Jumat, 19 Oktober 2012



Anthocyanin biosynthesis (maize and Arabidopsis genes)

In maize there are a number of regulatory genes that mediate transcriptional activation of the anthocyanin biosynthesis pathway genes (see Selinger and Chandler (1999) for a discussion of these regulatory loci). For example, R has been shown to regulate 3 enzymes involved in anthocyanin biosynthesis; chalcone synthase [EC 2.3.1.74] encoded by C2, dihydroflavonol 4-reductase (dihydroquercitin reductase) (DFR) [EC 1.1.1.219] encoded by A1, and flavonol 3-O-glucosyltransferase (UFGT or 3GT) [EC 2.4.1.91] encoded by bronze1 (Bz1). Regulation of the anthocyanin pathway in maize requires two classes of transcription factors; one class (B and R) contains a bHLH motif, and the other (C1 and P1) contains a Myb domain. To activate the genes of the anthocyanin pathway, a protein from each class must be expressed; neither alone is sufficient for induction (Lesnick and Chandler, 1998; and references cited therein). Cell lines of maize engineered to express the C1 and R accumulate two cyanidin derivatives that are similar to the predominant anthocyanin found in differentiated tissues. In contrast, expression of P causes accumulation of 3-deoxy flavonoids (Grotewold et al, 1998).
In Arabidopsis dihydroflavonol 4-reductase (DFR) [EC 1.1.1.219] is encoded by the tt3 locus. Flavonol synthase (FLS) may be encoded by tt6, chalcone synthase (CHS) [EC 2.3.1.74] by tt4, and flavonoid 3'-hydroxylase (F3'H) [EC 1.14.13.21] by tt7 (where tt refers to transparent testa mutants) (Pelletier et al, 1997).
The A2 locus in maize may encode a dioxygenase; thus anthocyanidin synthase (ANS) may actually represent a leucoanthocyanidin dioxygenase (LDOX). The LDOX of Arabidopsis responsible for converting leucopelargonidin to pelargonidin and leucocyanidin to cyanidin, has recently been cloned (Pelletier et al, 1997). UDP-flavonol 3-O-glucosyltransferase (UFGT or 3GT) [EC 2.4.1.91] would then further elaborate pelargonidin and cyanidin to pelargonidin-3-glucoside and cyanidin-3-glucoside, respectively.
The Bronze2 (Bz2) gene in maize encodes a glutathione S-transferase [EC 2.5.1.18] that performs the last genetically defined step in anthocyanin biosynthesis -- tagging cyanidin-3-glucoside with glutathione, allowing for transport to the vacuole via a tonoplast Mg-ATP-requiring GS-X pump (Marrs and Walbot, 1997; Lu et al, 1998; Alfenito et al, 1998). The equivalent locus to Bz2 in petunia is Anthocyanin9 (An9), although the maize Bz2 gene encodes a type III GST while the petunia An9 locus encodes a type I GST (Alfenito et al, 1998).
The 3-deoxyanthocyanidin phytoalexins (apigeninidin and leuteolinidin) accumulated by Sorghum bicolor in response to inoculation with the fungus Cochliobolus heterostrophus, are thought to be derived from naringenin. Fungal inoculation leads to repression of the transcription of the genes encoding F3H, DFR and ANS, leading to inhibition of light-induced accumulation of anthocyanin. In contrast, PAL and CHS are induced. This presumably diverts metabolic flux away from anthocyanin synthesis towards naringenin and 3-deoxyanthocyanidin synthesis to meet immediate biochemical needs for plant defense (Lo and Nicholson, 1998).
The flavan 3,4-diols produced by the action of DFR are not only the precursors of anthocyanins (whose synthesis is mediated by ANS and 3GT), but also catechins and condensed tannins, synthesized by flavan 3,4-diol reductase (FDR) and condensing and polymerizing enzymes. Consequently, antisense-DFR plants of birdsfoot trefoil exhibit reduced condensed tannin levels (Robbins et al, 1998).

Jumat, 12 Oktober 2012

Nikotin

Nicotine is an alkaloid found in the nightshade family of plants (Solanaceae) that acts as a nicotinic acetylcholine agonist and a monoamine oxidase inhibitor[citation needed]. The biosynthesis takes place in the roots and accumulation occurs in the leaves of the Solanaceae. It constitutes approximately 0.6–3.0% of the dry weight of tobacco and is present in the range of 2–7 µg/kg of various edible plants. It functions as an antiherbivore chemical; therefore, nicotine was widely used as an insecticide in the past and nicotine analogs such as imidacloprid are currently widely used.
Nicotine is a hygroscopic, oily liquid that is miscible with water in its base form. As a nitrogenous base, nicotine forms salts with acids that are usually solid and water soluble. Nicotine easily penetrates the skin. As shown by the physical data, free base nicotine will burn at a temperature below its boiling point, and its vapors will combust at 308 K (35 °C; 95 °F) in air despite a low vapor pressure. Because of this, most of the nicotine is burned when a cigarette is smoked; however, enough is inhaled to cause pharmacological effects.
Optical activity
Nicotine is optically active, having two enantiomeric forms. The naturally occurring form of nicotine is levorotatory with a specific rotation of [α]D = –166.4° ((−)-nicotine). The dextrorotatory form, (+)-nicotine is physiologically less active than (–)-nicotine. (−)-nicotine is more toxic than (+)-nicotine.[17] The salts of (+)-nicotine are usually dextrorotatory.
Biosynthesis

