alpha-Neoendorphin-like immunoreactivities (alpha-NE-IR) were demonstrated in tissues from three patients with medullary carcinoma of the thyroid (MCT). A large amount of alpha-NE-IR was detected in the extracts of primary tumors and metastatic lymphatic tissues by a highly sensitive and specific radioimmunoassay (RIA). Gel filtration analyses showed two different molecular-weight forms of the alpha-NE-IR: One eluted at the void fraction and the other at the position of [125I]-alpha-NE on Sephadex (Pharmacia Fine Chemical, Uppsala, Sweden) G-50 chromatography. Immunohistochemical examination revealed the presence of alpha-NE-IR in the C-cell carcinoma. These data presumably reflect that alpha-NE, the opioid peptide derived from preproenkephalin B, is synthesized in the MCT.
