Amyloid fibrils were studied from two different tissues of medullary carcinoma of the thyroid (MCT). The fibrils mainly consisted of a low molecular weight protein, AMCT, which was immunologically distinct and did not react with various antisera against known amyloid fibril proteins. A specific antiserum raised against the MCT amyloid proteins gave a reaction of identity with the degraded MCT amyloid fibrils from two patients, as well as with the isolated AMCT protein, but showed no reaction with other known amyloid proteins. The AMCT protein had a blocked N terminus, but the sequence analysis of a cyanogen bromide fragment revealed identity with human calcitonin in the 11 positions studied. Although the amino acid composition was similar, there were also distinct differences, and the mol wt of 5,700 daltons was considerably larger than that of calcitonin. For these reasons the AMCT protein may represent a prohormone of calcitonin.
An unusual protein AR was isolated from the amyloid fibril preparation derived from a patient with primary amyloidosis. Protein AR was unique in its antigenicity, and revealed no structural identity with any known amyloid proteins or with immunoglobulin chains or fragments. Thus a new third class of amyloid fibril proteins besides the immunoglobulin light-chain variable region fragments and the nonimmunoglobulin protein AS, has been characterized. A component antigenically related to protein AR was found in the serum of the patient.