Serum from humans was used to detect diamine oxidase (DAO) enzyme activity using enzyme-linked immunosorbent assay (ELISA), as described by Honzawa et al. (2011) (link). The human blood samples were kept at room temperature for 1 h before centrifugation for 20 min at 1,000 × g and were immediately stored at −80°C for subsequent quantification.
After sonication of the mouse caecal contents, the intestinal bacterial cell supernatant was used to determine lysine decarboxylase (LDC) enzyme activity using ELISA, as previously described (Lee and Cho, 2001 (link)). Feces samples (100 mg) suspended in 2.5 ml of cold phosphate-buffered saline were centrifuged at 1,000 × g for 20 min to obtain the supernatant and were centrifuged again (4,000 rpm, 20 min, 4°C) to obtain the bacterial cells. These cells resuspended in lysis solution (50 mM of Tris-HCl at pH 7.4, 2 mM of EDTA, and 100 mM of NaCl) were crushed using an ultrasonic cell disruptor (BRANSON, Branson, MO, United States) under the following conditions: 20 kHz, 150 W, and crushing for 60 s three times, 60 s apart at 4°C.
After sonication of the mouse caecal contents, the intestinal bacterial cell supernatant was used to determine lysine decarboxylase (LDC) enzyme activity using ELISA, as previously described (Lee and Cho, 2001 (link)). Feces samples (100 mg) suspended in 2.5 ml of cold phosphate-buffered saline were centrifuged at 1,000 × g for 20 min to obtain the supernatant and were centrifuged again (4,000 rpm, 20 min, 4°C) to obtain the bacterial cells. These cells resuspended in lysis solution (50 mM of Tris-HCl at pH 7.4, 2 mM of EDTA, and 100 mM of NaCl) were crushed using an ultrasonic cell disruptor (BRANSON, Branson, MO, United States) under the following conditions: 20 kHz, 150 W, and crushing for 60 s three times, 60 s apart at 4°C.