Results Sex - associated Difference in Electrophoretic Patterns of C 5 in Mouse Plasma

The sex-dependent differences in serum levels of complement components have been reported concerning C4 (1), C5 (2, 3), C6 (3), C4-binding protein (C4-bp) ~ (4), and sex-limited protein (Slp) (5) in the mouse complement system. The serum levels o f these components in adult mice are always higher in males than in females within each inbred strain and are strongly under hormonal control. With regard to C5, young male and female mice have the same serum levels of C5, and the levels increase with age, most markedly in males and less in females (2). After sexual maturity, the ratio of C5 in serum of males vs. females is greater than 2:1. Injection of testosterone increases the levels of C5 in both males and females, whereas castration or injection of estradiol decreases the levels in males to those in females (3, 6). These reports, however, reveal only the quantitative difference but not the qualitative one. During attempts to detect polymorphism of mouse C5 by isoelectric focusing (IEF), we found the difference in electrophoretic patterns of C5 between males and females. Females o f all strains tested had one C5 band, but males had two C5 bands, one o f which was identical to the female band. The expression of the other C5 band in males was shown to be influenced by sex hormones, indicating that this C5 is actually responsible for the sex-dependent difference.