Formation of the initial C3 convertase of the alternative complement pathway. Acquisition of C3b-like activities by spontaneous hydrolysis of the putative thioester in native C3.

Abstract

We present evidence suggesting that nonenzymatic, spontaneous hydrolysis of a thioester bond in native C3 constitutes the initial event in the activation of the alternative pathway.It has been known since the original description of C3 that the hemolytic activity of the isolated protein decays spontaneously in aqueous solution at a slow rate (1). This decay was greatly accelerated by chaotropic agents (2). It has also been known that treatment of C3 with simple amines such as hydrazine abolished its hemolytic activity (3). Recently, evidence has accumulated that strongly supports the existence of an intramolecular thioester bond in native C3 (4-8). Modification of the thioester by incorporation of 1 mol of methylamine destroyed the potential of C3 to bind to biological targets of complement, induced conformational changes in the molecule (9) and led to the expression of functional binding sites for complement proteins resembling those of enzymatically generated C3b (4, 10). Methylamine-modified C3 [C3(CHsNH2)] 1 was shown (4) to bind Factor B and properdin and was susceptible to cleavage and inactivation by Factors H and I.The present study was performed to investigate the spontaneous decay of the binding potential of native C3 and to determine whether hydrolysis of the thioester bond in C3 generates, without any further chemical or enzymatic modification, C3b-like functional properties. Because chaotropic agents facilitate access of water to relatively concealed groups by perturbing the tertiary structure of proteins, these agents were used to accelerate the spontaneous decay of C3. Chaotrope-treated C3 that has acquired C3b-like functions on the basis of this treatment will be tentatively designated C3(H20). This denotation proposes that this form of C3 contains, instead of the thioester bond, a free sulfhydryl and carboxyl group.

Anti-factor B autoantibody in dense deposit disease

“…Complement can be activated via three major pathways, of which the alternative pathway shows a continuous low-level activation in plasma. Spontaneous hydrolysis of the C3 internal thioester bond results in C3(H 2 O), which is able to bind factor B (Pangburn et al, 1981). After cleavage of factor B by factor D, C3(H 2 O)Bb forms the short-lived initial fluid-phase C3 convertase enzyme, which cleaves C3 into the anaphylatoxin C3a and the opsonin C3b that covalently binds to target surfaces and initiates the formation of the C3bBb convertase.…”

Decay-Accelerating Factor Must Bind Both Components of the Complement Alternative Pathway C3 Convertase to Mediate Efficient Decay

“…C is activated via several different mechanisms. The alternative pathway (AP) 3 of C activation turns over continuously at a low level in plasma, priming the system to react immediately when confronted by a pathogen (1). The AP also functions as an amplification loop for two other activation pathways, the classical pathway (CP) and the lectin pathway.…”

Post-bone marrow transplant thrombotic microangiopathy

“…[50][51][52] It was estimated by Sahu and Lambris 52 that~0.5% of total C3 exists at any given instant in this hydrolyzed form. The rate of C3 (H2O) formation has been measured between 0.2 and 0.4% per hour.…”

“…The rate of C3 (H2O) formation has been measured between 0.2 and 0.4% per hour. 50 Meehan et al 53 stained 1101 consecutive renal allograft biopsies for C4d over a period of 51 months. Only six out of 182 C4d+ biopsies were reported as TMA-positive.…”