Oxidation of 1,1,1,2-tetrafluoroethane in rat liver microsomes is catalyzed primarily by cytochrome P-450IIE1.
1,1,1,2-Tetrafluoroethane (R-134a), a nonozone-depleting alternative air-conditioning refrigerant and propellant for pharmaceutical preparations, is oxidatively defluorinated by rat hepatic microsomes. In this report we show that induction of cytochrome P-450IIE1 in rats, by pyridine administration, resulted in an 8-fold increase in the rate of R-134a metabolism by hepatic microsomes (Vmax 47 vs. 6 nmol F-/mg microsomal protein/15 min). Furthermore, when data were normalized for P-450 content, a 4-fold increase in R-134a metabolism was noted for IIE1-enriched microsome preparations. In contrast, phenobarbital and Aroclor 1254 decreased the specific activity of hepatic microsomes for this function. The microsomal content of P-450IIE1, as evaluated by Western blot, was elevated significantly only in microsomes from pyridine-treated rats. p-Nitrophenol and aniline, which are metabolized at high rates by rat P-450IIE1, decreased the rate of R-134a defluorination by hepatic microsomes; Dixon plot analysis indicated competitive inhibition with a Ki of 36 microM p-nitrophenol or 115 microM aniline. Pyridine also potently induced defluorination of R-134a catalyzed by rabbit liver microsomes. Studies with individual P-450 isozymes purified from rabbit liver showed that the phenobarbital- and polycyclic hydrocarbon-induced isozymes (IIB1 and IA2) defluorinated R-134a at negligible rates (1.9 and 0.4 nmol F-/nmol P-450/60 min, respectively). In contrast, P-450IIE1 catalyzed defluorination of R-134a at a relatively high rate (16.2 nmol F-/nmol P-450/60 min); isozyme IA1, which also is induced by nitrogen-containing heterocycles such as pyridine, was somewhat active (5.3 nmol F-/nmol P-450/60 min).(ABSTRACT TRUNCATED AT 250 WORDS)
Biomedical Science Department, General Motors Research Labs., Warren, MI 48090.
M J
MJ
Olson
S G
SG
Kim
C A
CA
Reidy
J T
JT
Johnson
R F
RF
Novak
D000814:Aniline Compounds / Q000378:metabolism; D000818:Animals; D001711:Biotransformation; D015153:Blotting, Western; D019392:Cytochrome P-450 CYP2E1; D003577:Cytochrome P-450 Enzyme System / Q000032:analysis / Q000276:immunology / Q000378:metabolism*; D004591:Electrophoresis, Polyacrylamide Gel; D005459:Fluorides / Q000032:analysis / Q000378:metabolism; D006845:Hydrocarbons, Fluorinated / Q000378:metabolism*; D066298:In Vitro Techniques; D007527:Isoenzymes / Q000378:metabolism; D008297:Male; D008862:Microsomes, Liver / Q000201:enzymology*; D009596:Nitrophenols / Q000378:metabolism; D010084:Oxidation-Reduction; D010089:Oxidoreductases, N-Demethylating / Q000032:analysis / Q000276:immunology / Q000378:metabolism*; D011725:Pyridines / Q000494:pharmacology; D051381:Rats; D011916:Rats, Inbred F344; D004980:Ethane / Q000031:analogs & derivatives; D005466:Fluorocarbons
D000814:Aniline Compounds; D019392:Cytochrome P-450 CYP2E1; D003577:Cytochrome P-450 Enzyme System; D005459:Fluorides; D006845:Hydrocarbons, Fluorinated; D007527:Isoenzymes; D009596:Nitrophenols; D010089:Oxidoreductases, N-Demethylating; D011725:Pyridines; C063006:norflurane; D004980:Ethane; D005466:Fluorocarbons; C024836:4-nitrophenol