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Thyroid hormone deiodination in the domestic cat
D J Foster1, K L Thoday, G J Beckett
1The University of Edinburgh, Department of Veterinary Clinical Studies, The Hospital for Small Animals, Easter Bush, Roslin, Midlothian EH25 9RG, UK.
Journal of Molecular Endocrinology
|February 5, 2000
Summary
Domestic cats possess type I iodothyronine deiodinase (IDI) in the liver and kidney. However, the feline IDI enzyme shows significantly lower activity with reverse T(3) (rT(3)) compared to rat IDI.
Area of Science:
- Endocrinology
- Comparative Physiology
- Biochemistry
Background:
- Thyroid hormones are crucial for metabolism and development.
- Type I iodothyronine deiodinase (IDI) plays a key role in thyroid hormone activation and inactivation.
- Understanding species-specific differences in IDI function is important for comparative physiology.
Purpose of the Study:
- To investigate the presence and activity of type I iodothyronine deiodinase (IDI) in domestic cats.
- To compare the kinetic properties of feline IDI with those of rat IDI, particularly regarding reverse T(3) (rT(3)) and thyroxine (T(4)) metabolism.
- To elucidate the physiological implications of feline IDI activity.
Main Methods:
- Affinity labeling using (125)I-bromoacetyl reverse T(3) ((125)I-BrAc-rT(3)) to identify IDI expression.
- Enzyme activity assays using (125)I-rT(3) and T(4) as substrates to determine kinetic parameters (K(m), V(max)).
- Comparison of feline enzyme kinetics with established rat IDI data.
Main Results:
- Type I iodothyronine deiodinase (IDI) was detected in the liver and kidney of domestic cats, but not in the thyroid.
- Feline hepatic and renal IDI exhibited significantly lower metabolic rates for rT(3) compared to rat hepatic IDI (approximately 0.2%).
- The K(m) for rT(3) of feline IDI was at least 500-fold greater than that of rat IDI, indicating poor substrate utilization.
- Feline and rat hepatic IDI demonstrated similar rates and K(m) values for T(4) metabolism.
Conclusions:
- Domestic cats express type I iodothyronine deiodinase (IDI) in the liver and kidney, similar to rats.
- The feline IDI enzyme has a markedly reduced capacity to metabolize reverse T(3) (rT(3)) compared to the rat enzyme under identical conditions.
- These findings suggest that under physiological conditions, feline IDI may not efficiently utilize rT(3) as a substrate, potentially impacting thyroid hormone metabolism in cats.