Metabolism of n-pentane by ethanol-inducible cytochrome P-450 in liver microsomes and reconstituted membranes

Abstract

The cytochrome P-450-dependent metabolism of n-pentane was studied by head-space gas chromatographic analysis of incubations with liver microsomes and reconstituted systems from rat and rabbit liver. The alkane was metabolized in liver microsomes from imidazole-treated rabbits at an apparent V,,, of 3 nmol mg-' min-and a K, of 35 pM and in liver microsomes from acetone-treated rats at an apparent V,,, of 10 nmol mg-' min-' and a K, of 9 pM. The rate of microsomal n-pentane metabolism was enhanced fivefold by acetone treatment of rats, compared to the rate observed in liver microsomes from control rats. The reaction was inhibited in microsomes by compounds that are known to interact with the acetone and ethanol-inducible form of liver microsomal cytochrome P-450, like acetone, imidazole, ethanol and benzene. Effective inhibition was also accomplished when microsomal incubations were performed in the presence of IgG against this form of cytochrome P-450 from either rat or rabbit liver. In reconstituted membrane vesicles containing NADPHcytochrome P-450 reductase, ethanol-inducible P-450 from rat liver (P-45Oj) was a fivefold more eficient catalyst of pentane metabolism than was the corresponding P-450 form from rabbit liver (P-450 LMeb). The rabbit enzyme metabolized the hydrocarbon with an apparent V,,, of 4 nmol nmol-' min-' and a K , of 8 pM, By contrast, phenobarbital-inducible P-450 LM2 or 3-methylcholanthrene-inducible P-450 LM4 from rabbit liver were quite ineffective catalysts of n-pentane metabolism. It is concluded that n-pentane constitutes a good substrate for cytochrome P-450 and, in particular, for the ethanol-inducible form of this hemoprotein. It is suggested that npentane is an important substrate for P-450 under in vivo conditions and that pentane measurements in expired air as an indicator of lipid peroxidation must be interpreted with caution.During lipid peroxidation, volatile alkanes like ethane and pentane are formed from polyunsaturated fatty acids [l]. Pentane is formed as the scission product from hydroperoxides of the linoleic acids (cf. [2]). Expired pentane or ethane has been widely used as a sensitive and non-invasive indicator of lipid peroxidation in vivo. However, Frommer et al. [3] showed in 1970 that n-pentane was metabolized by liver microsomes to 2-pentanol preferentially and that cytochrome P-450 probably constituted the catalyst. Recently, several investigators have shown that n-pentane and ethane are metabolized in vivo in rats [4-81, mice [5] and man [9], although the metabolism of ethane is smaller than that of n-pentane. Remmer and coworkers [4, 51 have studied the production of alkanes by liver microsomes and concluded that n-pentane was metabolized in vitro.Treatment of rabbits with acetone, ethanol or imidazole causes the induction of a specific form of liver microsomal cytochrome P-450 which metabolizes aliphatic alcohols [lo -131, acetone [14] and carbon tetrachloride [15]. This form of ethanol-inducible cytochrome P-450 , probably identical to isozyme 3a [12]), ...

Effect of phenethyl isothiocyanate on microsomalN-nitrosodimethylamine metabolism and other monooxygenase activities

“…When included in the incubations using microsomes from acetone-treated animals, PEITC acted as a competitive inhibitor with an apparent Ki of 1 PM. This value is lower than the apparent kinetic constants ( K , or Ki) of other previously reported substrates or inhibitors of this isozyme, having values ranging from 9 to 30 PM , Terelius and Ingelman-Sundberg 1986, and indicates a high affinity for binding at the active site. This probably contributed to the observed selective inhibition of microsomal NDMA demethylation compared to activities associated with other forms of P-450.…”

“…NDMA, an environmentally prevalent procarcinogen, is a potent liver tumorigenic agent in rats (Magee et al 1976). It has previously been demonstrated that cytochrome P450 IIEl-an enzyme inducible by ethanol, acetone, pyrazole, isoniazid, fasting, diabetes, and other factors-is involved in the metabolism of a variety of small organic compounds including aniline, acetone, butanol, carbon tetrachloride, chloroform, ethanol, ether, benzene, and n-pentane (Morgan et al 1982, Johansson and Ingelman-Sundberg 1985, Koop and Casazza 1985, Johansson et al 1986, Terelius and Ingelman-Sundberg 1986. Microsomal P450 IIEl is also responsible for the initial a-hydroxylation of NDMA at low concentrations (ca.…”

Breath Ethane Concentrations in Healthy Volunteers Correlate with a Systemic Marker of Lipid Peroxidation but Not with Omega-3 Fatty Acid Availability

“…Specifically, lipid hydroperoxides via a process known as β-scission, give arise to ethane or pentane, the former from fatty acids which have their first carbon-carbon double bond 3 carbon atoms from the terminal “omega” carbon (omega-3 or n-3 fatty acids) and the latter from omega-6 (n-6) fatty acids [ 1 ]. Since pentane can be metabolized in the liver to pentanol the interpretation of altered breath pentane concentrations is more complex that of ethane which does not undergo such metabolism [ 11 ]. Ethane is also highly volatile resulting in it being predominantly excreted in the breath where it can be assayed using thermal desorption gas chromatography mass spectrometry or light spectroscopy, it being found in concentrations ranging from approximately 0.5 to 5 PPB by volume in the exhalant of healthy individuals [ 6 , 12 ].…”

Emerging non‑invasive detection methodologies for lung cancer (Review)

“…The effects of lipid peroxidation may be reduced and regulated by antioxidants (57). Saturated alkanes including ethane and pentane are considered the final products of lipid peroxidation and have been widely recognized as volatile biomarkers of this reaction in breath analysis (110). The production of C3-C11 saturated alkanes is also associated with lipid peroxidation, though the production of branched alkanes does not originate from this mechanism (111).…”