Catecholamines Thyroid Hormones Are Made From Tyrosine

Catecholamines Are Synthesized in Final Form & Stored in Secretion Granules

Three amines—dopamine, norepinephrine, and epinephrine—are synthesized from tyrosine in the chromaffin cells of the adrenal medulla. The major product of the adrenal medulla is epinephrine. This compound constitutes about 80% of the catecholamines in the medulla, and it is not made in extramedullary tissue. In contrast, most of the norepinephrine present in organs innervated by sympathetic nerves is made in situ (about 80% of the total), and most of the rest is made in other nerve endings and reaches the target sites via the circu-

Sunlight

7-Dehydrocholesterol -

Vitamin D3

Other „ metabolites

25-Hydroxylase

-25-Hydroxycholecalciferol (25[OH]-D3)

-25-Hydroxycholecalciferol (25[OH]-D3)

24,25(OH)2-D3 KIDNEY

24,25(OH)2-D3 KIDNEY

7-Dehydrocholesterol

Vitamin D3

Figure 42-9. Formation and hydroxylation of vitamin D3. 25-Hydroxylation takes place in the liver, and the other hydroxylations occur in the kidneys. 25,26(OH)2-D3 and 1,25,26(OH)3-D3 are probably formed as well. The formulas of 7-dehydrocholesterol, vitamin D3, and 1,25(OH)2-D3 are also shown. (Modified and reproduced, with permission, from Ganong WF: Review of Medical Physiology, 20th ed. McGraw-Hill, 2001.)

7-Dehydrocholesterol

Vitamin D3

Figure 42-9. Formation and hydroxylation of vitamin D3. 25-Hydroxylation takes place in the liver, and the other hydroxylations occur in the kidneys. 25,26(OH)2-D3 and 1,25,26(OH)3-D3 are probably formed as well. The formulas of 7-dehydrocholesterol, vitamin D3, and 1,25(OH)2-D3 are also shown. (Modified and reproduced, with permission, from Ganong WF: Review of Medical Physiology, 20th ed. McGraw-Hill, 2001.)

lation. Epinephrine and norepinephrine may be produced and stored in different cells in the adrenal medulla and other chromaffin tissues.

The conversion of tyrosine to epinephrine requires four sequential steps: (1) ring hydroxylation; (2) decarboxylation; (3) side chain hydroxylation to form norepinephrine; and (4) N-methylation to form epinephrine. The biosynthetic pathway and the enzymes involved are illustrated in Figure 42-10.

A. Tyrosine Hydroxylase Is Rate-Limiting for Catecholamine Biosynthesis_

Tyrosine is the immediate precursor of catecholamines, and tyrosine hydroxylase is the rate-limiting enzyme in catecholamine biosynthesis. Tyrosine hydroxylase is found in both soluble and particle-bound forms only in tissues that synthesize catecholamines; it functions as an oxidoreductase, with tetrahydropteridine as a cofactor, to convert L-tyrosine to L-dihydroxyphenylalanine (L-dopa).

As the rate-limiting enzyme, tyrosine hydroxylase is regulated in a variety of ways. The most important mechanism involves feedback inhibition by the catecholamines, which compete with the enzyme for the pteridine cofac-tor. Catecholamines cannot cross the blood-brain barrier; hence, in the brain they must be synthesized locally. In certain central nervous system diseases (eg, Parkinson's disease), there is a local deficiency of dopamine synthesis. L-Dopa, the precursor of dopamine, readily crosses the blood-brain barrier and so is an important agent in the treatment of Parkinson's disease.

B. Dopa Decarboxylase Is Present in All Tissues

This soluble enzyme requires pyridoxal phosphate for the conversion of L-dopa to 3,4-dihydroxyphenylethyl-amine (dopamine). Compounds that resemble L-dopa, such as a-methyldopa, are competitive inhibitors of this reaction. a-Methyldopa is effective in treating some kinds of hypertension.

