The initial characterization of a thyroid iodotyrosine dehalogenase (tDh), which deiodinates mono-iodotyrosine and di-iodotyrosine, was made almost 50 years ago, but little is known about its catalytic and kinetic properties. A distinct group of dehalogenases, the so-called iodothyronine deiodinases (IDs), that specifically remove iodine atoms from iodothyronines were subsequently discovered and have been extensively characterized. Iodothyronine deiodinase type 1 (ID1) is highly expressed in the rat thyroid gland, but the co-expression in this tissue of the two different dehalogenating enzymes has not yet been clearly defined. This work compares and contrasts the kinetic properties of tDh and ID1 in the rat thyroid gland. Differential affinities for substrates, cofactors and inhibitors distinguish the two activities, and a reaction mechanism for tDh is proposed. The results reported here support the view that the rat thyroid gland has a distinctive set of dehalogenases specialized in iodine metabolism.
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- Author: P Villalobos x
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JC Solis-S, P Villalobos, A Orozco, and C Valverde-R
G. Thordarson, R. Villalobos, P. Colosi, J. Southard, L. Ogren, and F. Talamantes
ABSTRACT
The ability of mouse placental lactogen (mPL), mouse prolactin (mPRL), mouse GH (mGH) and ovine prolactin (oPRL) to stimulate synthesis of α-lactalbumin was tested in a primary culture of mouse mammary gland epithelial cells. Mammary tissue was obtained from 10-day pregnant Swiss Webster mice, enzymatically dissociated and the cells were cultured on floating collagen gels for 5 days. The basic culture medium consisted of Nutrient Mixture F12/Dulbecco's Modified Eagle's Medium (1:1, v/v), containing 10 mg insulin/1, 5 mg cortisol/l, 10 μg epidermal growth factor/l, 5 g bovine serum albumin/l and 50 mg gentamycin/l. Mouse PL, mPRL, mGH and oPRL were added to the basic medium in concentrations from l μg/l to l mg/l. Accumulation of α-lactalbumin in the culture medium was measured. For that purpose, mouse α-lactalbumin was purified from mammary tissue obtained from lactating Swiss Webster mice and a radioimmunoassay was developed. Mouse PL, mPRL and oPRL stimulated a dose-dependent increase in α-lactalbumin secretion. Mouse GH also caused a significant, but dose-independent, increase in α-lactalbumin secretion. Mouse PL showed the greatest activity in stimulating α-lactalbumin secretion. It was concluded that mPL is an important lactogenic hormone in the latter half of pregnancy in the mouse, when circulating mPRL concentrations are low.
J. Endocr. (1986) 109, 263–274
