Metabolic detoxification: Implications for thresholds

The fact that chemical carcinogenesis involves single, isolated, essentiall y irreversible molecular events as discrete steps. several of which must oc cur in a row to finally culminate in the development of a malignancy. rathe r suggests that an absolute threshold for chemical carcinogens may not exis t. However, practical thresholds may exist due to saturable pathways involv ed in the metabolic processing, especially in the metabolic inactivation, o f such compounds. An important example for such a pathway is the enzymatic hydrolysis of epoxides via epoxide hydrolases, a group of enzymes for which the catalytic mechanism has recently been established. These enzymes conve rt their substrates via the intermediate formation of a covalent enzyme-sub strate complex. Interestingly, the formation of the intermediate proceeds f aster by orders of magnitude than the subsequent hydrolysis, ie, the format ion of the terminal product. Under normal circumstances, this does not pose a problem, since the microsomal epoxide hydrolase (mEH). the epoxide hydro lases with the best documented importance in the metabolism of carcinogens, is highly abundant in the liver, the organ with the highest capacity to me tabolically generate epoxides. Computer simulation provides evidence that t he high amount of mEH enzyme is favorable for the control of the steady-sta te level of a substrate epoxide and can keep it extremely low. However, onc e the mEH is titrated out under conditions of extraordinarily high epoxide concentration, the epoxide steady-state level steeply rises, leading to a s udden burst of the genotoxic effect of the noxious agent. This prediction o f the computer simulation is nicely supported by experimental work. V79 Chi nese hamster cells that we have generically engineered to express human mEH at about the same level as that observed in human liver are completely pro tected from any measurable genotoxic effect of the model compound styrene o xide (STO) up to a dose of 100 mu M in the cell culture medium (toxicokinet ic threshold). In V79 cells that do not express mEH, STO leads to the forma tion of DNA strand breaks in a dose-dependent manner with no toxicokinetic threshold observable. Above 100 mu-M. the genotoxic effect of STO in the mE H-expressing cell line parallels the one in the parental cell line. Thus, t he saturable protection from STO-induced strand breaks by mEH represents a typical example of a practical threshold. However, it must be: pointed out that even in the presence of protective amounts of mEH. a minute but defini te level of STO is present that does not contribute sufficiently to the str and break formation to overcome the background noise of the detection proce dure. As pointed out above, absolute thresholds probably do not exist in ch emical carcinogenesis.