Ethylene

Endpoint summary

Description of key information

Ethylene is absorbed via the lungs and either exhaled unchanged or oxidised by CYP450. Ethylene oxide may react with macromolecules to form adducts or be hydrolysed by epoxide hydrolase.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Absorption rate - inhalation (%):

15

Additional information

Non-human information

Absorption

During exposure of rats by inhalation, uptake of ethylene was rapid and steady state concentrations in blood and tissues were attained within 12 hours (Guest et al., 1981; Eide et al., 1995). Approximately 15% of inhaled ethylene is absorbed, a significant proportion of that absorbed is eliminated unchanged in exhaled air; this results in a retained dose at steady state of approximately 3% (Csanady et al., 2000). At atmospheric concentrations below that which saturates metabolism, blood concentrations peak rapidly but then decline to a lower steady state concentration. This effect has also been observed with other alkenes and has been explained by a transient reduction in the activity of the enzymes responsible for metabolism (Fennel et al., 2004).

Distribution

Tissue:air partition coefficients for ethylene have been determined; with the exception of blood, values are similar in rat and human tissue. The human blood:air partition coefficient is approximately half that measured with rat blood; this is attributed to species difference in protein binding (Csanady et al., 2000). Measurements of total radioactivity (Guest et al., 1981), unchanged ethylene (Eide et al., 1995) or adducts (Eide et al., 1999; Rusyn et al., 2005; Walker et al., 2000; Wu et al., 1999) in tissues following exposure of rodents to ethylene by inhalation demonstrated distribution of ethylene or metabolites into all tissues studied.

Metabolism and adduct formation

A fraction of the systemically available ethylene can be metabolised to ethylene oxide, some of which is further metabolised by epoxide hydrolase to form ethane-1,2-diol . In vitro studies: Using a headspace vial technique Li et al. (2011) evaluated ethylene metabolism and ethylene oxide uptake using subcellular fractions (microsomes and cytosol) from mice, rats, and humans. Formation of ethylene oxide from ethylene (expressed as VmaxEO/Km) was 5 fold greater for mouse liver microsomes compared to samples from rats, and 4 fold greater for mice compared to humans. Negligible elimination of ethylene oxide was apparent in native or heat-inactivated mouse or rat liver and lung microsomal fractions, however, samples of native human liver microsomes showed detectable activity (absent after heat inactivation) indicating the presence of epoxide hydrolase mediated metabolism of ethylene oxide (Km = 13 mmol/l, Vmax = 14 nmol/min/mg). In native liver cytosol, glutathione mediated conjugation of ethylene oxide (expressed as VmaxEO/Km) was 5 fold greater for mice compared to rats, and 28 fold greater for mice compared to humans; for lung cytosol, ethylene oxide elimination was 3 fold greater in mouse samples than in rat samples (no human samples available). Ethylene oxide elimination by cytosol was decreased, but still detectable, in native and heat inactivated incubations performed in the presence of diethyl maleate. The authors suggest the results support the enzymatic (glutathione transferase-mediated) and non-enzymatic (direct conjugation and direct hydrolysis) elimination of ethylene oxide by cytosol. The results demonstrate species differences in the microsomal metabolism of ethylene to ethylene oxide, and also in the microsomal and cytosolic metabolism of ethylene oxide. The rate of metabolism of ethylene is affected by pre-treatment with enzyme inducers or inhibitors of cytochrome P450 (Guest et al., 1981; Filser and Bolt, 1983). Values for Vmax were 8.5, 13.7 and 0 µmol/h/kg in untreated rats and those treated with Aroclor 1254 or diethyldithiocarbamate respectively (Filser and Bolt, 1983).  In vivo studies: In the rat, approximately 50% of systemic ethylene oxide is conjugated with glutathione to form S-(2-hydroxyethyl) -glutathione (GSEO); this product has been measured in liver tissue and shown to follow a similar concentration-time profile to that of ethylene oxide (Filser, 2007). Peak blood concentrations of ethylene oxide were achieved within two hours of the start of exposure to ethylene; AUC values for ethylene oxide showed a non-linear dose response (Fennel et al., 2004). In rats and mice, metabolism of ethylene is saturated at exposure concentrations above 1,000 ppm (Filser and Bolt, 1983, Walker et al., 2000). The transient reduction in the activity of metabolising enzymes has been investigated and shown to result predominantly from effects on cytochrome P450 2E1 (Fennel et al., 2004). Filser et al. (2013) compared the conversion of ethylene to ethylene oxide in mice, rats and human volunteers exposed to defined concentrations of ethylene (1 – 10000 ppm). Formation of ethylene oxide was approx. 6-9-fold higher in rats, and approx. 3-4-fold higher in mice, relative to that found in humans i.e. clear species differences were present. Based on the slope of the AUC curves obtained by these authors, the extent of ethylene oxide formation in rats (92 nmol ethylene oxide.hr/l blood per ppm ethylene) and mice (41 nmol ethylene oxide.hr/l blood per ppm ethylene) was much greater than that of humans (see below). A small, dose dependant proportion of the ethylene oxide reacts with haemoglobin or DNA to form adducts; these provide sensitive markers of exposure. Eide et al. (1995) measured the haemoglobin adduct N-(2-hydroxyethyl) valine (HEval) in blood and the DNA adduct 7-ethylguanine in lymphocytes and liver of rats following exposure to 300 ppm ethylene for three consecutive days. A subsequent study showed the presence of low levels of the DNA adduct N7-(2-hydroxyethyl) guanine (7-HEG) in unexposed rats (Eide et al.,1999). Measured levels of ethylene oxide in blood, formed following exposure of rats and mice to ethylene, were used by Filer et al. (2013) to predict formation of N-(2-hydroxyethyl)valine N7-(2-hydroxyethyl)guanine in vivo. The results suggested higher levels of adduct formation in rats relative to mice, however the authors noted that there were some inconsistencies present between the predictions and measured values obtained by others. Wu et al. (1999) determined the dose-response curves for 7-HEG concentrations in tissues from rats and mice exposed by inhalation to ethylene oxide (0-100 ppm). Background concentrations were 0.2-0.3 pmol/µmol guanine in both species and increased linearly with atmospheric concentrations. Adduct concentrations were higher in rats than mice; values in rats were 5 to 13-fold higher than in control animals while in mice the increase was only 1 to 3-fold. The authors commented that the species difference may be due to either more efficient detoxification or DNA repair in mice. At ethylene exposure concentrations above 1,000 ppm, saturation of metabolism limits the adduct concentrations in rodents, resulting in a non-linear dose-response for both HEVal and 7-HEG. Comparison of 7-HEG concentrations showed that values in rodents exposed to 40 and 3,000 ppm ethylene were similar to rats exposed to 0.7-2.3 and 6.4-23.3 ppm ethylene oxide, respectively, or mice exposed to 3.0-8.8 and 6.7-21.5 ppm ethylene oxide, respectively (Walker et al.2000). Similar comparisons by Rusyn et al. (2005) found that exposure of rats to 40 ppm ethylene resulted in 38 to 65-fold lower concentrations of 7-HEG than exposure to 100 ppm of the metabolite ethylene oxide. Following exposure of rats to 100 ppm ethylene oxide, HEVal concentrations were greater than 10-fold higher than when rats were exposed to 3,000 ppm ethylene.

