The arginine carboxypeptidase involved in the proteolytic cleavage of the haemagglutinin of influenza A virus has been analysed by an assay employing a Sepharose-bound peptide containing radioactive arginine as a substrate. The enzyme activity has been extracted from purified virus with non-ionic detergents and has been separated from the haemagglutinin and from the neuraminidase by isoelectric focusing and by affinity chromatography. The carboxypeptidase present in virus grown in different host cells shows variations in its isoelectric point. It can be concluded from these observations that the carboxypeptidase is a host component incorporated into the virus envelope. When the enzyme is inhibited by 2-mercaptomethyl-3-guanidinoethyl-thiopropanoic acid, haemagglutinin with the arginine attached to the carboxy terminus of HA can be obtained. The observation that under these conditions the haemagglutinin has retained its haemolytic activity indicates that the carboxypeptidase does not play an essential role in the activation process.


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