As detailed information
about each of the test methods is already available in the scientific literature, this is not covered here. The laboratories in which the methods have been developed are indicated and key references are included for further reading. Skin sensitisers show a high diversity in terms of chemical and physiochemical properties. However the AOP considers, chemicals – or in case of pre-/pro-haptens, their respective metabolites – which act as sensitisers due to their ability to react Selleckchem LY294002 with skin proteins (haptenation). This common characteristic is used in a number of non-animal test methods to differentiate between sensitisers and non-sensitisers. Two in chemico assays focus on peptide reactivity using two model peptides as surrogates for cellular proteins. In addition, three cell line assays use the kelch-like ECH-associated protein 1 (Keap1) as an intracellular sensor to investigate the reactivity of the test substance. Covalently binding to cysteine residues of Keap 1 causes this repressor protein to delocalize from the LY2109761 transcription factor NF-E2 p45-related factor 2 (Nrf2) which can then bind to and activate antioxidant response element (ARE) containing promoters. Whilst all five protein reactivity methods reflect the well established importance of interaction between electrophilic haptens and nucleophilic target proteins, the cell line based assays address
in addition the induction of cytoprotective mechanisms (referring to AOP key event 2). KeratinoSens™ and LuSens furthermore provide the potential for keratinocyte metabolism of pro-haptens. The DPRA is a chemistry-based assay that evaluates reactivity of a test compound using two synthetic model peptides including a lysine or cysteine residue. A solution of peptide and test substance in a ratio of 1:10 for cysteine and 1:50 for lysine is incubated for 24 h. After the incubation
period, the remaining concentration of the free peptide is measured by high performance liquid chromatography (HPLC) with gradient elution and ultraviolet (UV) detection at 220 nm. Depending on the data obtained from triplicate reactions, averaged peptide depletion of cysteine, lysine or Phospholipase D1 both are used in classification tree models to identify substances as sensitising or non-sensitising. In addition, the prediction model allows the allocation of the protein to the reactivity classes minimal, low, moderate and high (Gerberick et al., 2004 and Gerberick et al., 2007). The PPRA was developed from the DPRA in order to better identify potential pro- and pre-haptens. Eight concentrations of chemical are tested – instead of one concentration as in the DPRA. The cysteine peptide is incubated for 24 h in the presence and absence of horseradish peroxidase/hydrogen peroxide (HRP/P), whilst the lysine peptide is used only without HRP/P.
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