EXPRESSION OF HUMAN CYTOCHROME P4501A1 (CYP1A1) IN SACCHAROMYCES-CEREVISIAE INHIBITS CELL-DIVISION

1. Saccharomyces cerevisiae cells, genetically engineered to express h uman cytochrome P4501A1 (CYP1A1), have a mean doubling time of 5 . 8 h , which is considerably slower than that of control yeast cells that h ave undergone the same transformation process but with a plasmid lacki ng CYP1A1 cDNA(3 . 3 h). 2. A smaller reduction in the rate of cell di vision is observed in yeast cells expressing the closely related human P450, CYP1A2. No reduction is seen with plaice CYP1A, despite similar levels of P450 expression and enzyme activity (ethoxyresorufin O-deet hylation) and no inhibition of growth is observed with yeast cells exp ressing higher levels of human CYP2D6. 3. Repeated culture of cells fr om a single CYP1A1 transformant colony results in a gradual loss of P4 50 expression and of CYP1A1-associated enzymatic activity over a 5-6 w eek period. In contrast, expression of human CYP2D6 by a single transf ormant colony is stable for at least 6 months. 4. The loss of CYP1A1 a ctivity from transformed cells is accompanied by a return to normal gr owth rate, similar to that of control cells. 5. Inhibition of CYP1A1 e nzyme activity during culture, by either type I (alpha-naphthoflavone) , type II (ellipticine) or mechanism-based (1-(1'propynyl)pyrene) CYP1 A inhibitors, does not affect growth rate, suggesting that some other property of human CPP1A1 protein is responsible for the growth inhibit ion observed.