Thyroxine 5′-Deiodinase Activity of Rat Kidney: Observations on Activation by Thiols and Inhibition by Propylthiouracil*

Abstract

Enzymatic 5'-deiodination of T4 in various tissues is known to be stimulated by thiols and inhibited by propylthiouracil (PTU). Dithiothreitol (DTT) stimulation of rat kidney T4 5'-deiodinase followed saturation kinetics. Inhibition of the enzyme by PTU (10 (-5) M) was overcome in a concentration-dependent manner by DTT, methimazole (MMI), and thiourea. Pretreatment of catalytically active kidney microsomal preparations with PTS caused persistent inhibition of enzyme activity, assayed in the absence of PTU. Similarly, injection of PTU in rats in a dosage of 5 microgram/100 g BW or greater significantly impaired T4 5'-deiodination in subsequently isolated kidney microsomal preparations. Reagents that cleave disulfide bonds (DTT and KCN), as well as the reductant, Na2S2O4, and the thioureylene, MII, partially restored activity to PTU-inactivated enzyme in a time- and concentration-dependent way. These observations suggest that PTU inhibition of T4 5'-deiodinase results from an interaction of PTU with the enzym, possibly via a PTU-enzyme disulfide which can be prevented or reversed by thiols or thioureylene agents, and raise the possibility that protein sulfhydryl groups may undergo oxidation during the reductive 5'-deiodination of L-T4.

Thyroid over-expression of type 1 and type 2 deiodinase may account for the syndrome of low thyroxine and increasing triiodothyronine during propylthiouracil treatment

“…Together with the findings on D1 and D2 expression, we suggest that this syndrome is caused by a paradoxical rise in thyroid stimulating antibodies after propylthiouracil treatment that stimulates D2 over-expression in the thyroid, increasing the ratio of T 3 to T 4 . We presume that the D1 over-expression which we also found, despite continued propylthiouracil treatment, is secondary to T 3 -mediated upregulation of D1 synthesis, which, in turn, will exacerbate the over-production of T 3 and the consumption of T 4 .…”

“…Reactions were stopped by the addition of 0.1 ml 5% (wt/vol) BSA in water followed by the addition of 0.5 ml 10% (wt/vol) trichloroacetic acid in water. 4 for 60 min at 37 8C with the indicated amounts of tissue homogenates. Samples were incubated in duplicate in the presence of 100 nmol/l unlabelled T 3 to block D3, and in the absence or presence of 100 nmol/l unlabelled T 4 to saturate D2, in 0.1 ml PED10.…”

“…A paradoxical response to propylthiouracil was first reported in a minority of patents in whom there was a high ratio of T 3 to T 4 , together with increasing goitre, undetectable thyrotrophin (TSH) and a normal or slightly elevated basic metabolic rate (5). In a later study, Hegedü s and colleagues (6) identified seven Graves' patients, out of 50 consecutively treated with propylthiouracil, in whom there was initial normalisation of serum T 3 and T 4 levels and then a rise of T 3 levels into the thyrotoxic range, together with undetectable TSH levels and lower than normal T 4 levels.…”

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“…T 3 levels are known to induce DIO1 expression at the transcriptional level (34), which further contributes to the progression of the disease by elevating T 3 levels. The treatment for this condition uses antithyroid drugs such as propylthiouracil (PTU) which act predominantly by inhibiting the enzyme thyroid peroxidase; however, at higher concentrations, PTU also specifically inhibits DIO1 by binding to the Sec residue (201), which is advantageous in the treatment of severe hyperthyroidism (177). Interestingly, PTU does not seem to have a significant inhibitory effect on DIO2 and DIO3; the reason for this is not fully understood (32).…”

Thyroid hormone export from cells: contribution of P-glycoprotein

“…Since MDCKII cells originate from dog renal epithelium, and kidney expresses type I deiodinase (Leonard & Rosenberg 1978), we sought evidence of metabolism of 125 I-T 3 in duplicate cultures of MDCK cells during uptake and efflux experiments by analysing the radioactivity present in the cells and the medium after incubation with the tracer for 45 min at room temperature. Intact hormone and metabolites were resolved by LH-20 Sephadex chromatography.…”