-450-dependent fatty acid monooxygenase in Bacillus megaterium. J Biol Chem 257 2147-2150... 

While isolated rat hepatocytes do not hberate appreciable amounts of 02 , simple quinones, such as 2,5-dimethyl-p-benzoquinone stimulate the formation of O2 up to 15 nmoles per min and 10 cells (Powis et al. 1981). Hepatocyte 02 formation stimulated by a variety of simple quinones and more complex antitumor quinones is maximal at a qui-none one-electron reduction potential (E, ) of -70 mV and quahtatively similar to the pattern of 02 formation seen with mitochondrial NADH ubi-quinone oxidoreductase and microsomal NADH-cytochrome bs reductase. 02 production, by microsomal NADPH-cytochrome P-450 reductase is maximal at a quinone E of -200 mV. Phenobarbital induction, which increases NADPH-cytochrome P-450 reductase, had no effect on O2 formation by hepatocytes. 

DJ Waxman, L Azaroff. Phenobarbital induction of cytochrome P-450 gene expression. 

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