Factor D̄ of the alternative pathway of human complement. Purification, alignment and N-terminal amino acid sequences of the major cyanogen bromide fragments, and localization of the serine residue at the active site

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Abstract
The serine esterase factor .hivin.D of the complement system was purified from outdated human plasma with a yield of 20% of the initial hemolytic activity found in serum. This represented an approximate 60,000-fold purification. The final product was homogeneous as judged by sodium dodecyl sulphate/polyacrylamide gel electrophoresis (with an apparent MW of 24,000), its migration as a single component in a variety of fractionation procedures based on size and charge and its N-terminal amino acid sequence analysis. The N-terminal amino acid sequence of the 1st 36 residues of the intact molecule was homologous with the N-terminal amino acid sequences of the catalytic chains of other serine esterases. Factor .hivin.D showed an especially strong homology (> 60% identity) with rat group-specific protease over the 1st 16 amino acid residues. This similarity is of interest since it is considered that both enzymes may be synthesized in their active, rather than zymogen, forms. The 3 major CNBr fragments of factor .hivin.D, which had apparent MW of 15,800, 6600 and 1700, were purified and aligned by N-terminal amino acid sequence analysis and amino acid analysis. By using factor .hivin.D labeled with di-[1,3-14C]isopropylphosphofluoridate, it was shown that the CNBr fragment of apparent MW 6600, which is located in the C-terminal region of factor .hivin.D, contained the active serine residue. The amino acid sequence around this residue was determined.