Soybean implicated in goiter & also inhibits thyroid - NATIONAL LIBRARY OF MEDICINE

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1: Biochem Pharmacol 1997 Nov 15;54(10):1087-96 Books

 

 

Anti-thyroid isoflavones from soybean: isolation, characterization, and

mechanisms of action.

 

Divi RL, Chang HC, Doerge DR.

 

National Center for Toxicological Research, Jefferson, AR 72079, USA.

 

The soybean has been implicated in diet-induced goiter by many studies.

The extensive consumption of soy products in infant formulas and in vegetarian

diets makes it essential to define the goitrogenic potential. In this report, it

was observed that an acidic methanolic extract of soybeans contains compounds

that inhibit thyroid peroxidase- (TPO) catalyzed reactions essential to thyroid

hormone synthesis. Analysis of the soybean extract using HPLC, UV-VIS

spectrophotometry, and LC-MS led to identification of the isoflavones genistein

and daidzein as major components by direct comparison with authentic standard

reference isoflavones. HPLC fractionation and enzymatic assay of the soybean

extract showed that the components responsible for inhibition of TPO-catalyzed

reactions coeluted with daidzein and genistein. In the presence of iodide ion,

genistein and daidzein blocked TPO-catalyzed tyrosine iodination by acting as

alternate substrates, yielding mono-, di-, and triiodoisoflavones. Genistein

also inhibited thyroxine synthesis using iodinated casein or human goiter

thyroglobulin as substrates for the coupling reaction. Incubation of either

isoflavone with TPO in the presence of H2O2 caused irreversible inactivation of

the enzyme; however, the presence of iodide ion in the incubations completely

abolished the inactivation. The IC50 values for inhibition of TPO-catalyzed

reactions by genistein and daidzein were ca. 1-10 microM, concentrations that

approach the total isoflavone levels (ca. 1 microM) previously measured in

plasma from humans consuming soy products. Because inhibition of thyroid hormone

synthesis can induce goiter and thyroid neoplasia in rodents, delineation of

anti-thyroid mechanisms for soy isoflavones may be important for extrapolating

goitrogenic hazards identified in chronic rodent bioassays to humans consuming

soy products.

 

PMID: 9464451 [PubMed - indexed for MEDLINE]

 

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