This section is from the book "A Text Book Of Materia Medica, Being An Account Of The More Important Crude Drugs Of Vegetable And Animal Origin", by Henry G. Greenish. Also available from Amazon: A Text Book of Materia Medica : Being an Account of the More Important Crude Drugs of Vegetable and Animal Origin.
Commercial pepsin is a mixture of the enzyme pepsin with other substances, obtained from the mucous membrane lining the stomach of the pig, sheep, or calf.
The surface of this mucous membrane exhibits multitudes of minute pits, each of which is about 0.2 mm. in diameter, and is the common orifice of two or three minute, elongated, tubular ducts. The pit together with the ducts debouching into it is termed a gastric gland. The duct is lined with two kinds of cells, viz. central cells and parietal cells, the former being the more numerous. The central cells secrete substances termed zymogens which are the precursors of enzymes but are not the enzymes themselves, the zymogens comprising pepsinogens and rennin zymogen. The pepsinogen is converted into pepsin and the rennin zymogen into rennin. The parietal cells secrete hydrochloric acid. These substances, probably with others, in aqueous solution constitute the gastric juice which is discharged from the glands into the stomach when meat is introduced into the mouth and masticated. Their action on the food is to convert insoluble proteids into soluble peptones and thus permit of digestion. Peptones differ from (soluble) proteids in not being coagulated by heat or precipitated by nitric acid, etc.
Commercial pepsin is obtained by stripping the mucous membrane from the stomach, mincing it and digesting it with water acidified with hydrochloric acid by which insoluble proteids are dissolved. From the filtered liquid it is precipitated by saturation with sodium chloride or ammonium sulphate, the peptones being left in solution. The pepsin is collected, dissolved in water and freed from sodium chloride, etc, by dialysis; it may then be precipitated by alcohol, or the aqueous solution may be evaporated to dryness in a vacuum and powdered.
Pepsin occurs in commerce as a pale yellowish powder, or in translucent scales or grains with a faint odour free from putrescence and a slightly saline bitterish taste. It is soluble in water, especially on the addition of a little hydrochloric acid. It contains the enzyme, pepsin, but does not consist of it. An acidified aqueous solution converts insoluble proteids into soluble parapeptone (or acid albumose), propeptone, and finally into peptone. Its action is inhibited by sodium chloride and by alcohol, and is completely destroyed at a temperature of 70°.
It is employed in dyspepsia caused by deficient gastric secretion.