Biochemistry
2nd stageDr.Lamees Majid Al-Janabi
Other Nitrogen-Containing Compounds
A. CatecholaminesDopamine, norepinephrine, and epinephrine are biologically active (biogenic) amines that are collectively termed catecholamines. Dopamine and norepinephrine function as neurotransmitters in the brain and the autonomic nervous system. Norepinephrine and epinephrine are also synthesized in the adrenal medulla. The catecholamines are synthesized from tyrosine.
Metabolism of the catecholamines by catechol-O-methyltranferase (COMT) and monoamine oxidase (MAO).
The metabolic products of these reactions are excreted in the urine as vanillylmandelic acid from epinephrine and norepinephrine, and homovanillic acid from dopamine.
B. Histamine
Histamine is a chemical messenger that mediates a wide range of cellular responses, including allergic and inflammatory reactions, gastric acid secretion, and possibly neurotransmission in parts of the brain. A powerful vasodilator, histamine is formed by decarboxylation of histidine in a reaction requiring pyridoxal phosphate .It is secreted by mast cells as a result of allergic reactions or trauma. Histamine has no clinical applications, but agents that interfere with the action of histamine have important therapeutic applications.
C. Serotonin
Serotonin, also called 5-hydroxytryptamine, is synthesized and stored at several sites in the body . By far the largest amount of serotonin is found in cells of the intestinal mucosa. Smaller amounts occur in the central nervous system, where it functions as a neurotransmitter, and in platelets.Serotonin is synthesized from tryptophan, which is hydroxylated in a reaction analogous to that catalyzed by phenylalanine hydroxylase. The product, 5-hydroxytryptophan, is decarboxylated to serotonin, which is also degraded by MAO.
Serotonin has multiple physiologic roles, including pain perception, affective disorders, and regulation of sleep, temperature, and blood pressure.
D. Creatine
Creatine phosphate (also called phosphocreatine), the phosphorylated derivative of creatine found in muscle, is a high-energy compound that can reversibly donate a phosphate group to adenosine diphosphate to form ATP . Creatine phosphate provides a small but rapidly mobilized reserve of high-energy phosphates that can be used to maintain the intracellular level of adenosine triphosphate (ATP) during the first few minutes of intense muscular contraction.[Note: The amount of creatine phosphate in the body is proportional to the muscle mass.]
Synthesis: Creatine is synthesized from glycine and the guanidino group of arginine, plus a methyl group from S-adenosylmethionine . Creatine is reversibly phosphorylated to creatine phosphate by creatine kinase, using ATP as the phosphate donor.
[Note: The presence of creatine kinase in the plasma is indicative of tissue damage, and is used in the diagnosis of myocardial infarction .]
Degradation: Creatine and creatine phosphate spontaneously cyclize at a slow but constant rate to form creatinine, which is excreted in the urine.
The amount of creatinine excreted is proportional to the total creatine phosphate content of the body, and thus can be used to estimate muscle mass. When muscle mass decreases for any reason (for example, from paralysis or muscular dystrophy), the creatinine content of the urine falls.
In addition, any rise in blood creatinine is a sensitive indicator of kidney malfunction, because creatinine normally is rapidly removed from the blood and excreted. A typical adult male excretes about 15 mmol of creatinine per day.
