Studies on gastric digestion during 1820-1840 led to the discovery of
pepsin as the agent which, in the presence of stomach
acid, causes the dissolution of nutrients such as meat or coagulated egg white. Soon afterward it was shown that these
protein nutrients were cleaved by
pepsin to diffusible products named
peptones. Efforts to isolate and purify
pepsin were spurred by its widespread adoption for the treatment of digestive disorders, and highly active preparations were available by the end of the nineteenth century. There was uncertainty, however, as to the chemical nature of
pepsin, for some preparations exhibited the properties of
proteins while other preparations failed to do so. The question was not settled until after 1930, when Northrop crystallized swine
pepsin and provided convincing evidence for its identity as a
protein. The availability of this purified
pepsin during the 1930s also led to the discovery of the first synthetic
peptide substrates for
pepsin, thus providing needed evidence for the
peptide structure of native
proteins, a matter of debate at that time. After 1945, with the introduction of new separation methods, notably chromatography and electrophoresis, and the availability of specific
proteinases, the amino acid sequences of many
proteins, including
pepsin and its precursor
pepsinogen, were determined. Moreover, treatment of
pepsin with chemical
reagents indicated the participation in the catalytic mechanism of two aspartyl units widely separated in the linear sequence. Studies on the kinetics of
pepsin action on long chain synthetic
peptides suggested that the catalytic site was an extended structure. Similar properties were found for other "
aspartyl proteinases," such as
chymosin (used in cheese making), some intracellular
proteinases (
cathepsins), and plant
proteinases. After 1975, the three-dimensional structures of
pepsin and many of its relatives were determined by means of x-ray diffraction techniques, greatly extending our insight into the mechanism of the catalytic action of these
enzymes. That knowledge has led to the design of new inhibitors of
aspartyl proteinases, which are participants in the maturation of human immunodeficiency virus and in the generation of
Alzheimer's disease.