The aim of the present study was to characterize human CYP2E1 turnover and examine the possible proteolytic pathways responsible for the rapid degradation of CYP2E1 in a transfected HepG2 cell line expressing human CYP2E1. Two methods were used to study the CYP2E1 turnover; after addition of cycloheximide, the half-life of the CYP2E1 in the intact cells was about 6 h as detected by PNP catalytic activity assay and immunoblot analysis of apoprotein content. CYP2E1 substrates or ligands such as 4-methylpyrazole, ethanol, glycerol, and dimethyl sulfoxide protected CYP2E1 against this rapid degradation, whereas CCl4 accelerated this process. The second procedure involved pulse-chase experiments after labeling CYP2E1 with [35S]methionine and immunoprecipitation with anti-human CYP2E1 IgG. The half-life of CYP2E1 was about 2.5 h, and the various substrates or ligands modified the turnover process within intact cells as described for the cycloheximide experiments. More than 20 different reagents including antioxidants, physiological metabolites, lysosomal inhibitors, and protease inhibitors were screened for possible effects on CYP2E1 proteolytic degradation. Dibutyryl cAMP had no effect on CYP2E1 activity or turnover. Among those reagents tested so far, the serine protease inhibitor 1-chloro-3-tosylamido-7-amino-2-heptanone hydrochloride exhibited some protection against CYP2E1 degradation. To demonstrate whether the proteasome complex is involved in this process, Czb-Ile-Glu(OtBu)-Ala-leucinal (PSI) as a cell penetrating aldehydic proteasome inhibitor and Czb-Leu-norleucinal (calpeptin inhibitor) as an aldehydic nonproteosomal protease inhibitor were used to examine their effect on both the normal and the CCl4-stimulated CYP2E1 proteolytic degradation pathways. Treatment with PSI at concentrations ranging from 5 to 80 microM resulted in a dose-dependent protection against the loss of both the normal CYP2E1 and the CCl4-modified CYP2E1. The maximum protection by PSI at a concentration of 80 microM after a 12-h chase period was about 60% in cells treated with 2 mM CCl4 or 75% in cells without CCl4 treatment. Calpeptin inhibitor afforded little or no protection against CYP2E1 degradation in the absence or presence of CCl4. PSI did not inhibit CYP2E1 catalytic activity, suggesting that it was not a ligand for CYP2E1. These results indicate that human CYP2E1 has a short half-life span and that substrates can significantly modify its turnover rate in intact HepG2 cells. The proteasome proteolytic pathway may be involved in the degradation process of both the normal and the CCl4-modified human CYP2E1 in this model.