Tyrosyl Free Radical Research Articles

The iron center in ribonucleotide reductase from Escherichia coli.

Ribonucleotide reductase from Escherichia coli consists of two nonidentical subunits, proteins B1 and B2. The active site is made up from both subunits. Protein B2 contains 2 iron atoms and a tyrosyl-free radical, which are essential for the enzymatic activity. The paramagnetic susceptibility of protein B2 has been measured over the temperature range 30-200 K. A deviation from the Curie law is observed at high temperatures, consistent with a structure of an antiferromagnetically coupled pair of high spin Fe(III) with an exchange coupling -J = 108(-20)+25 cm-1. Electronic spectra are resolved into components from the iron center and the radical. A band at 600 nm is clearly identified and shown to have contributions from both components. The electronic absorptions of the tyrosyl radical of protein B2 are closely similar to those reported for phenoxy radicals of tyrosine and tritertiary butyl phenol. Determinations by EPR of the amount of free radical suggest the possibility of more than one radical per active protein B2 molecule. Reconstitution of the active site from apoprotein B2 and Fe(II) is only observed in the presence of oxygen. With Fe(III), no reconstitution is obtained. The additional physical data on the iron center of protein B2 strengthen the analogy with oxidized forms of hemerythrin. The most likely structure is an antiferromagnetically coupled pair of high spin Fe(III), possibly with a bridging oxo-group.

Current concepts of thyroid physiology

In presenting a review of current concepts of thyroid physiology, we have examined iodide metabolism, hormonogenesis, hormone secretion and peripheral metabolism, and finally homeostatic regulation. Iodide (I) is actively taken up by thyroid cells. As an essential constituent of thyroid homeostatically maintained at about 80 μg/day by the regulatory action of TSH on the trapping process. In large doses, I temporarily inhibits binding of I to tyrosine and the release of thyroid hormone by the gland. Trapped I is oxidized by peroxidase to a free radical, which then combines covalently with a tyrosyl free radical in preformed thyroglobulin to form MIT. Iodotyrosine residues within thyroglobulin combine to from T 4 or T 3 , which is stored in the follicular lumen. Secretion of thyroid hormone into the bloodstream involves the endocytosis of colloid material by the thyroid cell, fusion with lysosomes containing proteolytic enzymes, and subsequent proteolysis of thyroglobulin. TSH stimulates secretion by a process involving the activation of adenyl cyclase, so raising the intracellular content of cyclic AMP. The cyclic AMP may in turn activate a mechanism involving cyclic-nucleotide-dependent-protein kinase, contractile proteins, and Ca ++ . Thyroid hormone circulates in blood mostly bound to specific circulating proteins; only 0.05% is free. The free hormone level is homeostatically maintained at a constant level, and is in equilibrium with an intracellular pool of thyroid hormone. The peripheral action of thyroxine may involve its conversion in the tissues to T 3 , or its binding to specific receptor proteins within the cell. Pituitary secretion of TSH is subject to a negative feedback by circulating free thyroid hormone, and the sensitivity of the feedback system is modulated by TRF released by the hypothalamus.