Protease from lactic acid bacteria is of great importance to flavor and texture quality of fermented foods. An acidic protease from Pediococcus pentosaceus 220 was purified to homogeneity with a 11.5-fold increase in specific activity and 13.4% of recovery by precipitation with ammonium sulfate ( . The enzyme activity was also strongly inhibited by Sodium dodecyl sulfate and EDTA. It could be deduced that the purified enzyme was an acidic metalloprotease.

RationaleAccurate measurement of trace compounds in blood samples is important in clinical diagnosis and life science. Ambient ionization mass spectrometry, however, suffers from the matrix effect when dealing with complex samples such as blood. Therefore, it is important to reduce the matrix effects in blood samples.MethodsA low‐cost and disposable Teflon tube was used as a platform to precipitate the protein in blood. The analytes are extracted into organic solvent, and the precipitated protein can be adsorbed by the chromatography paper inserted. Therefore, the Teflon tube after precipitation can be directly subjected to paper spray ionization mass spectrometry, achieving one‐step analysis of blood.ResultsHigh sensitivity and satisfactory stability were achieved for pharmaceuticals, acids, and endogenic metabolites in blood. The absolute signal intensities of characteristic product ions of the tested analytes were 8–20 times higher after protein precipitation than those obtained using paper spray. Detection limits and quantitative performance were evaluated for three drugs: carbamazepine, metformin, and tioconazole. In addition, the limits of detection and quantitation were improved 9–14‐ and 8–12‐fold, respectively.ConclusionsProtein precipitation coupled to paper spray with a tube and then to mass spectrometry was successfully achieved and applied in the one‐step analysis of trace compounds in blood samples. The experimental results showed that this method was sensitive, stable, convenient, and economic for the direct analysis of blood.