Inhibition by lysine and arginine of the conversion of C3 and B in the serum and a purified system

Abstract

When human serum was incubated at 45°C for 30 min, C3 and B were converted to C3b and Bb. Molecular weights of purified C3 and B were shown not to change after incubation at 50°C. Spectropolarimetry indicated that the secondary structures of C3 and B changed after incubation at higher temperature. The titration of SH groups in the C3 molecule showed the liberation of an SH group. These results show that the alternative complement pathway is activated at raised temperatures without known activators such as zymosan or lipopolysaccharide (LPS). This may be due to the accelerated interaction of conformationally changed components of the alternative pathway such as C3 and B. Using this system, effects of various substances on the interaction of C3 and B in serum and the purified system were investigated. The addition of arginine and lysine resulted in the inhibition of the conversion of C3 and B in the serum at elevated temperature. Other amino acids such as anionic amino-acids and NaCl did not influence the conversion. In the purified system, only arginine and lysine prevented the conversion of C3 and B, when C3, B and D were incubated in the presence of Mg ++ and amino-acids. Since lysine and arginine did not inhibit the enzymatic activity of D , these data suggest that arginine and lysine prevent the interaction of C3 and B in the serum at elevated temperatures.