Increased Oxidation of Dimethylnitrosamine in Pericentral Microsomes after Pyrazole Induction of Cytochrome P‐4502El

The cytochrome P‐45011El (CYPPE1) isozyme activates several tox ins and procarcinogens. Recent studies employing immunohisto‐chemical and immunoanalysis techniques have shown that this iso zyme is predominantly localized In the pericentral zone of the liver acinus. Experiments were conducted to evaluate whether micro somes isolated from the pericentral region of the liver display ele vated catalytic activity towards effective substrates for CYP2E1 such as dimethylnitrosamine (DMN) as compared with periportal micro somes. Rats were treated with pyrazole to induce CYP2E1 and hepatocytes prepared from penportal or pericentral zones of the livers by the digitonin‐collagenare procedure. Microsomes isolated from these hepatocytes had similar total P‐450 contents; however, the microsomes from the pericentral hepatocytes displayed an in creased DMSO binding spectrum suggesting an increased content of CYP2E1. Low K_m DMN demethylase activity (but not high K_m activity) as well as the oxidation of aniline and p‐nitrophenol were 2‐to 3‐fold higher in pericentral compared to periportal microsomes. The oxidation of DMN by both microsomal preparations, as well as the increased rates obtained with the pericentral microsomes, was sensitive to inhibition by carbon monoxide as well as to other CYP2E1 substrates such as ethanol, pyrezole, or 4‐methylpyrazole. Antl‐CYP2E1 IgG inhibited the oxidation of DMN by both microsomal preparations 75% to 85% and prevented most of the increase found with the pericentral microsomes. Oxidation of aniline and p‐nitro phenol was elevated in pericentral hepatocytes compared with per iportal hepatocytes to the same extent as in the isolated microsomes. these results show that microsomes isolated from the pericentral region of the liver are more reactive in oxidizing effective Substrates for the CYP2E1 isozyme, including the potent procarcinogen, DMN, and suggest that the pericentral toxicity associated with a variety of liver toxins known to be activated by P‐450 may relate to the en hanced catalytic activity of CYP2E1 in this zone of the liver.