Carboxypeptidase-resistant peptides

As described in Section 1.6.1 exopeptidases cleave at N- and C-termini and include aminopeptidases, carboxypeptidases and dipeptidyl peptidase, whereas endopeptidases cleave at an internal peptide bond and include enkephalinase and cathepsin B. Small peptides are relatively resistant to the action of endopeptidases but their activity is significant for large peptides. 

The action of these two pancreatic exopeptidases on synthetic substrates, proteins, and peptides has been reviewed in detail by Neurath (1960). The specificity requirements which were deduced from studies with synthetic peptides have been confirmed by studies with polypeptides. The structural requirements of specific substrates for both types of carboxy-peptidase are analogous except for the nature of the amino acids which contain the free, ionized a-carboxyl group at the terminus of the substrate. Carboxypeptidase B hydrolyzes most rapidly those bonds formed by terminal lysyl and arginyl residues, whereas carboxypeptidase A hydrolyzes terminal bonds formed by a variety of aromatic, neutral, or acidic amino acids. Of the natural amino acids only carboxyl-terminal prolyl residues are resistant to the action of the enzyme. The rate of hydrolysis depends upon the nature of the side chains of the amino acids which form the susceptible bonds. Thus, differences in the rate of hydrolysis of different substrates may vary several thousandfold. The methods for application of these peptidases to hydrolysis of proteins have been discussed in detail by Canfield and Anfinsen (1963). 

Note that all have V-tcrminal sarcosine and are therefore resistant to amino-peptidases. The second peptide has C-terminal D-Phe whereas the third has C-terminal (3-diphenyl-alanine. Both are resistant to carboxypeptidases. All three compounds are antagonists of angiotensin II. 

Most of these peptidolipids contain one or several D-amino acids and some unusual amino acids such as t/ireo-3-hydroxy-L-glutamic acid and cysteic acid. The peptide chain of imacidin C includes D-a//othreonine and the much rarer L-a//othreonine. The presence of such amino acids probably plays a role in increasing the resistance of these substances towards proteolytic enzymes. For instance, imacidic acid or imacidin is not cleaved by any of eight different proteolytic enzymes (trypsin, chymotrypsin, pepsin, thermolysine, pronase, carboxypeptidases A and B, papain) 124). This could be very useful for protection of the bacterial cell or for conservation of the molecule inside the host tissues. 

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