Denatured TBG does not bind iodothyronines but can be detected with antibodies that recognize the primary structure of the molecule (23). In euthyroid adults with normal TBG concentration, about one-third of the molecules carry thyroid hormone, mainly T4. When fully saturated, it carries about 20 µg of T4/dl of serum. The biologic half-life is about 5 days, and the volume of distribution is similar to that of albumin (34,35) (Table 1). TBG is cleared by the liver. Loss of sialic acid accelerates its removal through interaction with the asialo-glycoprotein receptors reducing the half live by 500-fold (21). However, it is unknown whether desialylation is a required in the normal pathway of TBG metabolism.
*The amino acids arginine , methionine and phenylalanine are considered essential for reasons not directly related to lack of synthesis. Arginine is synthesized by mammalian cells but at a rate that is insufficient to meet the growth needs of the body and the majority that is synthesized is cleaved to form urea. Methionine is required in large amounts to produce cysteine if the latter amino acid is not adequately supplied in the diet. Similarly, phenylalanine is needed in large amounts to form tyrosine if the latter is not adequately supplied in the diet.
Enzymes, which play an essential kinetic role in biological reactions, are composed of large protein molecules . Enzymes facilitate the rate of reactions by acting as catalysts and lowering the activation energy barrier between the reactants and the products of the reactions. All chemical reactions that occur during the digestion of food and the metabolic processes in tissues require enzymes. Therefore, enzymes are vital to the overall function of the body, and thereby indicate the fundamental and significant role of proteins.