Thyroid hormone deiodination in the domestic cat.
We have investigated thyroid hormone deiodination in the liver, kidney and thyroid of the domestic cat. Affinity labelling with (125)I-bromoacetyl reverse T(3) (125)(I-BrAc-rT(3) 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 (125)I-rT(3) and T(4) as substrate. Feline hepatic and renal IDI metabolised rT(3) 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 T(4) at a similar rate and had similar K(m) values (1.35 microM and 2.25 microM, 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 rT(3) as a substrate under physiological conditions.
Foster
D J
DJ
The University of Edinburgh, Department of Veterinary Clinical Studies, The Hospital for Small Animals, Easter Bush, Roslin, Midlothian EH25 9RG, UK.
Thoday
K L
KL
Beckett
G J
GJ
D000345:Affinity Labels; D000818:Animals; D002415:Cats; D007453:Iodide Peroxidase / Q000378:metabolism*; D007455:Iodine / Q000378:metabolism*; D007700:Kinetics; D013379:Substrate Specificity; D013974:Thyroxine / Q000378:metabolism*; D014284:Triiodothyronine / Q000378:metabolism*
D000345:Affinity Labels; D014284:Triiodothyronine; D007455:Iodine; D007453:Iodide Peroxidase; D013974:Thyroxine