Tyrosine

Tyrosine

TYROSINE HYDROXYLASE

TYROSINE HYDROXYLASE

Dopa

DOPA

DECARBOXYLASE

Dopa

DOPA

DECARBOXYLASE

the conversion of dopamine to norepinephrine occurs in this organelle.

D. Phenylethanolamine-W-methyltransferase (PNMT) Catalyzes the Production of Epinephrine_

PNMT catalyzes the N-methylation of norepinephrine to form epinephrine in the epinephrine-forming cells of the adrenal medulla. Since PNMT is soluble, it is assumed that norepinephrine-to-epinephrine conversion occurs in the cytoplasm. The synthesis of PNMT is induced by glucocorticoid hormones that reach the medulla via the intra-adrenal portal system. This special system provides for a 100-fold steroid concentration gradient over systemic arterial blood, and this high intra-adrenal concentration appears to be necessary for the induction of PNMT.

DOPAMINE

DOPAMINE

DOPAMINE ß-HYDROXYLASE

NOREPINEPHRINE

PNMT

NOREPINEPHRINE

EPINEPHRINE

H CH

EPINEPHRINE

Figure 42-10. Biosynthesis of catecholamines. (PNMT, phenylethanolamine-W-methyltransferase.)

C. Dopamine P-Hydroxylase (DBH) Catalyzes the Conversion of Dopamine to Norepinephrine

DBH is a monooxygenase and uses ascorbate as an electron donor, copper at the active site, and fUmarate as modulator. DBH is in the particulate fraction of the medullary cells, probably in the secretion granule; thus,

T3 & T4 Illustrate the Diversity in Hormone Synthesis

The formation of triiodothyronine (T3) and tetra-iodothyronine (thyroxine; T4) (see Figure 42-2) illustrates many of the principles of diversity discussed in this chapter. These hormones require a rare element (iodine) for bioactivity; they are synthesized as part of a very large precursor molecule (thyroglobulin); they are stored in an intracellular reservoir (colloid); and there is peripheral conversion of T4 to T3, which is a much more active hormone.

The thyroid hormones T3 and T4 are unique in that iodine (as iodide) is an essential component of both. In most parts of the world, iodine is a scarce component of soil, and for that reason there is little in food. A complex mechanism has evolved to acquire and retain this crucial element and to convert it into a form suitable for incorporation into organic compounds. At the same time, the thyroid must synthesize thyronine from tyro-sine, and this synthesis takes place in thyroglobulin (Figure 42-11).

Thyroglobulin is the precursor of T4 and T3. It is a large iodinated, glycosylated protein with a molecular mass of 660 kDa. Carbohydrate accounts for 8-10% of the weight of thyroglobulin and iodide for about 0.2-1%, depending upon the iodine content in the diet. Thyroglobulin is composed of two large subunits. It contains 115 tyrosine residues, each of which is a potential site of iodination. About 70% of the iodide in thyroglobulin exists in the inactive precursors, monoiodotyrosine (MIT) and diiodotyrosine (DIT), while 30% is in the iodothyronyl residues, T4 and T3. When iodine supplies are sufficient, the T4:T3 ratio is about 7:1. In iodine deficiency, this ratio decreases, as does the DIT:MIT ratio. Thyroglobulin, a large molecule of about 5000 amino acids, provides the confor-

PNMT

FOLLICULAR SPACE WITH COLLOID

FOLLICULAR SPACE WITH COLLOID

Figure 42-11. Model of iodide metabolism in the thyroid follicle. A follicular cell is shown facing the follicular lumen (top) and the extracellular space (at bottom). Iodide enters the thyroid primarily through a transporter (bottom left). Thyroid hormone synthesis occurs in the follicular space through a series of reactions, many of which are peroxidase-mediated. Thyroid hormones, stored in the colloid in the follicular space, are released from thyroglobulin by hydrolysis inside the thyroid cell. (Tgb, thyroglobulin; MIT, monoiodotyrosine; DIT, diiodotyro-sine; T3, triiodothyronine; T4, tetraiodothyronine.) Asterisks indicate steps or processes that are inherited enzyme deficiencies which cause congenital goiter and often result in hypothyroidism.

