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Description of key information

Key value for chemical safety assessment

Bioaccumulation potential:
no bioaccumulation potential

Additional information

TFE is metabolised in rodents by conjugation with glutathione in a reaction catalysed by glutathione S-transferases. The glutathione conjugate is metabolised to the equivalent cysteine conjugate. which is further metabolised via the mercapturic acid pathway and by renal and hepatic C-S lyases. Metabolism by C-S lyases leads to a number of acylating intermediates that react with the amino group of cysteine or are hydrolysed to difluoroacetic acid. In vitro studies indicate that the same pathways exist in humans. However, in humans, the C-S lyase activity is much lower than that seen in rodents. As a consequence, it is unlikely that humans will be more sensitive than rats or mice to the effects of TFE. An Assessment Factor of 1 can be used to describe "Other Interspecies Differences" when deriving the DNEL.

Discussion on bioaccumulation potential result:

In vivo studies

Humans

The metabolism of TFE has not been studied in humans in vivo. Using PBPK modelling, uptake has been calculated to be poor; approximately 1% of TFE entering the airways passes into the systemic circulation (Green and Mainwaring 1998). Evidence of exposure and metabolism has been demonstrated by the presence of fluoride ion in the urine of exposed workers (Xu et al, 1992). Exposure via the skin or other routes is not considered to be significant because of the volatility and slight solubility (in aqueous and organic solvents) of TFE.

Animals

The chemical properties of TFE preclude its synthesis in a radiolabelled form. Consequently, there are no quantitative in vivo data describing uptake, distribution and excretion. As in humans uptake in the rat has been calculated to be approximately 1% of the inhaled dose (Green and Mainwaring, 1998).

Metabolism of TFE was first demonstrated by the presence of fluoride ion in the urine of TFE exposed rats and hamsters (Dilley et al, 1974; Schneider 1983). Subsequently, the urine of rats and mice exposed to 6000 ppm (25 000 mg/m3) TFE for 6 hours was analysed by F-19 NMR and a number of metabolites identified (Odum and Green, 1984; Green, 2000). Based on the fluorine signals in the NMR spectra , difluoroacetic acid was identified as the major metabolite in the rat, accounting for >90% of all fluorine containing metabolites found in urine. Trace amounts of N-acetyl-S-(1,1,2,2 -tetrafluoroethyl)-L-cysteine, N-difluorothionoacetyl-S-(1,1,2,2 -tetrafluoroethyl)-L-cysteine and N-difluoroacetyl-S-(1,1,2,2 -tetrafluoroethyl)-L-cysteine were also present. In mouse urine, the mercapturate, N-difluorothionoacetyl-S-(1,1,2,2 -tetrafluoroethyl)-L-cysteine and N-difluoroacetyl-S-(1,1,2,2 -tetrafluoroethyl)-L-cysteine were present in similar amounts. The concentration of difluoroacetic acid was approximately half that of the combined total of cysteine conjugates (Green 2000). Fluoride ion excretion was approximately 1.7 -fold greater in rats than in mice. Cysteineylglycine and cysteine conjugates of TFE have been identified in the bile of exposed rats (Odum and Green, 1984).

Rats dosed in vivo with the cysteine conjugate of TFE, S-(1,1,2,2 -tetrafluoroethyl)-L-cysteine, yielded the same urinary metabolites as those seen in rats exposed to TFE itself, confirming a single metabolic pathway through glutathione conjugation (Commandeur et al 1988; 1991; Green, 2000). As with TFE the major metabolite was difluoroacetic acid. Following an intraperitoneal dose of either deuterated N-acetyl-S-(1,1,2,2 -tetrafluoroethyl)-L-cysteine or S-(1,1,2,2 -tetrafluoroethyl)-L-cysteine, only 2 -3% of the dose appeared in the urine as mercapturates in the 24 hour period following dosing, suggesting extensive metabolism of these cysteine conjugates in vivo (Commandeur et al 1991).

