Failure of medullary carcinoma of the thyroid to respond to doxorubicin therapy

We describe 3 patients with metastatic medullary carcinoma of the thyroid who were treated with doxorubicin hydrochloride (Adriamycin). Serum calcitonin was measured before and after doxorubicin therapy. Doxorubicin failed to arrest the progression of the disease in any of the patients. Although serum calcitonin levels dropped in 1 patient during therapy, they remained markedly elevated in all 3 patients. From the present series it appears that medullary thyroid carcinoma often does not have a response to doxorubicin.

Catalytic mechanism of p-hydroxybenzoate hydroxylase with p-mercaptobenzoate as substrate

p-Hydroxybenzoate hydroxylase (EC 1.14.13.2) from Pseudomonas fluorescens catalyzes in vivo the hydroxylation of p-hydroxybenzoate by molecular oxygen to form 3,4-dihydroxybenzoate. p-Mercaptobenzoate is also a substrate of the enzyme, but instead of being converted to the expected product, 3-hydroxy-4-mercaptobenzoate, the disulfide, 4,4'-dithiobisbenzoate, is formed. To find what mechanistic information this unusual reaction provided, steady state kinetic analyses, combined with rapid reaction studies of the changes in the enzyme-bound FAD, were carried out with the separate half-reactions involved in catalysis. Most of the kinetic measurements were made with a stopped-flow spectrophotometer designed for working anaerobically and connected on line to a minicomputer. Initial rate studies, upon varying systematically the concentrations of p-mercaptobenzoate, NADPH, and oxygen showed that the enzyme interacted with the substrates in the same manner as it does with p-hydroxybenzoate in place of the mercaptan. That is, a ternary complex is formed between enzyme, mercaptobenzoate, and NDAPH, followed by reaction and release of NADP+. Then a second ternary complex is formed between enzyme, mercaptobenzoate, and oxygen followed by reaction, liberation of product, and return to the resting state of the enzyme. Rapid reaction studies showed that the first half-reaction was analagous to that with the natural substrate. The enzyme-flavin is reduced to the 1,5-dihydroflavin by NADPH, and the rate of reaction is dramatically enhanced in the presence of mercaptobenzoate. The rate enhancement with this enzyme correlates well with the presence of a dianion form of the substrate on the enzyme. Examination of the second half-reaction showed that the reduced flavin on the enzyme formed transient intermediates upon reaction with oxygen, which were analogous to the intermediates in reactions where the enzyme forms an hydroxylated product. The oxidation of p-mercaptobenzoate by H2O2 in free solution resulted in the same disulfide as formed in the enzymatic reaction, only orders of magnitude slower. A sulfenic acid was probably the initial oxidation product from p-mercaptobenzoate, and this reacted very fast, and nonenzymatically, with mercaptobenzoate to form the disulfide and H20. The significance of the enzyme reaction with oxygen when complexed with p-mercaptobenzoate is discussed in relation to the mechanism of hydroxylation.

Primary empty sella syndrome with visual field defects

A patient with an enlarged, asymmetric sella turcica and visual field defects suggestive of a pituitary or parasellar tumor underwent extensive roentgenographic and pituitary function studies. No abnormalities in pituitary luteinizing hormone, follicle-stimulating hormone, thyroid-stimulating hormone, ACTH, prolactin or vasopressin secretion were detected. Growth hormone secretion was provoked by arginine infusion but not by hypoglycemia. Pneumoencephalography revealed air in the sella turcica, and no evidence of tumor. Thus, an enlarged sella turcica in a patient with visual field defects but normal pituitary function may suggest the presence of an "empty sella syndrome."

Failure of cyprohepatdine to inhibit vasopressin-stimulated cortisol release in a patient with Cushing's disease

A case of a 21-year-old woman with Cushing's disease due to a pituitary tumor is described. The patient was treated with cyprohepatadine for 4 weeks immediately following pituitary alpha-particle irradiation. A standard vasopressin test to measure ACTH-mediated cortisol release was performed four times: prior to pituitary irradiation, after irradiation, after 4 weeks of cyproheptadine therapy, and off cyproheptadine for 2 weeks. Cyproheptadine failed to modify vasopressin-stimulated cortisol release in the patient described. This study suggests that cyproheptadine, which has previously been shown to decrease ACTH secretion, probably acts principally at the hypothalamic, rather than at the pituitary level.