Figure 42-11. Model of iodide metabolism in the thyroid follicle. A follicular cell is shown facing the follicular lumen (top) and the extracellular space (at bottom). Iodide enters the thyroid primarily through a transporter (bottom left). Thyroid hormone synthesis occurs in the follicular space through a series of reactions, many of which are peroxidase-mediated. Thyroid hormones, stored in the colloid in the follicular space, are released from thyroglobulin by hydrolysis inside the thyroid cell. (Tgb, thyroglobulin; MIT, monoiodotyrosine; DIT, diiodotyro-sine; T3, triiodothyronine; T4, tetraiodothyronine.) Asterisks indicate steps or processes that are inherited enzyme deficiencies which cause congenital goiter and often result in hypothyroidism.

mation required for tyrosyl coupling and iodide organi-fication necessary in the formation of the diaminoacid thyroid hormones. It is synthesized in the basal portion of the cell and moves to the lumen, where it is a storage form of T3 and T4 in the colloid; several weeks' supply of these hormones exist in the normal thyroid. Within minutes after stimulation of the thyroid by TSH, colloid reenters the cell and there is a marked increase of phagolysosome activity. Various acid proteases and peptidases hydrolyze the thyroglobulin into its constituent amino acids, including T4 and T3, which are discharged from the basal portion of the cell (see Figure 42-11). Thyroglobulin is thus a very large prohormone.

Iodide Metabolism Involves Several Discrete Steps

The thyroid is able to concentrate I- against a strong electrochemical gradient. This is an energy-dependent process and is linked to the Na+-K+ ATPase-dependent thyroidal I- transporter. The ratio of iodide in thyroid to iodide in serum (T:S ratio) is a reflection of the activity of this transporter. This activity is primarily controlled by TSH and ranges from 500:1 in animals chronically stimulated with TSH to 5:1 or less in hy-pophysectomized animals (no TSH). The T:S ratio in humans on a normal iodine diet is about 25:1.

The thyroid is the only tissue that can oxidize I- to a higher valence state, an obligatory step in I- organifica-tion and thyroid hormone biosynthesis. This step involves a heme-containing peroxidase and occurs at the luminal surface of the follicular cell. Thyroperoxidase, a tetrameric protein with a molecular mass of 60 kDa, requires hydrogen peroxide as an oxidizing agent. The H2O2 is produced by an NADPH-dependent enzyme resembling cytochrome c reductase. A number of compounds inhibit I- oxidation and therefore its subsequent incorporation into MIT and DIT. The most important of these are the thiourea drugs. They are used as antithyroid drugs because of their ability to inhibit thyroid hormone biosynthesis at this step. Once iodination occurs, the iodine does not readily leave the thyroid. Free tyrosine can be iodinated, but it is not incorporated into proteins since no tRNA recognizes iodinated tyrosine.

The coupling of two DIT molecules to form T4—or of an MIT and DIT to form T3—occurs within the thyroglobulin molecule. A separate coupling enzyme has not been found, and since this is an oxidative process it is assumed that the same thyroperoxidase catalyzes this reaction by stimulating free radical formation of iodotyrosine. This hypothesis is supported by the observation that the same drugs which inhibit I- oxidation also inhibit coupling. The formed thyroid hor mones remain as integral parts of thyroglobulin until the latter is degraded, as described above.

A deiodinase removes I- from the inactive mono-and diiodothyronine molecules in the thyroid. This mechanism provides a substantial amount of the I- used in T3 and T4 biosynthesis. A peripheral deiodinase in target tissues such as pituitary, kidney, and liver selectively removes I- from the 5' position of T4 to make T3 (see Figure 42-2), which is a much more active molecule. In this sense, T4 can be thought of as a prohormone, though it does have some intrinsic activity.