Elimination

Rats exposed to [¹⁴C]-ethylene by inhalation exhaled small amounts of unchanged ethylene along with some [¹⁴C]-CO₂. Polar metabolites and conjugates are eliminated predominantly in urine with smaller amounts in faeces (Guest et al., 1981).

Human Information

Exposure of healthy human volunteers by inhalation to initial concentrations of 5 or 50 ppm ethylene for 2 hours showed that the metabolism of ethylene is not saturated over this concentration range. The alveolar retention of inhaled ethylene was 2±0.8% (Filser et al.,1992). Measurement of the level of ethylene oxide present in blood from a group of four volunteers exposed to 5, 20 or 50 ppm ethylene for 2 hr was used by Filser et al. (2013) to calculate the extent of ethylene oxide formation. The results indicated that a mean value of 1.4 nmol ethylene oxide.hr/l blood was formed per ppm ethylene, a value which contrasts with the (30-65 fold) greater conversion of ethylene to ethylene oxide recorded in rodents.

Tornqvist et al.(1989) demonstrated a statistically significant increase in HEVal concentrations in fruit store workers exposed to ethylene (0.03-3.35 ppm; average 0.3 ppm) when compared to a control population. A similar effect was seen in both smokers and non-smokers, although concentrations of HEVal were higher in both control and exposed smokers than in comparable groups of non-smokers. The authors estimated that approximately 3% of atmospheric ethylene is converted to ethylene oxide by metabolism. Workers exposed occupationally to ethylene in a plastics factory were divided into groups according to their level of exposure. Concentrations of HEVal were 15 pmol/g (range 9-32) in the control group and 110 pmol/g (range 56-200) in the group exposed to approximately 4 mg/m3 (3.5 ppm) ethylene; the fraction of inhaled ethylene converted to ethylene oxide was estimated to be 0.5% (Granath et al.,1996).

Endogenous production of ethylene

Filser and Bolt (1983) identified that control rats excreted low concentrations of ethylene oxide, indicating endogenous production of ethylene. Csanady et al.(2000) determined that the rate of endogenous production of ethylene in rats was 11.5 nmol/h/kg bodyweight resulting in a steady state blood concentration of 0.57nmol/l. The values in humans were 33 nmol/h/70kg bodyweight resulting in a steady state concentration of 0.097nmol/l; this is similar to that resulting from occupational exposure to 0.126 ppm ethylene for 8 h/day, 5 days/week.