Chem.-Biol. Interactions, 39 (1982) 279--299 279 Elsevier/North-Holland Scientific Publishers Ltd. ALTERATIONS IN THE ENZYME ACTIVITY AND POLYPEPTIDE COMPOSITION OF RAT HEPATIC ENDOPLASMIC RETICULUM DURING ACUTE EXPOSURE TO 2.ACETYLAMINOFLUORENE M.A. KADERBHAI, T.K. BRADSHAW a and R.B. FREEDMAN Biological Laboratory, University of Kent at Canterbury, Canterbury, Kent CT2 7NJ and aShell Toxicology Laboratory, Sittingbourne Research Centre, Sittingbourne ME9 8AG (United Kingdom) (Received August 6th, 1981) (Revision received October 23rd, 1981) (Accepted October 28th, 1981) SUMMARY Studies have been made of the morphology, enzyme activity and protein composition of liver endoplasmic reticulum in rats exposed to acute doses of the carcinogen, 2-acetylaminofluorene (2-AAF). Electron microscopic examination revealed numerous ultrastructural changes in the hepatocyte; most consistent alterations were the disorganisation of endoplasmic reticulum system with apparent increase of smooth endoplasmic reticulum. Administra- tion of 2-AAF to rats immediately depressed microsomal glucose-6-phos- phatase activity and eventually induced epoxide hydratase activity 6--7-fold over control activity. The induction was time-dependent and maximal rates of induction were observed at dosages greater than 40 mg/kg body wt. The treatment also induced cytochrome bs content, NADH and NADPH cyto- chrome c reductase activities (1.0--1.5-fold). Only very small changes in the total content of cytochrome P-450 were noted. Sodium dodecyl sulphate- polyacrylamide gel electrophoresis (SDS-PAGE) of microsomal proteins from 2-AAF pretreated animals showed time<lependent induction of two polypeptides which differed slightly in migration, in the region of Mr = 48 000; the faster-migrating induced polypeptide has been identified as epoxide hydratase. Two-dimensional PAGE analysis of microsomal pro- teins from 2-AAF exposed rats showed a reproducible deletion of a protein with molecular weight in the region of 67 000. The basis for the alterations in the protein composition of endoplasmic reticulum in response to 2-AAF treatment is discussed. Abbreviations: 2-AAF, 2-acetylaminofluorene; ER, endoplasmic reticulum; PN, preneo- plastic; SDS-PAGE, sodium dodecyl sulphate-polyacrylamide gel electrophoresis. 0009--2797/82/0000--0000/$02.75 © 1982 Elsevier/North-Holland Scientific Publishers Ltd.