SEVERAL HORMONES ARE MADE FROM LARGER PEPTIDE PRECURSORS

Formation of the critical disulfide bridges in insulin requires that this hormone be first synthesized as part of a larger precursor molecule, proinsulin. This is conceptually similar to the example of the thyroid hormones, which can only be formed in the context of a much larger molecule. Several other hormones are synthesized as parts of large precursor molecules, not because of some special structural requirement but rather as a mechanism for controlling the available amount of the active hormone. PTH and angiotensin II are examples of this type of regulation. Another interesting example is the POMC protein, which can be processed into many different hormones in a tissue-specific manner. These examples are discussed in detail below.

Insulin Is Synthesized as a Preprohormone & Modified Within the p Cell

Insulin has an AB heterodimeric structure with one in-trachain (A6-A11) and two interchain disulfide bridges (A7-B7 and A20-B19) (Figure 42-12). The A and B chains could be synthesized in the laboratory, but attempts at a biochemical synthesis of the mature insulin molecule yielded very poor results. The reason for this became apparent when it was discovered that insulin is synthesized as a preprohormone (molecular weight approximately 11,500), which is the prototype for peptides that are processed from larger precursor molecules. The hydrophobic 23-amino-acid pre-, or leader, sequence directs the molecule into the cisternae of the endoplasmic reticulum and then is removed. This results in the 9000-MW proinsulin molecule, which provides the conformation necessary for the proper and efficient formation of the disulfide bridges. As shown in Figure 42-12, the sequence of proinsulin, starting from the amino terminal, is B chain—connecting (C) peptide—A chain. The proinsulin molecule undergoes a series of site-specific peptide cleavages that result in the formation of equimo-lar amounts of mature insulin and C peptide. These enzymatic cleavages are summarized in Figure 42-12.

Figure 42-12. Structure of human proinsulin. Insulin and C-peptide molecules are connected at two sites by dipeptide links. An initial cleavage by a trypsin-like enzyme (open arrows) followed by several cleavages by a car-boxypeptidase-like enzyme (solid arrows) results in the production of the heterodimeric (AB) insulin molecule (light blue) and the C-peptide.

Figure 42-12. Structure of human proinsulin. Insulin and C-peptide molecules are connected at two sites by dipeptide links. An initial cleavage by a trypsin-like enzyme (open arrows) followed by several cleavages by a car-boxypeptidase-like enzyme (solid arrows) results in the production of the heterodimeric (AB) insulin molecule (light blue) and the C-peptide.

Parathyroid Hormone (PTH) Is Secreted as an 84-Amino-Acid Peptide

The immediate precursor of PTH is proPTH, which differs from the native 84-amino-acid hormone by having a highly basic hexapeptide amino terminal extension. The primary gene product and the immediate precursor for proPTH is the 115-amino-acid preproPTH. This differs from proPTH by having an additional 25-amino-acid amino terminal extension that, in common with the other leader or signal sequences characteristic of secreted proteins, is hydrophobic. The complete structure of preproPTH and the sequences of proPTH and PTH are illustrated in Figure 42-13. PTH1-34 has full biologic activity, and the region 25-34 is primarily responsible for receptor binding.

The biosynthesis of PTH and its subsequent secretion are regulated by the plasma ionized calcium (Ca2+) concentration through a complex process. An acute decrease of Ca2+ results in a marked increase of PTH

mRNA, and this is followed by an increased rate of PTH synthesis and secretion. However, about 80-90% of the proPTH synthesized cannot be accounted for as intact PTH in cells or in the incubation medium of experimental systems. This finding led to the conclusion that most of the proPTH synthesized is quickly degraded. It was later discovered that this rate of degradation decreases when Ca2+ concentrations are low, and it increases when Ca2+ concentrations are high. Very specific fragments of PTH are generated during its prote-olytic digestion (Figure 42-13). A number of proteolytic enzymes, including cathepsins B and D, have been identified in parathyroid tissue. Cathepsin B cleaves PTH into two fragments: PTH^g and PTH37-84. PTH37-84 is not further degraded; however, PTH1-36 is rapidly and progressively cleaved into di-and tripeptides. Most of the proteolysis of PTH occurs within the gland, but a number of studies confirm that PTH, once secreted, is proteolytically degraded in other tissues, especially the liver, by similar mechanisms.

