Pepsin is the principal digestive enzyme of gastric juices. It digests proteins at peptide bonds only, and is active in the low pH of the stomach. NF grade pepsin has the ability to digest 3,000 or more times its weight in fresh egg albumin. This is called pepsin 1:3000. Concentrated pepsin can be 1:20,000 or more. Pepsin is used most often as a digestive aid, often in combination with HCl, to improve its activity. Pepsin is the first in a series of enzymes that digest proteins. In the stomach, protein chains bind in the deep active site groove of pepsin and are broken into smaller pieces. Then, a variety of proteases and peptidases in the intestine finish the job. The small fragments - amino acids and dipeptides - are then absorbed by cells for use as metabolic fuel or construction of new proteins.