Alterations in the enzyme activity and polypeptide composition of rat hepatic endoplasmic reticulum during acute exposure to 2-acetylaminofluorene

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. Administration of 2-AAF to rats immediately depressed microsomal glucose-6-phosphatase 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 b₅ content, NADH and NADPH cytochrome 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-dependent induction of two polypeptides which differed slightly in migration, in the region of M_r = 48 000; the faster-migrating induced polypeptide has been identified as epoxide hydratase. Two-dimensional PAGE analysis of microsomal proteins 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.