Tryptophan hydroxylase antibodies used in the diagnosis of carcinoid

The expression of tryptophan hydroxylase, the rate-limiting enzyme in the biosynthesis of serotonin, is described in a case of a 35 year-old patient with metastatic jejunal carcinoid. Immunohistochemically, monoclonal anti-tryptophan hydroxylase antibodies positively identified liver metastases of a neuroendocrine tumor. The cellular distribution of tryptophan hydroxylase was restricted exclusively to the cytoplasm of carcinoid cells, where it was found in large amounts. By means of immunoblotting, anti-tryptophan hydroxylase antibodies detected in samples from carcinoid tissue two closely migrating polypeptide bands with molecular weights of 26 kDa and 29 kDa, respectively. These two protein bands appear to represent proteolytically degraded polypeptides, since tryptophan hydroxylase is known for its extreme unstability in vitro. In our case, the immunohistochemical and biochemical identification of tryptophan hydroxylase in liver lesions of a neuroendocrine tumor permitted the correct diagnosis of a metastatic carcinoid.