Biological Origin of Naturally Occurring Peptides

Uncommon Amino Adds Found in Naturally Occurring Peptides 

Some naturally occurring peptides, as we have said previously, contain certain uncommon amino acids. It should be noted here that the 

The dual presence, of linkages other than the classical a-peptide bond, and of uncommon amino acids in numerous naturally occurring peptides, enables us to predict that, if enzymes exist which hydrolyze such peptides, these enzymes should differ by their specificity from the peptidases which hydrolyze peptides resulting from the partial degradation of proteins. Actually, the supposition is true, for several such enzymes are known, which we shall at least mention in this review. 

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