Pepsin

PEPSIN

Biological Source

It is the enzyme prepared from the mucous membrane of the stomach of various animals like pig, sheep, or calf. The commonly used species of pig is Sus scrofa Linn, belonging to family Suidae.

The stomach consists of an outer muscular layer and an inner mucous layer. The inner surface is covered with a single layer of epithelial cells which also lines the piths present on them. The piths are about 0.2 mm in diameter, and each pith has two to three narrow tubular ducts opening at the base. The epithelial layer is made of either the parietal cell or the central cell. The central cells are mainly covered with almost cubical shape and secrete pepsinogen and rennin zymogen, whereas the parietal cells are round or oval shaped cells, and they secrete the hydrochloric acid to activate the zymogen to produce rennin and pepsin. Pepsin is the first in a series of enzymes that digest proteins. Pepsin binds with protein chains and breaks it up into small pieces. Pepsin cleaves proteins preferentially at carboxylic groups of aromatic amino acids such as phenylalanine and tyrosine but does not cleave at bonds containing amino acids like valine or alanine. Pepsin mainly cleaves C-terminal to F, L, and E, and it does not cleave at V-, A-, or G-terminals. Structurally, the active site is located in a deep cleft within the molecule. Optimal activity of pepsin is at pH of 1.8 –3.5, depending on the isoform. They are reversibly inactivated at about pH 5 and irreversibly inactivated at pH 7–8.

Preparation

The mucous membrane is separated from the stomach either by the process of stripping or it is scrapped off, and it is placed in acidified water for autolysis at 37°C for 2 hours. The liquid obtained after autolysis consist of both pepsin and peptone. It is then filtered and sodium or ammonium salts are added to the liquid till it is half saturated. At this point only the pepsin separates out, and the peptone remains in the solution. The precipitates are collected and subjected to dialysis for the separation of salts. Remaining amount of pepsin if any in the aqueous solution is precipitated by the addition of alcohol into it. The pepsin is collected and dried at low temperature.

Description

Pepsin occurs in pale yellow colour, they are odourless or with very faint odour, translucent grains and slightly bitter in taste. It is soluble in dilute acids, water, and physiological salt (NaCl) solution. It is best active at a temperature of 40°C with pH 2–4. Pepsin is unstable above pH 6. The enzyme gets denatured at a temperature of 70°C and in the presence of alcohol and sodium chloride. Pepsin can be stored for 1–2 years at 2–8°C.

Uses

It is used in the deficiency of gastric secretion. Pepsin is also used in the laboratory analysis of various proteins; in the preparation of cheese, and other protein-containing foods.