Acidic endopeptidase, the inactive precursor of which, pepsinogen, is produced in the stomach mucosa. Pepsin cleaves proteins preferentially at carboxylic groups of aromatic amino acids such as phenylalanine and tyrosine. Highly active pepsine is preferably used for detection of trichina in pre-digested muscle meat of animals.

1 FIP unit (≈ Ph. Eur. unit) of Pepsin activity will, during incubation at 25 ±0.1 °C for one minute, cause decomposition of haemoglobin to an extent comparable with the colour caused by reaction of 1 micromole of tyrosine with the same dye. 2 000 FIP-Units correspond to 1: 10 000 NF-U and 1: 12 500 BP-U.

Irreversible inactivated in alkaline medium. Solution is stable in temperatures of up to 60 °C, and also in higher concentrations of urea and guanidine. pH optimum: 1-2. Isoelectric point: 1.0