Penicillamine hormonal

Certain amino acids and their derivatives, although not found in proteins, nonetheless are biochemically important. A few of the more notable examples are shown in Figure 4.5. y-Aminobutyric acid, or GABA, is produced by the decarboxylation of glutamic acid and is a potent neurotransmitter. Histamine, which is synthesized by decarboxylation of histidine, and serotonin, which is derived from tryptophan, similarly function as neurotransmitters and regulators. /3-Alanine is found in nature in the peptides carnosine and anserine and is a component of pantothenic acid (a vitamin), which is a part of coenzyme A. Epinephrine (also known as adrenaline), derived from tyrosine, is an important hormone. Penicillamine is a constituent of the penicillin antibiotics. Ornithine, betaine, homocysteine, and homoserine are important metabolic intermediates. Citrulline is the immediate precursor of arginine. 

The most frequent offenders are cytostatic agents and anticoagulants, but hair loss can occur with a variety of common drugs, including hormones, anticonvulsants, amantadine, amiodarone, captopril, cholesterol-lowering drugs, cimetidine, colchicine, etretinate, isotretinoin, ketoconazole, heavy metals, lithium, penicillamine, valproic acid, and propranolol. 

After parturition the mother s thyroid function tests were normal, as were thyroglobulin and thyroperoxidase TSH receptor antibodies were undetectable. Her thyroid was not palpable, but a scan showed diffuse enlargement and uptake was increased. She developed a multinodular goiter 4 years later, but free thyroxine and TSH concentrations remained normal. Three more children born to mothers using penicillamine all had subclinical hypothyroidism. The authors hypothesized that penicillamine inhibits thyroid hormone iodination and coupling reactions catalysed by thyroid peroxidase, and concluded that in utero exposure to penicillamine may cause congenital goitrous hypothyroidism and persistent subclinical hypothyroidism in older children. 

In addition to these common amino acids which make up the protein molecule, a number of other amino acids exists which are not present in the protein but are biochemically important. These may be a-, jS-, y-, or 5-substi-tuted. Two important examples are the neurotransmitter y-aminobutyric acid (GABA) and the thyroid hormone (1-28, Chap. 1) precursor 2,5-diiodotyrosine. Other examples include j -alanine (precursor of the vitamin pantothenic acid), jS-cyanoalanine (a plant amino acid), and penicillamine (a clinically useful metal chelating agent). 

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