Abstract
Objective—To isolate and characterize factor I of the bovine complement system.
Sample Population—Serum obtained from the blood of beef cattle.
Procedures—Serum samples were fractionated to yield factor I by means of sequential precipitation, ionexchange, and gel-filtration chromatography. The protein was identified throughout the procedure on the basis of its ability to degrade the α'-chain of bovine C3b in the presence of bovine factor H. Electrophoresis in polyacrylamide gels was used to assess the degradation of C3b and determine the molecular weights of factor I and its component polypeptide chains.
Results—Bovine factor I had an apparent molecular weight of 94 kd and consisted of 2 disulfide-bonded polypeptides that had apparent molecular weights of 51 and 42 kd (under reducing conditions). Factor H was required for the factor I cleavage of the α'-chain of bovine C3b into iC3b. A similar cofactor effect was provided by trypsinized bovine erythrocytes or erythrocyte ghosts. Bovine properdin was prepared and shown to be a single polypeptide chain of 58 kd in the reduced form.
Conclusions and Clinical Relevance—Bovine factor I can be purified from serum by a simple 4-step procedure. It is structurally and functionally comparable to factor I of other species, and its purification completes the isolation and characterization of all the soluble components of the bovine alternative complement pathway. (Am J Vet Res 2003;64:989–993)