Figure 42-13. Structure of bovine preproparathyroid hormone. Arrows indicate sites cleaved by processing enzymes in the parathyroid gland (1-5) and in the liver after secretion of the hormone (4-5). The biologically active region of the molecule is flanked by sequence not required for activity on target receptors. (Slightly modified and reproduced, with permission, from Habener JF: Recent advances in parathyroid hormone research. Clin Biochem 1981;14:223.)

Figure 42-13. Structure of bovine preproparathyroid hormone. Arrows indicate sites cleaved by processing enzymes in the parathyroid gland (1-5) and in the liver after secretion of the hormone (4-5). The biologically active region of the molecule is flanked by sequence not required for activity on target receptors. (Slightly modified and reproduced, with permission, from Habener JF: Recent advances in parathyroid hormone research. Clin Biochem 1981;14:223.)

Angiotensin II Is Also Synthesized From a Large Precursor

The renin-angiotensin system is involved in the regulation of blood pressure and electrolyte metabolism (through production of aldosterone). The primary hormone involved in these processes is angiotensin II, an octapeptide made from angiotensinogen (Figure 42-14). Angiotensinogen, a large a2-globulin made in liver, is the substrate for renin, an enzyme produced in the juxtaglomerular cells of the renal afferent arteriole. The position of these cells makes them particularly sensitive to blood pressure changes, and many of the physiologic regulators of renin release act through renal baroreceptors. The juxtaglomerular cells are also sensitive to changes of Na+ and Cl- concentration in the renal tubular fluid; therefore, any combination of fac tors that decreases fluid volume (dehydration, decreased blood pressure, fluid or blood loss) or decreases NaCl concentration stimulates renin release. Renal sympathetic nerves that terminate in the juxtaglomerular cells mediate the central nervous system and postural effects on renin release independently of the baroreceptor and salt effects, a mechanism that involves the P-adrenergic receptor. Renin acts upon the substrate angiotensino-gen to produce the decapeptide angiotensin I.

Angiotensin-converting enzyme, a glycoprotein found in lung, endothelial cells, and plasma, removes two carboxyl terminal amino acids from the decapep-tide angiotensin I to form angiotensin II in a step that is not thought to be rate-limiting. Various nonapeptide analogs of angiotensin I and other compounds act as competitive inhibitors of converting enzyme and are used to treat renin-dependent hypertension. These are

Angiotensinogen Asp-Arg-Val-Tyr-Ile-His-Pro-Phe-His-Leu-Leu (~400 more amino acids)

I RENIN I

Angiotensin I Asp-Arg-Val-Tyr-Ile-His-Pro-Phe-His-Leu t

I CONVERTING ENZYM~j~|

ANGIOTENSIN II Asp-Arg-Val-Tyr-Ile-His-Pro-Phe t

I AMINOPEPTIDASE | Angiotensin III Arg-Val-Tyr-Ile-His-Pro-Phe I ANGIOTENSINASES | Degradation products

Figure 42-14. Formation and metabolism of angiotensins. Small arrows indicate cleavage sites.

referred to as angiotensin-converting enzyme (ACE)

inhibitors. Angiotensin II increases blood pressure by causing vasoconstriction of the arteriole and is a very potent vasoactive substance. It inhibits renin release from the juxtaglomerular cells and is a potent stimulator of aldosterone production. This results in Na+ retention, volume expansion, and increased blood pressure.

