Metabolites in Urine that Interfere with the Sandell-Kolthoff Assay for Urinary Iodine

The Sandell-Kolthoff (SK) assay is the main analytical method used to monitor population iodine nutrition in low- and middle-income countries. This assay can distinguish between populations that are iodine-deficient (median urinary iodine levels below 100 ppb), iodine-sufficient (median urinary iodine levels between 100 and 300 ppb), and iodine- excessive(median urinary iodine levels above 300 ppb). However, the analysis of urine samples with the SK reaction is technically challenging, partly because urine samples must be rigorously pretreated to remove interferents. In the literature, the only urinary metabolite that has been identified as an interferent is ascorbic acid. In this study, we used the microplate SK method to screen thirty-three of the major organic metabolites present in urine. We identified four previously unknown interferents: citric acid, cysteine, glycolic acid and urobilin. For each interferent, we investigated the following factors: (1) nature of interference-positive or negative, (2) threshold concentration for interference, and (3) possible mechanisms of interference. While this paper does not attempt to provide an exhaustive list of all interferents, knowledge of the main interferents allows for targeted removal.

The thyroid hormone converting enzyme human deiodinase 1 is inhibited by gold ions from inorganic salts, organic substances, and by small-size nanoparticles

The selenocysteine-containing enzyme class deiodinases (DIO) consists of three isoforms. DIOs play a role in regulation of thyroid hormone (TH) signaling through the removal of iodide from TH leading to TH that interact with the hypothalamic-pituitary-thyroid (HPT) axis with differing potency. Some gold-containing organic substances are known to inhibit many selenoenzymes, including DIOs. It is, however, unclear whether the Au-containing substances or the Au ions are causing the inhibition. In this study, five organic and inorganic gold substances as well as three gold nanoparticles (AuNPs) were tested for their potential to inhibit DIO1. The enzyme activity was tested using human liver microsomes as an enzyme source and reverse T3 as a substrate; iodide release was measured by the Sandell-Kolthoff method. The three organic gold substances aurothioglucose, auranofin and sodium aurothiomalate inhibited DIO1 with IC₅₀ between 0.38 and 0.75 μM while their structural analogues lacking the gold ion did not. Likewise, the two tested gold salts, Au(I) and Au(III) chloride, showed a concentration-dependent inhibition of the DIO1 with IC₅₀ values of 0.95 and 0.57 μM. Further, AuNPs of different sizes (100, 30 and 5 nm diameter) were tested with only the 5 nm AuNPs leading to inhibition with an IC₅₀ of 8 × 10¹⁴ AuNP/L. This inhibition was not caused by the Au ions released by the AuNP into the incubation media. The exact mechanism of inhibition of DIO1 by 5 nm AuNPs should be further examined. In conclusion, the microsomal DIO1 assay demonstrated the inhibition of DIO1 by gold ions originating from different gold-containing substances, but not by Au released from AuNPs; rather DIO1 is inhibited by 5 nm, but not larger, AuNPs.

[Current status of urinary iodine analysis and its clinical interest]

Iodine deficiency especially during pregnancy causes neurological disorders in the fetus. Actually, about two billion people are suffering from iodine deficiency through the world. The assessment of iodine intake is based on the determination of urinary iodine in the target population. Despite the complexity of the urinary matrix, a reliable assay must be achieved. After a review of pathophysiological interest of urinary iodine, various techniques for its measurement and their analytical performance were analyzed in order to select a reliable technique and enhance results interpretation.