In vitro studies

Rat

A number of in vitro studies have investigated the metabolism of TFE and provided a partial explanation for the formation of the metabolites seen in vivo. TFE is metabolised by addition of glutathione across the double bond to give S-(1,1,2,2 -tetrafluoroethyl) glutathione without liberation of fluoride (Odum and Green, 1984). The reaction is catalysed by hepatic microsomal and cytosolic glutathione S-transferases and occurs at very similar rates (1.0 - 1.3 nmol/min/mg microsomal protein) in rat and mouse (Green 2000). There is no evidence for oxidation of TFE by cytochrome P450 enzymes (Odum and Green 1984).

S-(1,1,2,2 -tetrafluoroethyl)-L-cysteine, the metabolite of TFE found in rodents, has been shown to be a substrate for human and renal cysteine conjugate C-S lyases (Green and Odum, 1985; Green, 2000, 2001). The initial products of the reaction are believed to be a thiol, pyruvate and ammonia. Further reactions of the thiol liberate fluoride ion and lead to the formation of an acylating species, difluorothionoacyl fluoride, which reacts with S-(1,1,2,2 -tetrafluoroethyl)-L-cysteine to give N-difluorothionoacetyl-S-(1,1,2,2 -tetrafluoroethyl)-L-cysteine, or following hydrolysis and rearrangement, to give N-difluoroacetyl-S-(1,1,2,2 -tetrafluoroethyl)-L-cysteine and difluoroacetic acid (Commandeur et al, 1988, 1989, 1996). In the rat liver and kidney fractions, metabolism of S-(1,1,2,2 -tetrafluoroethyl)-L-cysteine by C-S lyases was approximately 4 -fold higher in the kidney than that in the liver.

Comparison between rodents and humans

S-(1,1,2,2 -tetrafluoroethyl)-L-cysteine, the metabolite of TFE found in rodents, has been shown to be a substrate for human renal cysteine conjugate C-S lyase (McCarthy et al 1994; Hawksworth et al 1996; Green 2001). Green (2001) compared the metabolism of TFE and its conjugates in liver and kidney fractions from the rat, mouse and human. The rates of conjugation of TFE with glutathione were measured in liver fractions and the metabolism of the S-(1,1,2,2 -tetrafluoroethyl)-L-cysteine by C-S lyases and N-acetyltransferases was compared in kidney fractions. The de-acetylation of N-acetyl-S-(1,1,2,2 -tetrafluoroethyl)-L-cysteine by renal acylases was also compared (see Table A). The highest rates of C-S lyase metabolism were found in mouse liver and rat kidney. Human C-S lyase activities were significantly lower than those in rodents.As a consequence, it is unlikely that humans will be more sensitive than rats or mice to the effects of TFE. An Assessment Factor of 1 can be used to describe "Other Interspecies Differences" when deriving the DNEL.

Table A: Metabolism of TFE and its cysteine conjugates in liver and kidney fractions from rats, mice and humans.

   GSTa  C-S lyaseb       N-acetyl transferaseb  Acylasec  
 Organ/  Vi  Km  Vmax  Km  Vmax  Km  Vmax
 Species  (nmol/min/  (mM)  (nmol/min/  (mM)  (nmol/min/  (mM)  (nmol/min/
    mg protein)    mg protein)    mg protein)    mg protein)
 Liver              
 Rat  94  2.0  5.9  2.0  3.9  0.3  37
 Mouse  79  3.0  40  7.0  69  0.2  18
 Human  87  5.4  1.7  4.9  3.5  0.3  48
 Kidney              
 Rat  ND  2.6  21.9  2.9  91  0.4  216
 Mouse  ND  5.9  4.0  9.0 48   1.0  248
 Human  ND  5.0  3.4  4.2  56  0.4  91

a Glutathione S-transferase (GST) activity was measured with TFE

b C-S lyase and N-acetyl transferase activities were measured with S-(1,1,2,2 -tetrafluoroethyl)-L-cysteine

c Acylase activity was measured with N-acetyl-S-(1,1,2,2 -tetrafluoroethyl)-L-cysteine

ND not determined