Elevated levels of histaminase activity were found in the serum of four patients with metastatic medullary carcinoma of the thyroid gland, and high activity of this enzyme was observed in each of 10 medullary-carcinoma tissue specimens from seven patients. The elevated enzyme activity appeared to be specific to histaminase since the activity of other amine oxidases was not increased in serum or tissue. In two patients with metastatic medullary carcinoma a total inhibition of serum histaminase activity was achieved with oral doses of aminoguanidine sulfate; the long duration of drug effect suggested irreversible inhibition of the enzyme. Measurements of histaminase activity afford a possible new diagnostic approach to this type of thyroid carcinoma and may provide a means for following the course of the disease. DESPITE the known existence of the enzyme histaminase since 1929,1 its biologic role has remained essentially undetermined. The only clinical state in which the presence of the enzyme has been studied in detail is normal pregnancy, in which a progressive rise in histaminase activity in serum is seen throughout the first six months.2, 3 The development in our laboratory of a new and sensitive radioassay for histaminase activity4 led us to investigate this enzyme in a variety of clinical situations. With this method the rise in histaminase activity in serum during pregnancy was found to be detectable as early as nine.
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