Fig. 5.

Contribution of MASP-1 and MASP-2 to C3–convertase formation through C2 cleavage. The C2 cleaving ability of MASP-1 and MASP-2 was determined through measuring C3 deposition on C4b-saturated plate using the MASP-1– and MASP-2–specific inhibitors. When MASP-1 was inhibited by SGMI-1, the level of C3 deposition dropped to 43.5% of the noninhibited experiment. In accordance with this result, when MASP-2 was inhibited by SGMI-2, the level of C3 deposition dropped to 61%. The two independent measurements unequivocally show that MASP-1 is the major contributor of the C2 cleavage. When applying both inhibitors at the same time the level of C3 deposition is reduced to 3.3%.