direct peptides are small molecules that react with specific amino acids to form peptide bonds. These peptides are used for a variety of applications including skin sensitization testing, food safety testing and drug discovery research. The DPRA is a chemistry-based test which uses synthetic cysteine or lysine peptides to evaluate the potential of chemical compounds to cause contact dermatitis or respiratory allergies in humans.
A sample of the test substance is incubated with the peptide and analyzed by high pressure liquid chromatography (HPLC) using UV detection. The peptide is depleted from the reaction mixture and the relative amount of the non-depleted peptide compared to the total peptide eluate from the HPLC is determined. The results are used to classify the test substance into one of the following UN GHS categories:
In chemico skin sensitization testing, the DPRA is modified by performing multiple incubations with increasing concentrations of the peptide to determine the reaction kinetics of the peptide. The results are used to identify the point in time when the peptide is fully depleted from the reaction mixture and a DPRA result can then be obtained for each concentration of the test substance tested.
The DPRA is commonly performed on essential oils to evaluate their skin sensitization potential. However, it is also possible to perform a DPRA on other chemicals that are known or suspected of being able to cause allergic reactions in humans.
During the DPRA, the peptides are stored in a solvent with nitrocellulose to prevent degradation and ensure that the peptides remain active for use during the testing process. The peptide stock solutions are prepared in acetonitrile and the working calibration solution is made up of 20% acetonitrile: buffer at pH = 7.5 to make up a 0.206 mM Cysteine peptide or 0.0167 mM Lysine peptide.
For each DPRA run, one sample is prepared without the peptide and is analyzed to check for interference from the test chemical (a phenomenon called co-elution). This can be assessed by examining the UV chromatogram of the peptide run at 220 nm and calculating the area ratio of the lysine or cysteine peptide compared with the acetonitrile peak at 258 nm.
For this study, the DPRA was performed on a set of seven different essential oils (Table 1) that were purchased from a supplier in Australia. The DPRA results were then compared with the published results of skin sensitization testing of these same essential oils in order to determine the validity and reliability of the DPRA as a screening tool for the potential allergenicity of these chemicals. The DPRA results indicate that all of the tested oils were not considered to be potent sensitizers, although the level of potency that the essential oil was classified as is still a concern and further testing will need to be conducted. The DPRA results are promising and may be useful in the evaluation of other chemicals for their potential to cause skin or respiratory allergies in humans.