Abstract
THE fourth component of complement (C4) in human serum is a glycoprotein with a molecular weight of 209,000 and consists of three covalently linked polypeptide chains1–3. A three-chain structure in proteins is exceptional and there is a possibility that C4 is synthesised as one or two chains which are subsequently cleaved. This hypothesis is in agreement with Hall and Colten4, who presented evidence that polysomes from homogenates of guinea pig liver synthesised a precursor form of C4. This pro-C4 has a single polypeptide chain of the same molecular weight as intact serum C4. We report here the presence in human serum and plasma of a functionally inactive protein with antigenic properties of human C4, which does not dissociate in reducing conditions into the α, β and γ chains characteristic of haemolytically active serum C4. This molecule probably represents a circulating precursor form of C4, or material that has escaped from the cell without being processed.
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