Diamine-induced dissociation of the first component of human complement, C1

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Abstract
Lys can inhibit spontaneous and antibody-dependent C1 [complement component 1] activation but Lys does not prevent autoactivation of purified C1r. Lys (20 mM), 1,2-diaminoethane, 1,3-diaminopropane, 1,4-diaminobutane or 1,5-diaminopentane are able to dissociate C1 into its 2 entities, C1q and the Ca-dependent C1r2-C1s2 complex. Ig-ovalbumin insoluble complexes bearing C1 are also dissociated by Lys and the above-mentioned diamines used at the same concentration: C1q remains bound to the immune complexes; the C1r2-C1s2 complex is partially solubilized. The effect of Lys or diamines is not due to a competition with Ca for Ca-binding sites, as increasing concentrations of Ca even slightly increase the dissociation due to the amines. The dissociative effect is dependent on the C chain length of the diamines, with an optimum for 1,3-diaminopropane. It is also dependent on the relative cis-position of the amino groups in the diamines. Polyamines such as spermine and spermidine are also able to dissociate C1 with even a higher efficiency than Lys and putrescine. Thus, a diamine-induced structural inhibition of C1 is demonstrated, of potential interest for a pharmacological control of complement activation.