Protein nitrogen unit precipitation procedure for allergenic extracts: collaborative study.

Protein nitrogen unit (NU) determination is one of the methods used to test and label the concentration of allergenic extracts. This recently standardized method is applicable to all allergenic extracts. One PNU/mL is equivalent to 1 X 10(-5) mg nitrogen determined to be in the material precipitated from 1 mL allergenic extract by phosphotungstic acid (PTA), a protein precipitant. The nitrogen is quantitated by the Kjeldahl method or another analytical method of equivalent accuracy and precision. A collaborative study of the optimized PNU precipitation method in which 6 samples were analyzed in duplicate by 6 laboratories using the Kjeldahl method for the determination of nitrogen yielded a mean of 0.1358 mg N/mL, a repeatability standard deviation and coefficient of variation of 0.0071 mg N/mL and 5.23%, respectively, and a reproducibility standard deviation and coefficient of variation of 0.0188 mg N/mL and 13.84%, respectively. The method has been adopted official first action.

Optimization of parameters in protein nitrogen unit precipitation procedure for allergenic extracts.

This work establishes the optimum conditions for the protein nitrogen unit (PNU) phosphotungstic acid (PTA) precipitation procedure for allergenic extracts. The volume of extract analyzed, the dilution of the extract prior to precipitation, the percent PTA in the precipitating solution, the washing of the precipitate, and the volume of concentrated HCl initially added to the extract were optimized, as they were found to be the factors which affected the amount of PTA precipitate obtained. Varying the digestion time, the digestion temperature, and the percent HCl in the 15% PTA precipitating solution did not cause detectable differences in the PNU values of the extracts studied.