Liver microsomes formation

Mansuy, D. and M. Fontecave (1983). Reduction of benzyl halides by liver microsomes Formation of 478 nm-absorbing sigma-alkyl-ferric cytochrome P450 complexes. Biochem. Pharmacol. 32, 1871-1879. 

S. E. Murphy (2000). 2 -Hydroxylation of nicotine by cytochrome P450 2A6 and human liver microsomes Formation of a lung carcinogen precursor. Proc. Natl. Acad. Sci. USA 97, 12493-12497. 

Zhang Z, Chen Q, Li Y, Doss GA, Dean BJ, Ngui JS, Silva EM, Kim S, Wu JY, Dininno F, Hammond ML, Steams RA, Evans DC, Baillie TA, Tang W (2005) In vitro bioactivation of dihydrobenzoxathiin selective estrogen receptor modulators by cytochrome P450 3A4 in human liver microsomes formation of reactive iminium and quinone type metabolites. Chem Res Toxicol 18 675-685... 

Kitada, M., Igarashi, K., Hirose, S. Kitagawa, H. (1979). Inhibition by polyamines of lipid peroxide formation in rat liver microsomes. Biochemical Biophysical Research Communications, 87, 388-92. 

The rates of formation of these metabolites in liver microsomes from Aroclor pretreated rats is summarized in Table III. 

Table III. Rates of Formation of NNN Metabolites by F-344 Rat Liver Microsomes ...

Only limited metabolic studies have been carried out on NPIP. It undergoes a-hydroxylation by rat liver microsomes to give 5-hydroxypentanal, a process analogous to the formation of... 

Screening is usually carried out with liver microsomes from humans, rats, mice, dogs and monkeys and liver S9 fraction from aroclor 1254-induced rats. The incubation is typically mn with a volume of 0.2-1. OmL in a microcentrifuge or a glass tube. Different incubation conditions are used for CYP and UGT reactions. The incubation mixture for formation of oxidative metabolites and/or GSH conjugates contains ... 

Figure 9.4 Formation of M3 via biotranformation of BMS-1 with monkey liver microsomes... 

It was recently reported that. >97% of BaP 4,5-epoxide metabolically formed from the metabolism of BaP in a reconstituted enzyme system containing purified cytochrome P-450c (P-448) is the 4S,5R enantiomer (24). The epoxide was determined by formation, separation and quantification of the diastereomeric trans-addition products of glutathione. Recently a BaP 4,5-epoxide was isolated from a metabolite mixture obtained from the metabolism of BaP by liver microsomes from 3-methylcholanthrene-treated Sprague-Dawley rats in the presence of the epoxide hydrolase inhibitor 3,3,3-trichloropropylene oxide, and was found to contain a 4S,5R/4R,5S enantiomer ratio of 94 6 (Chiu et. al., unpublished results). However, the content of the 4S,5R enantiomer was <60% when liver microsomes from untreated and phenobarbital-treated rats were used as the enzyme sources. Because BaP 4R,5S-epoxide is also hydrated predominantly to 4R,5R-dihydro-... 

Finally, we have studied the metabolism of a series of PAH with decreasing IP. In these metabolic studies with Aroclor-induced rat liver microsomes, the formation of quinones was measured in the presence of NADPH or cumene hydroperoxide as cofactor. 

Ionization Formation of Quinone by Aroclor-induced Rat Liver Microsomes with ... 

Recent studies suggest that many factors may affect hydroxyl radical generation by microsomes. Reinke et al. [34] demonstrated that the hydroxyl radical-mediated oxidation of ethanol in rat liver microsomes depended on phosphate or Tris buffer. Cytochrome bs can also participate in the microsomal production of hydroxyl radicals catalyzed by NADH-cytochrome bs reductase [35,36]. Considering the numerous demonstrations of hydroxyl radical formation in microsomes, it becomes obvious that this is not a genuine enzymatic process because it depends on the presence or absence of free iron. Consequently, in vitro experiments in buffers containing iron ions can significantly differ from real biological systems. 

On the other hand, microsomes may also directly oxidize or reduce various substrates. As already mentioned, microsomal oxidation of carbon tetrachloride results in the formation of trichloromethyl free radical and the initiation of lipid peroxidation. The effect of carbon tetrachloride on microsomes has been widely studied in connection with its cytotoxic activity in humans and animals. It has been shown that CCI4 is reduced by cytochrome P-450. For example, by the use of spin-trapping technique, Albani et al. [38] demonstrated the formation of the CCI3 radical in rat liver microsomal fractions and in vivo in rats. McCay et al. [39] found that carbon tetrachloride metabolism to CC13 by rat liver accompanied by the formation of lipid dienyl and lipid peroxydienyl radicals. The incubation of carbon tetrachloride with liver cells resulted in the formation of the C02 free radical (identified as the PBN-CO2 radical spin adduct) in addition to trichoromethyl radical [40]. It was found that glutathione rather than dioxygen is needed for the formation of this additional free radical. The formation of trichloromethyl radical caused the inactivation of hepatic microsomal calcium pump [41]. 

Manno et al. [43] observed the formation of superoxide during the oxidation of arylamines by rat liver microsomes. Noda et al. [44] demonstrated that microsomes are able to oxidize hydrazine into a free radical. In contrast, hepatic cytochrome P-450 apparently oxidizes paracetamol (4 -hydroxyacetanilide) to A-acetyl-p-benzoquinone imine by a two-electron mechanism [45]. Younes [46] proposed that superoxide mediated the microsomal S -oxidation of thiobenzamide. 

The above sequence mimics the proposed biosynthesis of Ervatamia alkaloids and in this context Thai and Mansuy (190) set out to determine whether an enzyme preparation would be able to promote the same transformation. By incubation of dregamine hydrochloride with a suspension of liver microsomes from a rat pretreated with phenobarbital (as a good inducer of P-450 cytochromes) in the presence of NADPH and 02, 20-epiervatamine (45) was formed together with the major metabolite Nl -demethyldregamine. It is well known that microsomal reaction on tertiary amines results in Af-oxide formation or N-deal-kylation. Thus it is likely that 45 was derived either from a rearrangement of dregamine JV4-oxide, catalyzed by the iron cytochrome P-450 or from one-electron oxidation of 30. 

Mansuy, D., Beaune, P., Crestell, T., Lange, M., and Leroux, M. 1977. Evidence for phosgene formation during liver microsomal oxidation of chloroform. Biochem. Biophys. Res. Comm. 79 513-517. 

In liver microsomes, the formation of 1-, 2-, and 3-hexanol from -hexane was best described kinetically by a 2-enzyme system, while for lung microsomes, single-enzyme kinetics were indicated for each metabolite. For conversion to 1-hexanol, apparent Km values were 0.4 and 300 M, and Vmax values were 0.09 and 1.2 nmol/mg protein/min, respectively. For conversion to 2-hexanol, apparent Km values were 6 and 1,100 M, and Vmax values were 1 and 4.6 nmol/mg protein/min respectively. Insufficient information was available to estimate the high-affinity activity for 3-hexanol, the low-affinity activity had an apparent Km of 290 M and a Vmax of 0.5 nmol/mg protein/min. In the lung, Km values were 9,... 

---