Effect of cortisol on the in vitro hepatic conversion of thyroxine to triiodothyronine in brook charr (Salvelinus fontinalis Mitchill)

General and Comparative Endocrinology
M M VijayanJ F Leatherland

Abstract

In vitro studies with liver homogenates of brook charr (Salvelinus fontinalis) demonstrate that the conversion of thyroxine (T4) to triiodothyronine (T3) is dependent upon temperature, pH, substrate, and homogenate concentration, supporting the hypothesis of an enzymatic hepatic T4 to T3 conversion. Dithiothreitol has a marked stimulatory effect on the rate of conversion. The kinetics of the conversion, determined by Lineweaver-Burke analysis suggest a Michaelis-Menten (Km) constant of 1.3 x 10(-8) M, and a maximum velocity (Vmax) of 0.42 pmol/mg protein/h. In brook charr given intraperitoneal implants of hydrogenated coconut oil containing cortisol at levels of 0, 5, and 50 mg/kg body weight, and sampled after 56 days, T4 to T3 conversion by homogenates of hepatic tissue in vitro was significantly (P less than 0.01) higher in the fish implanted with 50 mg/kg of cortisol than in either of the other two groups. However, plasma cortisol, L-thyroxine (T4) and triiodo-L-thyronine (T3) concentrations, plasma T3:T4 ratios and hepatosomatic index were not significantly different in the three treatment groups.

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Citations

Sep 28, 2000·Comparative Biochemistry and Physiology. Part B, Biochemistry & Molecular Biology·A OrozcoC Valverde
Jan 31, 2002·Comparative Biochemistry and Physiology. Part A, Molecular & Integrative Physiology·Aurea OrozcoCarlos Valverde-R
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Nov 7, 2003·Comparative Biochemistry and Physiology. Part B, Biochemistry & Molecular Biology·G Tripathi, Priyanka Verma
Sep 1, 2005·Thyroid : Official Journal of the American Thyroid Association·Aurea Orozco, Carlos Valverde-R
Oct 22, 2008·The Journal of Endocrinology·Edwin J W GevenPeter H M Klaren
Feb 21, 2013·Comparative Biochemistry and Physiology. Part A, Molecular & Integrative Physiology·Hon Jung LiewGudrun De Boeck
Oct 8, 2014·Comparative Biochemistry and Physiology. Part A, Molecular & Integrative Physiology·Samaneh PoursaeidGlen Van Der Kraak
Oct 24, 2007·General and Comparative Endocrinology·F J ArjonaP H M Klaren
Mar 3, 2011·General and Comparative Endocrinology·M C Subhash Peter
Jan 1, 1990·Fish Physiology and Biochemistry·J F LeatherlandT J Lam

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