C cell (parafollicular cell) ?Immunoreactive thyroglobulin: Purification, identification and immunological characterization

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Abstract
In relation to our earlier finding that the thyroglobulin-like material responsible for the cytochemical immunoreaction of C cells was obtained in the peak I fraction of Bio-Gel A-5 m, which included faster sedimenting components of thyroglobulin, the present study has identified the positive reacting component and clarified its immunochemical and immunohistochemical properties. The peak I fraction of dog and hog thyroglobulin was chromatographed on a Bio-Gel A-50 m column. Antiserum to the faster eluted peak I 1 ′ only immunoreacted with C cells. The peak I 1 ′ was then refiltered on Bio-Gel A-150 m column. Antiserum to peak I 1 ″ fraction of both species which was eluted in the first part had high immune specificity for C cells. When 4–30% and 2–16% continuous gradient gels of polyacrylamide were employed, peak I 1 ″ represented a single electrophoretic band corresponding to the component with the largest molecular weight in thyroglobulin. The protein was named C-thyroglobulin. The molecular weight was approximately 2,600,000, four times as large as 19 S, as calculated by relative mobility on the 2–16% gradient gel. In double diffusion tests, anti-peak I 1 ″ antiserum produced two immunoprecipitin lines with its own antigen. The reaction was different from that of anti-19 S antiserum which formed a single line. On immunoperoxidase staining, anti-peak I 1 ″ antiserum reacted to C cells in exactly the same way as anti-calcitonin antiserum. When anti-peak I 1 ″ antiserum was absorbed with calcitonin, the subsequent reaction of the C cells was greatly decreased. The absorption of anticalcitonin antiserum with increased amounts of peak I 1 ″ abolished the C cell reaction. On the basis of these observations, the possibility that C-thyroglobulin is a biosynthetic precursor of calcitonin exists.