Catecholamines conjugation

Urinary catecholamines represent a quantitatively small but diagnostically important component of the catecholamine excretion products. Catecholamines are excreted in the urine as free amines and as glucuronide and sulfate conjugates. As with plasma measurements, total urinary catecholamines (conjugated and unconjugated forms) may be measured by... 

Analysis of D oligomers and catecholamine conjugates isolated from insect cuticle... 

Verlander, M. et al. Some novel approaches to the design and thesis of peptide-catecholamine conjugates in Polymers in Medicine Biomedical and Pharmacological Applications (eds.) Chiellini, E., Guisti, P., Plenum Press, 1983 in press... 

APPLICATION OF THE CONGENER APPROACH TO THE DESIGN AND SYNTHESIS OF PEPTIDE-CATECHOLAMINE CONJUGATES... 

Scheme 4 General Synthetic Route to Peptide Catecholamine Conjugates...

M. Goodman, Some novel approaches to the design and synthesis of peptide-catecholamine conjugates. Biopolymers (1983), in press. 

Conjugations can also be brought about by sulfotransferases (SULTs) and glutathi-one-S-transferases (GSTs), both of which exist in a number of isoenzymic forms. Amines and alcohols are sulfate acceptors and SULTs are important in steroid hormone and catecholamine metabolism and like the UGTs require the sulfate to be activated prior to its incorporation into the target molecule (Figure 6.32). In this case, sulfate is activated at the expense of two molecules of ATP to form the final sulfate carrier PAPS O -phosphoadenosine-S -phosphosulfate). 

Other components of the urine are conjugates with sulfuric acid, glucuronic acid, glycine, and other polar compounds that are synthesized in the liver by biotransformation (see p. 316). In addition, metabolites of many hormones (catecholamines, steroids, serotonin) also appear in the urine and can provide information about hormone production. The proteohormone chorionic gonadotropin (hCG, mass ca. 36 kDa), which is formed at the onset of pregnancy, appears in the urine due to its relatively small size. Evidence of hCG in the urine provides the basis for an immunological pregnancy test. 

Conjugation with glucuronyl residues is of great importance for the metabolic fate of bilirubin (S3), steroids (L5, M2, R8), catecholamines (W17) and other hydrophobic compounds (D8, D9). Neonatal accumulation of bilirubin in man and rats may trigger maturation of UDP-glucuronyltransferase (Bl, B2, T6). Delayed maturation of the enzyme, or its partial or total deficiency, are critical factors in the development of kernicterus (P6). Compared to other species partial deficiency of the... 

The methyl transferases (MTs) catalyze the methyl conjugation of a number of small molecules, such as drugs, hormones, and neurotransmitters, but they are also responsible for the methylation of such macromolecules as proteins, RNA, and DNA. A representative reaction of this type is shown in Figure 4.1. Most of the MTs use S-adenosyl-L-methionine (SAM) as the methyl donor, and this compound is now being used as a dietary supplement for the treatment of various conditions. Methylations typically occur at oxygen, nitrogen, or sulfur atoms on a molecule. For example, catechol-O-methyltransferase (COMT) is responsible for the biotransformation of catecholamine neurotransmitters such as dopamine and norepinephrine. A-methylation is a well established pathway for the metabolism of neurotransmitters, such as conversion of norepinephrine to epinephrine and methylation of nicotinamide and histamine. Possibly the most clinically relevant example of MT activity involves 5-methylation by the enzyme thiopurine me thy Itransf erase (TPMT). Patients who are low or lacking in TPMT (i.e., are polymorphic) are at... 

Most hormones have a half-life in the blood of only a few minutes because they are cleared or metabolized very rapidly. The rapid degradation of hormones allows target cells to respond transiently. Polypeptide hormones are removed from the circulation by serum and cell surface proteases, by endocytosis followed by lysosomal degradation, and by glomerular filtration in the kidney. Steroid hormones are taken up by the liver and metabolized to inactive forms, which are excreted into the bile duct or back into the blood for removal by the kidneys. Catecholamines are metaboli-cally inactivated by O-methylation, by deamination, and by conjugation with sulfate or glucuronic acid. 

Catechol oxidation catalyzed by peroxidases can be used not only for the synthesis of sulfur-substituted catechols but also for the preparation of synthetic compounds related to pheomelanins, which contain benzothiazine units. In fact, the quinone undergoes an extremely easy nucleophilic addition by thiols. For example, treating the neurotransmitter dopamine with cysteine, in the presence of HRP/H2O2, gives rise to 2-S- and 5-5-cysteinyl-catecholamine and a smaller amount of the 2-S,5-S,-di-cysteinyl-catecholamme conjugate [48, 49] (Fig. 6.3e). 

Catechol Amine Biosynthesis and MetaboUsm, Assay of Enzymes of Catecholamines and Catecholamine Metabolites, Estimation of Total (Free + Conjugated), in... 

The primary metabolites of dopamine are homovanillic acid and dihydroxyphenylacetic acid (75%) and norepinephrine (25%). The primary metabolites of epinephrine and norepinephrine are vanilylmandelic acid and 3-methoxy-4-hydroxy-phenethyleneglycol. Catecholamine metabolites and their conjugates are excreted in urine. 

Normetanephrine andmetanephrine are metabolic products of norepinephrine and epinephrine, respectively, and are formed by the action of catechol-0-methyltransferase without deamination. As a result of active neuronal reuptake and deamination of norepinephrine, normetanephrine normally represents <5% of the total norepinephrine excretion products in urine. Metanephrine, however, even with its lower urinary concentration relative to normetanephrine, represents a major excretion product of epinephrine. The metanephrines are excreted in both conjugated and unconjugated forms. Unlike the catecholamines, total metanephrine excretion is not significantly influenced by diet. As a result, the total metanephrines are routinely measured after acid hydrolysis or sulfatase pretreatment. 

Vanillylmandehc Acid (VMAj is a major catecholamine metabolite formed by the actions of catechol-0-methyl-transferase and MAO. It is excreted by the kidney and represents an average of 40% to 50% of the urinary excretion production of norepinephrine and epinephrine. Norepinephrine is the major source of VMA, with metabolism through MHPG as the major pathway. VA4A is not significantly conjugated and therefore is measured without a hydrolysis step. VMA was first isolated and identified in the urine of a patient with a pheochromocytoma, and its analysis is commonly performed to detect the presence of pheochromocytomas and neuroblastomas. 

Puyo AM, Levin GM, Armando I, Barontini MB. Free and conjugated plasma catecholamines in pheochromocytoma patients with and without sustained hypertension. Acta Endocrinol (Copenh) 1986 113 111-7. 

Figure 7.1 Pathways of synthesis and metabolism of catecholamines with enzymes catalyzing various reactions. (1) Tyrosine hydroxylase (2) aromatic amino acid decarboxylase (3) phenylamine-P-hydroxylase (4) phenylethanolamine-A-methyltransferase (5) monoamine oxidase plus aldehyde dehydrogenase (6) catechol-O-methyltransferase (7) conjugating enzymes about 95% phenolsulfo-transferase and 5% phenolglucuronatetransferase (in human). DOPA, dihydroxyphenylalanine DOMA, dihydroxymandelic acid DHPG, dihydroxyphenylglycol DOPAC, dihydroxyphenylacetic acid HVA, homovanillic acid MHPG, methoxyhydroxylphenylglycol VMA, vanilmandelic acid...

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