Lactoperoxidase-catalysed surface iodination and sucrose-gradient ultracentrifugation were used to investigate the structure of human complement component C1. 1. Proenzymic subcomponents C1r and C1s associated to form a trimeric C1r2-C1s complex (7.6 S) in the presence of EDTA, and a tetrameric Clr2-C1s2 complex (9.1 S) in the presence of Ca2+. Iodination of the 9.1 S complex led to a predominant labelling of C1r (70%) over C1s (30%), essentially located in the b-chain moiety of C1r and in the a-chain moiety of C1s. 2. Reconstruction of proenzymic soluble C1 (15.2 S) from C1q, C1r and C1s was partially inhibited when C1s labelled in its monomeric form was used and almost abolished when iodinated C1r was used. Reconstruction of fully activated C1 was not possible, whereas hybrid C1q-C1r2-C1s2 complex was obtained. 3. Iodination of proenzymic or activated C1 bound to IgG-ovalbumin aggregates led to an equal distribution of the radioactivity between C1q and C1r2-C1s2. With regard to C1q, the label distribution between the three chains was similar whether C1 was in its proenzymic or activated form. Label distribution in the C1r2-C1s2 moiety of C1 was the same as that obtained for isolated C1r2-C1s2, and this was also true for the corresponding activated components. However, two different labelling patterns were found, corresponding to the proenzyme and the activated states.
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