Thyroid hormone deiodination in the domestic cat

Abstract

We have investigated thyroid hormone deiodination in the liver, kidney and thyroid of the domestic cat. Affinity labelling with 125I-bromoacetyl reverse T3 (125I-BrAc-rT3) demonstrated that liver and kidney, but not the thyroid, express type I iodothyronine deiodinase (IDI), results that were confirmed by measuring the activity of the IDI using 125I-rT3 and T4 as substrate. Feline hepatic and renal IDI metabolised rT3 at approximately 0·2% of the rate of rat hepatic IDI under identical assay conditions. The K(m) of the feline enzyme was at least 500-fold greater than that of rat IDI. However, feline and rat hepatic IDI metabolised T4 at a similar rate and had similar K(m) values (1·35 μM and 2·25 μM, respectively). This study demonstrates that cats and rats express IDI in the liver and kidney in similar concentrations; however, the feline enzyme appears unable to utilise rT3 as a substrate under physiological conditions.