Safety evaluation of certain food additives - ipcs inchem

494 FURAN-SUBSTITUTED ALIPHATIC HYDROCARBONS
metabolite with sulfhydryl trapping agents, including cysteine (10 mmol/l) and GSH
(10 mmol/l), showed a marked decrease in microsomal protein binding, suggesting
that sulfhydryl conjugation plays a role in the detoxication of acetylacrolein. Cysteine
was the better trapping agent for the prevention of microsomal protein binding when
compared with GSH, semi-carbazide, lysine or N-acetylcysteine. The authors
postulated that cysteine forms a stable cyclic conjugate with ,-unsaturated
aldehydes, whereas the ability of GSH to form stable conjugates with ,unsaturated
aldehydes varies (Esterbauer et al., 1975, 1976; Ravindranath &
Boyd, 1985).
Other in vitro experiments support the conclusion that CYP-mediated
oxidation of 2-methylfuran is directly related to its toxicity. This was studied in
hepatocytes isolated from adult male Wistar rats that were untreated or treated with
phenobarbitone (0.1% in drinking-water for 5 days) or -naphthoflavone (80 mg/kg
bw by intraperitoneal injection daily for 3 days). The cultured hepatocytes were
incubated with 2-methylfuran at 0, 100, 300, 600 or 1000 μmol/l (0, 8.2, 24.6, 49.3
and 82.1 μg/ml, respectively) for 24 h. The median lethal concentrations (LC50
values) for untreated, phenobarbitone-treated or -naphthoflavone-treated
hepatocytes were 794, 34 and 57 μmol/l (65.2, 2.8 and 4.7 μg/ml), respectively,
indicating that enzyme induction increased the toxicity of 2-methylfuran (Hammond
& Fry, 1991).
Male Sprague-Dawley rats (150–200 g) were administered a single dose of
50, 100, 200 or 400 mg 2-methylfuran/kg bw in sesame oil by intraperitoneal
injection and were sacrificed 24 h later. The 50 mg/kg bw group did not show any
evidence of liver necrosis, but they exhibited endothelial injury, with bleeding of the
endothelium into the vascular lumen of the central veins. Animals given 100, 200 or
400 mg 2-methylfuran/kg bw showed a dose-related increase in the severity of
hepatocellular injury (e.g. eosinophilic cytoplasm, vacuolation), centrilobular
necrosis, and necrosis and sloughing of the bronchiolar epithelium, which, at the
high dose, resulted in complete obliteration of numerous respiratory and terminal
bronchioles. Dose-related increases in serum glutamic pyruvic transaminase (GPT)
were observed up to 200 mg 2-methylfuran/kg bw; however, the levels of serum
GPT in the animals given 50 mg 2-methylfuran/kg bw were not significantly higher
than those of the control rats. At a dose of 100 mg/kg bw, GSH levels in liver were
reduced by 32% at 0.5 h and 20% at 4 h. Depletion was not found in the lungs.
Tissue distribution and covalent binding studies were conducted over a period of
0.5–24 h after an intraperitoneal dose of 100 mg [ 14 C]2-methylfuran/kg bw. Maximal
covalent binding was observed in the liver at 4 h. At all time points, binding of the
label was greatest in liver, followed by kidney (Ravindranath et al., 1986).
Free GSH levels in the liver, lungs and kidneys, investigated over a period
of 0.5–36 h after administration of 100 mg 2-methylfuran/kg bw, were initially
decreased (67.5% of control in the liver and 87% of control in the kidneys at 0.5 h),
but then reached or exceeded control levels within 8–24 h (137% of control in the
kidneys and 130% of control in the lungs at 12 h). The radiolabelled [ 14 C]2methylfuran
covalently bound to protein was detected at the highest concentration
in the liver, followed by the kidney, lung and blood. Liver and kidney DNA also
showed covalent binding of 14 C label, with a 2-fold increase in binding in the liver