Comparison of complement fixation and radioassay techniques to detect solubilized human thyroid microsomal antigenic activity.

Although recently the human thyroid microsomal antigen (M-Ag) has been possibly identified as the thyroid peroxidase, its nature remained unknown over almost three decades. One of the difficulties encountered in the identification of M-Ag derived from the conflicting data obtained in the attempts to solubilize active antigenic material from thyroid subcellular fractions. In particular, following detergent treatment, M-Ag could not be detected by complement fixation, while a full recovery of the antigen has been observed using a radioassay technique. In the present investigation, the antigenic activity of Triton X-100 solubilized thyroid microsomes was assessed in parallel by complement fixation and radioassay methods employing the same anti-microsomal antibody (anti-M Ab) preparation for antigen detection. In untreated microsomes antigenic activity was detected by both methods. In contrast, detergent solubilized M-Ag was detected by radioassay, but could not be detected by complement fixation. These data indicate that detergent solubilization diminishes the complement fixing capacity of M-Ag, while the solubilized antigen can still be fully detected by its binding reaction with the autoantibody, and explain the discrepant results obtained in previous studies.