Identification and origin of potential background carcinogens : endogenous isoprene and oxiranes, dietary acrylamide

Abstract: The primary aims of this thesis are to discover carcinogens in the general population by identification of products of reaction with hemoglobin (Hb adducts) in blood from humans without known exposure, and tracing the origin of the adducts. Chemicals and/or their metabolites forming Hb adducts mostly also react with DNA and may thus cause mutations, a key event in carcinogenesis. Factors, that may affect in vivo doses, that is cooking of food, degree of unsaturation of dietary fatty acids, and intestinal micro-flora, were investigated. Analysis of adducts to N-termini in Hb, by gas chromatography-tandem mass spectrometry (GC/MS-MS), was applied as a tool.Due to large endogenous production of isoprene the in vivo formation of reactive monoepoxides was studied. Two adducts originating from the monoepoxides were identified in rodents exposed to isoprene or isoprene monoepoxides. Relatively high in vivo doses of monoepoxides were found in mice compared to rats after treatment with isoprene, although the detoxification rates were about equal in both species. Despite an effective metabolism to monoepoxides as shown in the mouse, corresponding background adduct levels were very low in both rodents and humans, probably showing a very low cancer risk increment from endogenous isoprene.Hb adduct levels signify ubiquitous background doses of the mutagens/carcinogens ethylene oxide (EO) and propylene oxide (PO) in humans. Levels of these adducts were earlier shown to be lower in germ-free (GF) than in conventional (CV) mice. Detoxification was shown to be faster in CV mice, but, evidently due to still faster production the in vivo doses of EO and PO were higher in CV than in GF mice. The influence of unsaturated fatty acids was small. Formation of EO and PO and also of a peroxidation indicator in GF mice, that is in the absence of micro-flora, points towards ethene and propene, peroxidatively formed in tissues, being the precursors of the oxiranes. The same mechanism, amplified by changes of the host metabolism caused by the microflora, occurs also in CV animals.Background Hb adducts from acrylamide show a general occurrence, in humans at levels possibly associated with a relatively high cancer risk. Acrylamide is present in tobacco smoke. In wild animals lower adduct levels than in humans are observed. These facts indicate a role of cooking in acrylamide exposure, a hypothesis tested in animal feeding experiments. Rats fed heated feed showed increase of several adducts including un about 10 times increase of the adduct levels from acrylamide, identified by MS-MS in comparison with authentic standards, with confirmation by analysis of acrylamide in heated feed. In follow-up studies with heated foods, high acrylamide formation was found in carbohydrate-rich foodstuffs as compared to low levels in protein foods. Acrylamide was formed above 120°C in a temperature-dependent manner.These studies give support to the use of Hb adducts for the detection and identification of endogenous and exogenous genotoxic factors which are potential contributors to background cancer incidence, and for identification of determinants of their in vivo doses.