Molecular Size and Subunit Structure of the Fourth Component of Guinea Pig Complement

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Abstract
The fourth component (C4) of human complement is a glycoprotein with a m.w. of 209,000 daltons and consists of three subunits of m.w. 93,000, 78,000, and 33,000 daltons linked by disulfide bridges (1, 2). The size and structure of the corresponding complement protein in guinea pig serum has not yet been elucidated, although the sedimentation coefficient of guinea pig C4 (8S20,w) is known to be similar to that of human C4 (3). The m.w. of guinea pig C4 has been estimated by gel filtration to be 180,000 (4). We now report isolation of C4 from guinea pig serum by a one-step immunoabsorbent method. Its molecular size and subunit structure were similar to that of human C4. Normal guinea pig liver in culture synthesized and secreted C4 with size and subunit structure identical to serum C4. Materials and Methods. A 14C-labeled l-amino acid mixture consisting of equal amounts of 14C-leucine (270 mCi/mmol), 14C-valine (225 mCi/mmol), 14C-isoleucine (270 mCi/mmol), and 14C-lysine (270 mCi/mmol) was obtained from New England Nuclear, Boston, Mass.