urea cycle

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cycle

 [si´k'l]
a succession or recurring series of events.
cardiac cycle a complete cardiac movement, or heart beat, including systole, diastole, and the intervening pause.
Cardiac cycle. From Applegate, 2000.
cell cycle the cycle of biochemical and morphological events occurring in a reproducing cell population; it consists of the S phase, occurring toward the end of interphase, in which DNA is synthesized; the G2 phase, a relatively quiescent period; the M phase, consisting of the four phases of mitosis; and the G1 phase of interphase, which lasts until the S phase of the next cycle.
citric acid cycle tricarboxylic acid cycle.
estrous cycle the recurring periods of estrus in adult females of most mammalian species and the correlated changes in the reproductive tract from one period to another.
hair cycle the successive phases of the production and then loss of hair, consisting of anagen, catagen, and telogen.
menstrual cycle see menstrual cycle.
ovarian cycle the sequence of physiologic changes in the ovary involved in ovulation; see also ovulation and reproduction.
reproductive cycle the cycle of physiologic changes in the reproductive organs, from the time of fertilization of the ovum through gestation and childbirth; see also reproduction.
sex cycle (sexual cycle)
1. the physiologic changes that recur regularly in the reproductive organs of nonpregnant female mammals.
2. the period of sexual reproduction in an organism that also reproduces asexually.
tricarboxylic acid cycle the cyclic metabolic mechanism by which the complete oxidation of the acetyl portion of acetyl-coenzyme A is effected; the process is the chief source of mammalian energy, during which carbon chains of sugars, fatty acids, and amino acids are metabolized to yield carbon dioxide, water, and high-energy phosphate bonds. Called also citric acid cycle, Krebs cycle, and TCA cycle.
 Central pathways of metabolism: How the body produces energy from the energy-containing nutrients using the tricarboxylic acid cycle. From Davis and Sherer, 1994.
urea cycle a cyclic series of reactions that produce urea; it is a major route for removal of the ammonia produced in the metabolism of amino acids in the liver and kidney.

u·re·a cy·cle

the sequence of chemical reactions, occurring primarily in the liver, that results in the production of urea; the key reaction is the hydrolysis of l-arginine by arginase to l-ornithine and urea; l-ornithine is then converted to l-citrulline by a carbamoylation reaction, then to l-argininosuccinate by an amination reaction involving l-aspartic acid; finally, a lyase-dependent step generates arginine and fumarate.

urea cycle

n.
The sequence of chemical reactions occurring in the liver that results in the production of urea. The key reaction is the hydrolysis of arginine by arginase to ornithine and urea.

urea cycle

a series of complex enzymatic reactions by which ammonia is detoxified in the liver. In the series of steps for disposing of the ammonia molecule, a waste product of protein metabolism, five enzymatic reactions occur as NH2 radicals are combined with carbon and oxygen atoms from carbon dioxide to form urea, which is excreted. The amino acid arginine is synthesized during the same process. Also called Krebs-Henseleit cycle.

urea cycle

see ORNITHINE CYCLE.

Krebs,

Sir Hans Adolph, German biochemist in England and Nobel laureate, 1900-1981.
Krebs cycle - together with oxidative phosphorylation, the main source of energy in the mammalian body and the end toward which carbohydrate, fat, and protein metabolism are directed. Synonym(s): tricarboxylic acid cycle
Krebs-Henseleit cycle - the sequence of chemical reactions, occurring primarily in the liver, that results in the production of urea. Synonym(s): urea cycle
Krebs-Ringer solution - a modification of Ringer solution.

cycle

a succession or recurring series of events.

cardiac cycle
a complete cardiac movement, or heartbeat, including systole, diastole, and the intervening pause.
The cycle includes eight separate phases: (1) isovolumetric contraction; (2) maximum ejection; (3) reduced ejection; (4) protodiastole (onset of ventricular relaxation); (5) isovolumetric relaxation; (6) rapid flow; (7) diastasis (onset of atrial contraction); (8) atrial systole.
cell cycle
the cycle of biochemical and morphological events occurring in a dividing cell population; it consists of the S phase, occurring toward the end of interphase, in which DNA is synthesized; the G2 phase, for gap 2, the interval between S and M; the M phase, for mitosis, consisting of the four phases of mitosis; and the G1 phase, which lasts from the end of M until the start of S phase of the next cycle. Fully differentiated cells are nondividing and are said to be in G0.
Enlarge picture
Cell cycle. By permission from Booth DM, Small Animal Clinical Pharmacology and Therapeutics, Saunders, 2000
citric acid cycle
estrus cycle
see estrous cycle.
Krebs cycle
ovarian cycle
the sequence of physiological changes in the ovary involved in ovulation. See also ovulation and reproduction.
reproductive cycle
the cycle of physiological changes in the reproductive organs, from the time of fertilization of the ovum through gestation and parturition. See also reproduction.
sex cycle, sexual cycle
1. the physiological changes recurring regularly in the reproductive organs of female mammals when pregnancy does not supervene.
2. the period of sexual reproduction in an organism that also reproduces asexually.
tricarboxylic acid cycle
urea cycle
a cyclic series of reactions that produce urea, a major route for removal of the ammonia produced in the metabolism of amino acids in the liver and kidney. See also urea.

urea

1. the diamide of carbonic acid found in urine, blood and lymph, the chief nitrogenous constituent of urine, and the chief nitrogenous end product of protein metabolism; it is formed in the liver from amino acids and from ammonia compounds.
2. a pharmaceutical preparation of urea occasionally used to lower intracranial pressure.
3. industrial urea is used as a fertilizer and feed additive for ruminants. Overfeeding or accidental access to large amounts can cause fatal poisoning.

urea cycle
see urea cycle.
urea cycle enzyme deficiency
urea hydrogen peroxide
see carbamide peroxide.
urea nitrogen
the urea concentration of serum or plasma, conventionally specified in terms of nitrogen content and called blood urea nitrogen (BUN), an important indicator of renal function.
urea poisoning
causes tremor, dyspnea, abdominal pain, incoordination, bellowing, convulsions and death in 2 to 4 hours. Due to hyperammonemia.
References in periodicals archive ?
Section Titles: Amino Acid Catabolism in Cheese; Ecology of Cheese; Autolysis; Mechanisms of Proteolysis; Accelerated Cheese Ripening; Cheese Texture Development and Functionality; Low Fat Cheese; Inclusion of Whey Proteins and Membrane Technology; Cheese Analogues; Heat Treatment