In some species, angiotensin II is converted to the heptapeptide angiotensin III (Figure 42-14), an equally potent stimulator of aldosterone production. In humans, the plasma level of angiotensin II is four times greater than that of angiotensin III, so most effects are exerted by the octapeptide. Angiotensins II and III are rapidly inactivated by angiotensinases.

Angiotensin II binds to specific adrenal cortex glomerulosa cell receptors. The hormone-receptor interaction does not activate adenylyl cyclase, and cAMP does not appear to mediate the action of this hormone. The actions of angiotensin II, which are to stimulate the conversion of cholesterol to pregnenolone and of corticosterone to 18-hydroxycorticosterone and aldosterone, may involve changes in the concentration of in-tracellular calcium and of phospholipid metabolites by mechanisms similar to those described in Chapter 43.

Complex Processing Generates the Proopiomelanocortin (POMC) Peptide Family

The POMC family consists of peptides that act as hormones (ACTH, LPH, MSH) and others that may serve as neurotransmitters or neuromodulators (endorphins) (see Figure 42-15). POMC is synthesized as a precursor molecule of 285 amino acids and is processed differently in various regions of the pituitary.

The POMC gene is expressed in the anterior and intermediate lobes of the pituitary. The most conserved sequences between species are within the amino terminal fragment, the ACTH region, and the P-endorphin region. POMC or related products are found in several other vertebrate tissues, including the brain, placenta, gastrointestinal tract, reproductive tract, lung, and lymphocytes.

The POMC protein is processed differently in the anterior lobe than in the intermediate lobe. The intermediate lobe of the pituitary is rudimentary in adult humans, but it is active in human fetuses and in pregnant women during late gestation and is also active in many animal species. Processing of the POMC protein in the peripheral tissues (gut, placenta, male reproductive tract) resem-

POMC (1-134)

I\ \ \ \ \ \ \ \ \

\ \ \

\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ 1

ACTH (1 -39)

ß-LPH (42-134)

III 1

1 11

1 1 1

a-MSH CLIP y-LPH ß-Endorphin

a-MSH CLIP y-LPH ß-Endorphin

ß-MSH Y-Endorphin

a-Endorphin (104-117)

Figure 42-15. Products of pro-opiomelanocortin (POMC) cleavage. (MSH, melanocyte-stimulating hormone; CLIP, corticotropin-like intermediate lobe peptide; LPH, lipotropin.)

bles that in the intermediate lobe. There are three basic peptide groups: (1) ACTH, which can give rise to a-MSH and corticotropin-like intermediate lobe peptide (CLIP); (2) P-lipotropin (P-LPH), which can yield Y-LPH, P-MSH, and P-endorphin (and thus a- and Y-endorphins); and (3) a large amino terminal peptide, which generates Y-MSH. The diversity of these products is due to the many dibasic amino acid clusters that are potential cleavage sites for trypsin-like enzymes. Each of the peptides mentioned is preceded by Lys-Arg, Arg-Lys, Arg-Arg, or Lys-Lys residues. After the prehormone segment is cleaved, the next cleavage, in both anterior and intermediate lobes, is between ACTH and P-LPH, resulting in an amino terminal peptide with ACTH and a P-LPH segment (Figure 42-15). ACTH1-39 is subsequently cleaved from the amino terminal peptide, and in the anterior lobe essentially no further cleavages occur. In the intermediate lobe, ACTH1-39 is cleaved into a-MSH (residues 1-13) and CLIP (18-39); P-LPH (42-134) is converted to Y-LPH (42-101) and P-endorphin (104134). P-MSH (84-101) is derived from Y-LPH.

There are extensive additional tissue-specific modifications of these peptides that affect activity. These modifications include phosphorylation, acetylation, glycosylation, and amidation.

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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  • olga
    What are thyroud hormones and catecholamines made from?
    24 days ago

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