Comparison between the monomeric and binary-complex forms of procarboxypeptidase A from whole pig pancreas

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Abstract
Two different forms of procarboxypeptidase A (I and II) were obtained from pig pancreas extracts. The MW values, the pattern found on polyacrylamide-gel electrophoresis in the presence of sodium dodecyl sulfate, and the sedimentation coefficients indicate that form I is a binary complex formed by 2 different subunits, whereas form II is a monomer. The carboxypeptidase A-precursor subunit of form I and the form II monomer are very similar with respect to MW value, amino acid composition and fragmentation by CNBr and iodosobenzoic acid. The activation process of both forms is unspecific with respect to the activating enzyme, the peptide released during activation is unusually long (MW .apprx. 12,500) and, in the case of the binary complex, the activation with trypsin follows a rather complex pattern, suggesting that the accompanying subunit of form I might play a modulating role in the activation process. Although the appearance of enzymic activity is slow, a protein with an MW equivalent to that of active carboxypeptidase A is found very early in the activation process. Both zymogens are glycoproteins (so far no carbohydrate has been reported in any procarboxypeptidase A) and both contain 2 strongly bound Zn2+ ions/molecule. Other chemical and physical properties were also determined.