Studies are in progress to characterize the biochemical and biological properties of a plasma sialoglycoprotein (sgp120) that we first identified by its capacity to interact with the activated fourth component (C4b) of the classical complement pathway (CCP). Results to date support the idea that one form of this protein, sgp120-A, regulates the activation of the CCP. Sgp120 is also cleaved by kallikrein of the intrinsic coagulation-kinin generating pathway to produce protein fragments that may possess biological activity. To obtain unequivocal data to support sgp120 as a new plasma protein and to gain information on its primary structure and function, studies are planned to complete cloning of the gene responsible for its synthesis. Excellent agreement was obtained by computer isoelectric point prediction analysis of the combined translated amino acid sequence of D and G clones with the splice site (pI = 6.36, 758 residues) and without (pI = 5.76, 723 residues) to that obtained by direct electrophoretic analysis of desialidated sgp120-A/I, respectively. This supports our hypothesis that the larger C4b binding form, sgp120-A, is a product of a spliced gene such as in clone D, the insert representing the C4b binding domain. Antibody was produced to a peptide deduced from the C-terminal portion of the splice site from clone D and will be used to study the binding of sgp120-A to surface bound C4b as well as to the free component. Pigtryin (PT), a postulated analog of human inter-alpha-trypsin inhibitor (IATI) was shown to have 83% identity to sgp120 over 97% of its entire partially reported sequence. This translated to a peptide match of 76% identity with 81% similarity. Thus, PT is most likely the pig analog of human sgp120 and not IATI. The mRNA of PT, identified as an acute phase reactant in a pig model of cardiogenic shock, is increased > 5 fold. Using a newly developed sensitive ELISA for sgp120 we have identified 2-3 fold increases in the serum sgp120 levels of burn patients suggesting that sgp120 like other complement components is an acute phase reactant.
