Structure-activity analysis of microsomal antigen/thyroid peroxidase.

Area of Science:

• Immunology
• Endocrinology
• Biochemistry

Background:

• Thyroid microsomal autoantibodies are implicated in autoimmune thyroid diseases like Graves' and Hashimoto's.
• The primary autoantigen is thyroid peroxidase (TPO), an enzyme involved in thyroid hormone synthesis.
• Understanding the specific binding sites of autoantibodies is crucial for diagnosing and potentially treating these conditions.

Purpose of the Study:

• To investigate the interaction between thyroid microsomal autoantibodies and thyroid microsomal antigen/TPO.
• To determine if the peroxidase-active site and major autoantigenic sites reside on the same TPO fragments.
• To elucidate the role of disulfide bridges in autoantibody binding to TPO.

Main Methods:

• Analysis of sera from 30 patients with Graves' or Hashimoto's diseases.
• Comparison of antibody reactivity to intact solubilized microsomal antigen and water-soluble trypsin fragments.
• Immunoprecipitation of 125I-labelled microsomal antigen followed by SDS-PAGE and autoradiography.

Main Results:

• Microsomal antibodies showed high correlation (r = 0.96) in reactivity towards intact antigen and trypsin fragments containing TPO activity.
• Human microsomal antigen (Mr = 110,000) possesses an intrachain disulfide bridge.
• Trypsin cleavage yields water-soluble fragments (Mr = 100,000, 73,000, 68,000) retaining binding activity, dependent on disulfide bridge integrity.

Conclusions:

• The major autoantigenic site(s) and peroxidase-active site of thyroid microsomal antigen are likely located on the same trypsin-cleaved fragments.
• The integrity of the disulfide bridge is essential for the binding of thyroid microsomal autoantibodies to TPO.
• These findings contribute to understanding the structural basis of autoantibody recognition in autoimmune thyroid diseases.