Activation of Rabbit C3: Studies of the Generation of Cleavage Products in vitro and of Their Metabolism in vivo

Abstract

The cleavage of purified, functionally active rabbit C3 by cobra venom factor and trypsin was analysed by reducing and non-reducing sodium dodecyl sulphate electrophoresis and autoradiography. The specific aim of the study was to compare these reactions to those that occur with human C3. Analysis showed that the pattern of breakdown was very similar to that for the human protein: while the ß-chain remained intact, there was step-wise degradation of the α-chain to form C3a, C3b, iC3b and C3c, all of which could be identified by gel analysis. The metabolic behaviour of three of these cleavage products, C3a, C3b and iC3b, was then examined in vivo using dual isotope techniques. Rabbits were studied simultaneously with I-C3 and I-labelled C3 breakdown products. Analysis of plasma and urine radioactivity for the subsequent 72 h showed that all three breakdown proteins had rapid rates of catabolism in vivo compared to the native molecule. Specifically, 93 and 98% of C3b and iC3b, respectively, were eliminated from the plasma compartment within 10 h of injection. C3a was completely eliminated within 10 h. By comparison, native C3 showed a half-life of 29 ± 3 h (mean ± SD) and a fractional catabolic rate of 4.30 ± 0.75%/h. The data support the use of this species in studies of complement behaviour in models of human immune disease and further clarify the basis for changes in plasma C3 concentration that accompany active immune complex- and antibody-mediated activity, in vivo.

The metabolism of isoforms of human adiponectin: studies in human subjects and in experimental animals

“…Thus C3, the third component of human complement, has a T1/2 in the rabbit of approximately 37 h, but 70 h in man (21). In comparison, rabbit C3 has a T1/2 of 29 h when reinjected into rabbits (21). The comparability of our data on the behaviour of the adiponectin-related molecule C1q in the rabbit to published turnover data on C1q in man (22,23) supports our conclusions on the metabolic behaviour of adiponectin.…”

“…As well, adiponectin was relatively long-lived in comparison with other hormones, with an average half-life in the rabbit of approximately 14 h. This figure may be underestimated, as human proteins typically exhibit more rapid metabolism in the rabbit than in man. Thus C3, the third component of human complement, has a T1/2 in the rabbit of approximately 37 h, but 70 h in man (21). In comparison, rabbit C3 has a T1/2 of 29 h when reinjected into rabbits (21).…”

Secreted production of collagen‐inspired gel‐forming polymers with high thermal stability in Pichia pastoris

“…Whereas act‐C3 is detectable for only a limited period after deposition,16 C4d, the stable split product of C4, can be detected for 4 to 8 days after deposition,17 giving a better indication of complement activation over time. Moreover, because C4 figures in the classical and lectin pathway but not in the alternative complement pathway, C4d detection gives information on the pathways involved in C3 activation.…”

Response of the alternative complement pathway to an oral fat load in first-degree relatives of subjects with type II diabetes

“…Metabolic studies in vivo have shown that C3-derived breakdown products equilibrate rapidly between the intravascular and extravascular compartments. 25 Proposed links between C3 and lipid metabolism depend on mechanisms that include the production of ASP and deposition of C3 in atherosclerotic lesions. The ASP hypothesis postulates that chylomicrons act as a catalyst to produce ASP in the capillary via the alternative pathway of complement.…”