Nicholas M. Alexander

Methylene blue competes with paraquat for reduction by flavo-enzymes resulting in decreased superoxide production in the presence of heme proteins

Methylene blue competes 100 to 600 times more effectively than paraquat for reduction by three different flavo-containing enzymes; xanthine oxidase, NADH cytochrome c reductase, and NADPH cytochrome c reductase. Paraquat and methylene blue both interact with deflavo xanthine oxidase, indicating that neither electron acceptor reacted at the FAD site of the enzyme where molecular oxygen is reduced to superoxide. As the paraquat radical also directly reduced acetylated cytochrome c the hemeprotein could not be utilized for measuring superoxide production in the presence of the herbicide. In the presence of cytochrome c the methylene blue caused a sharp decrease in both paraquat-induced superoxide and hydroxyl radical production.

Oxidation and oxidative cleavage of tryptophanyl peptide bonds during iodination

Abstract Tryptophan peptides are oxidized to the oxindole by various chemical and peroxidase catalyzed iodination procedures in the pH range 4.0–7.5. This oxidation results in significant cleavage of tryptophanyl peptide bonds at pH 4–5. Thus, destruction of some biologically active peptides and proteins during iodination may be due to the modification of essential tryptophan residues.

Carbamazepine (Tegretol) inhibits in vivo iodide uptake and hormone synthesis in rat thyroid glands

Decreased serum concentrations of T3 and T4 occur in patients treated with the anticonvulsant drug carbamazepine (CBZ), but with rare exception, these patients remain euthyroid. The mechanism that accounts for diminished hormone levels is unknown, and our objective was to study the direct effect of CBZ on iodide uptake and hormone synthesis in thyroid glands of CBZ-treated and pair-fed control rats. Chronic ingestion (per os) of CBZ in male rats reduced the four hour thyroid 131I-iodide uptake by approximately 60%. This inhibition occurred after the animals had received sufficient CBZ to attain plasma CBZ concentrations of 0.8 microgram/ml. Continued treatment with CBZ ranging from 560 to 800 mg/kg/day for 14 days did not result in further inhibition of iodide uptake even though the plasma CBZ concentrations had increased 6-20 fold. No inhibition of iodide uptake was apparent when the animals initially received CBZ ranging from 40 to 152 mg/kg body weight for 22 days when there were no detectable levels of plasma CBZ. Overall growth rates of CBZ-treated rats were slightly (6-10%) less than the pair-fed control animals. Plasma T4 concentrations were reduced by 18% (p less than 0.05) in the CBZ-fed animals, while T3 concentrations were diminished by 53% (p less than 0.01). CBZ appeared to alter thyroidal iodide transport because the thyroid:plasma iodide ratios were decreased by 26% in the drug-treated rats. The distribution of radioiodine in thyroidal iodoamino acids was essentially the same in both groups of rats but the absolute quantities of radioiodine were more than 2.5 times greater in the control rats. CBZ failed to inhibit peroxidase-catalyzed iodide and guaiacol oxidation in vitro. No differences in thyroid gland morphology were noted in the two groups of animals, but weights of the glands from the CBZ-fed animals increased by 25% after 42 days. It is concluded that CBZ, or its metabolites, has a direct inhibitory effect on iodide utilization and hormone synthesis by the thyroid gland.

Semi-automated competitive protein binding analysis of serum thyroxine on reusable sephadex columns and its advantages over radioimmunoassay

Competitive protein-binding analysis of serum thyroxine on small, reusable, Sephadex columns has been further studied and improved. The improved, semi-automated procedure results in reduced working time and costs. It has also been established that triiodothyronine crossreacts only 1/6 to 1/9 as well as thyroxine, and can be ignored because it represents only about 1/80 of the total serum iodothyronine content. The economic and methodological advantages of the improved method over radioammunoassay and other displacement assays are discussed.

Myeloma immunoglobulin interferes with serum thyroxine analysis by homogeneous enzyme immunoassay

Myeloma immunoglobulin paraproteins interfere with a homogeneous enzyme immunoassay (EMIT) for serum thyroxine. The EMIT assay failed to detect any hormone in three hyperproteinemic sera from multiple myeloma patients, although thyroxine in these sera was accurately measured by our competitive protein-binding radio-assay on small, re-usable Sephadex columns. The interference was due to turbidity of the paraproteins in the EMIT reaction mixture, resulting in an increased absorbance and a marked underestimation of hormone concentration. Thyroxine was detected (82 to 107% recovery) by the EMIT assay in ethanol extracts of myeloma sera. With 82 other sera there was an excellent correlation (r = 0.985, slope = 0.912, Y intercept (EMIT) = 6.8) of the EMIT assay with our competitive radioassay. Thus, although the EMIT thyroxine assay possesses many desirable features and it is an attractive alternative method to competitive radioassays, its susceptibility to interferences by immunoglobulin paraproteins is a troublesome liability.

Superoxide dismutase activity (erythrocuprein) in Wilson's disease

Abstract Erythrocyte superoxide dismutase (erythrocuprein) levels were determined in cells from normal subjects and from patients with Wilsons disease. The concentration of this copper-containing enzyme was essentially identical in both groups, even though serum ceruloplasmin was markedly reduced, or absent, in Wilsons disease. The observed concentration of the dismutase confirms previous results by others using immunochemical procedures. Extended therapy with D-penicillamine resulted in a 25 to 43% decrease in superoxide dismutase activity, and an 81 to 99% decrease in serum ceruloplasmin. Our results indicate that erythrocuprein levels are independent of serum ceruloplasmin concentration.

Inhibition of Thyroid Peroxidase (TPO) and Lactoperoxidase (LPO) by Goitrin and Ricinine

The plant toxins, goitrin and ricinine, inhibit TPO and LPO. Goitrin (0.3 mM) completely inhibits the peroxidase-catalyzed oxidation of iodide and is nine times more potent than ricinine. Both toxins also inhibit the peroxidase-catalyzed oxidation of guaiacol, although less efficiently than iodide oxidation. Goitrin is oxidized by H2O2 in the presence, but not in the absence, of peroxidase, whereas ricinine is not oxidized. Iodide stimulates (twofold) the rate of goitrin oxidation. Spectra obtained with mixtures of LPO, H2O2, and goitrin indicate that LPOoxid oxidizes goitrin and is converted back to native LPO. Thus, both LPOoxid and LPO-Ioxid oxidize goitrin but LPO-Ioxid is more effective. Because of their well known toxicity, neither of these toxins derived from Brassicae plants (goitrin) and castor beans (ricinine) is considered a serious dietary problem.

Normal Peroxidase Activity in Pendred's Syndrome.

Excerpt Familial goiter due to defective iodide organification has been divided into three genetic types. Type A, with cretinism, lacks peroxidase activity completely. Type C, with euthyroidism, ap...