Purification and partial characterization of the third component of the complement system from porcine serum (C3) and of a crystallizable degradation product of the fourth component of the complement system from human serum (C4). Study of the tryptic digestion products of human and porcine C3 and of human C4.

The third component of the porcine complement system (C3) was isolated. The protein with 190000 molecular weight is composed of two chains alpha and beta of Mr 116000 and 74000. The amino-acid composition is very similar to that of human C3. A degradation product of human C4 was also isolated and characterized. This protein was crystallized. The degradation product is characterized by a molecular weight of 128000; the major part of the alpha chain is lost. Native human and porcine C3 and human C4 were submitted to tryptic digestion. The digestion products were isolated and characterized in terms of molecular weight, subunit organization and amino-acid composition. The present study confirms the high homology among these proteins.