Restriction of the Substrate Specificity of C1 Human by Heat Inactivation or Cell Binding

Fluid phase or cell-bound activated first component of human complement (C1̄) utilizes the fourth (C4) and to a lesser degree the second (C2) components, as well as N-α-acetylglycyl-l-lysine methylester (AGLMe), N-acetyl-l-arginine methylester (AAMe), p-tosyl-l-arginine methylester (TAMe) and N-acetyl-l-tyrosine methylester (ATMe). The hydrolysis of either natural or synthetic substrates is decreased when C1̄ is cell bound (EAC1̄). Inactivation of C4 and C2 by C1̄ or EAC1̄ is inhibited by TAMe and AAMe, but not ATMe. C2 destruction by C1̄ in the presence of C4 exhibits a similar inhibition profile with these esters.

Heat-inactivated C1̄ loses the ability to split C2, TAMe, or ATMe, but not C4 or AAMe. C4 destruction by heat-inactivated C1̄ is prevented by AAMe. C1̄ can be blocked by C1̄INH so as to lose its activity against C2 and TAMe, but not C4 or AAMe.