Detection of two variants of complement component C3 in C3-deficient guinea pigs distinguished by the absence and presence of a thiolester

The complement system is an essential part of the innate defense, and C3 is an integral part of this powerful system. In previously identified complem ent C3 deficient guinea pigs only approx. 5% of the normal serum C3 level i s detectable. No differences were found between in vitro C3 protein synthes is and C3 mRNA levels of cells from C3-deficient and wild-type animals and the amino acid sequences of both C3 proteins are identical as deduced from cDNA sequencing. Previously, the principal inability to form a C3 thioleste r was discussed as a possible reason for this CS-deficiency. Here we report the isolation of two functionally different C3 species from the C3-deficie nt animals. Only one of these C3 proteins exhibits normal hemolytic activit y and contains a thiolester group. The second C3 species is exclusively pre sent in CS-deficient animals and lacks a thiolester, explaining its failure to express hemolytic activity. The presence of a second C3 species lacking a thiolester structure only in C3-deficient animals indicates that the sta bility of the thiolester may play a role in C3 deficiency. However further analysis of the in vitro stability of the thiolesters of C3 from normal and C3-deficient guinea pigs revealed no differences. A decreased in vivo thio lester stability might lead to the presence of C3 with and without a thiole ster or alternatively the expression of two isoforms of C3 in these animals . Considering the central role of C3 in host defense, the mechanisms of C3 thiolester formation require further analysis. (C) 2000 Elsevier Science Lt d. All rights reserved.