C3-like activity in C3-deficient dog serum

A complete absence of the third component of complement has been demonstrated in a colony of Brittany spaniels. The deficiency is inherited in an autosomal recessive fashion and is clinically characterized by an increased susceptibility to infection and renal disease. Despite having no detectable antigenic C3, C3-deficient dog serum has approximately 10% of normal serum C3 hemolytic activity. This report characterizes the nature of the C3-like activity in C3-deficient dog serum. The activity is inhibited by heating serum to 56 degrees C for 30 min and by incubation in 0.01 M EDTA. The activity is also sensitive to both methylamine and D-76, in a dose-dependent fashion. The lytic activity in C3-deficient dog serum appears to be complement mediated. Furthermore, lysis of EAC1,4,oxy2 cells by deficient dog serum is inhibited by methylamine. Although lysis is boosted by the addition of purified C5, a greater boost is obtained in the absence of methylamine. Therefore, there exists a methylamine-sensitive protein other than C4 in C3-deficient dog serum which interacts with C5. It is possible that this protein may be a newly discovered complement protein for which appropriate conditions for its detection have not previously been available.