Enzymic methods for determining V-terminal sequences

Although the Edman method is by far the most common method of sequencing peptides from the /V-terminus, some enzymic methods are used occasionally. Several aminopeptidases are available commercially, which differ in their specificities. One aminopeptidase from porcine kidney preferentially releases amino acids such as leucine with hydrophobic side-chains. This enzyme does not release /V-terminal Arg or Lys or any amino acid that is followed by Pro. Another enzyme, aminopeptidase M, which is obtained from the microsomal fraction of porcine kidney cells, is less specific and perhaps more useful. It is advisable to examine aliquots of the hydrolysate at intervals by chromatography to determine the order in which amino acids are being released. 

Another type of enzyme, termed a dipeptidyl aminopeptidase, releases dipeptides rather than amino acids from the /V-terminus. Cathepsin C is one such enzyme and it will remove dipeptides consecutively from the /V-terminus of a peptide until either Lys or Arg becomes the /V-terminal amino acid or until Pro is in position 2 or 3 in the chain. Thus two dipeptides, Asp—Arg and Val—Tyr are cleaved from angiotensin II  

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