Pharmacokinetics
As nicotine enters the body, it is distributed quickly through the bloodstream and crosses the blood–brain barrier reaching the brain within 10–20 seconds after inhalation. The elimination half-life of nicotine in the body is around two hours.
The amount of nicotine absorbed by the body from smoking depends on many factors, including the types of tobacco, whether the smoke is inhaled, and whether a filter is used. For chewing tobacco, dipping tobacco, snus and snuff, which are held in the mouth between the lip and gum, or taken in the nose, the amount released into the body tends to be much greater than smoked tobacco.[clarification needed][citation needed] Nicotine is metabolized in the liver by cytochrome P450 enzymes (mostly CYP2A6, and also by CYP2B6). A major metabolite is cotinine.
Other primary metabolites include nicotine N'-oxide, nornicotine, nicotine isomethonium ion, 2-hydroxynicotine and nicotine glucuronide. Under some conditions, other substances may be formed such as myosmine.
Glucuronidation and oxidative metabolism of nicotine to cotinine are both inhibited by menthol, an additive to mentholated cigarettes, thus increasing the half-life of nicotine in vivo.

In the central nervous system
By binding to nicotinic acetylcholine receptors, nicotine increases the levels of several neurotransmitters – acting as a sort of "volume control". It is thought that increased levels of dopamine in the reward circuits of the brain are responsible for the apparent euphoria and relaxation, and addiction caused by nicotine consumption. Nicotine has a higher affinity for acetylcholine receptors in the brain than those in skeletal muscle, though at toxic doses it can induce contractions and respiratory paralysis.[30] Nicotine's selectivity is thought to be due to a particular amino acid difference on these receptor subtypes.
Tobacco smoke contains anabasine, anatabine, and nornicotine.[citation needed] It also contains the monoamine oxidase inhibitors harman and norharman.These beta-carboline compounds significantly decrease MAO activity in smokers.MAO enzymes break down monoaminergic neurotransmitters such as dopamine, norepinephrine, and serotonin. It is thought that the powerful interaction between the MAOI's and the nicotine is responsible for most of the addictive properties of tobacco smoking.The addition of five minor tobacco alkaloids increases nicotine-induced hyperactivity, sensitization and intravenous self-administration in rats.
Chronic nicotine exposure via tobacco smoking up-regulates alpha4beta2* nAChR in cerebellum and brainstem regions but not habenulopeduncular structures.Alpha4beta2 and alpha6beta2 receptors, present in the ventral tegmental area, play a crucial role in mediating the reinforcement effects of nicotine.
In the sympathetic nervous system
Nicotine also activates the sympathetic nervous system,[40] acting via splanchnic nerves to the adrenal medulla, stimulates the release of epinephrine. Acetylcholine released by preganglionic sympathetic fibers of these nerves acts on nicotinic acetylcholine receptors, causing the release of epinephrine (and norepinephrine) into the bloodstream. Nicotine also has an affinity for melanin-containing tissues due to its precursor function in melanin synthesis or due to the irreversible binding of melanin and nicotine. This has been suggested to underlie the increased nicotine dependence and lower smoking cessation rates in darker pigmented individuals. However, further research is warranted before a definite conclusive link can be inferred.
Effect of nicotine on chromaffin cells.

In adrenal medulla
By binding to ganglion type nicotinic receptors in the adrenal medulla nicotine increases flow of adrenaline (epinephrine), a stimulating hormone and neurotransmitter. By binding to the receptors, it causes cell depolarization and an influx of calcium through voltage-gated calcium channels. Calcium triggers the exocytosis of chromaffin granules and thus the release of epinephrine (and norepinephrine) into the bloodstream. The release of epinephrine (adrenaline) causes an increase in heart rate, blood pressure and respiration, as well as higher blood glucose levels.
Nicotine is the natural product of tobacco, having a half-life of 1 to 2 hours. Cotinine is an active metabolite of nicotine that remains in the blood for 18 to 20 hours, making it easier to analyze due to its longer half-life.
Sumber: en.wikipedia.org
The Metabolism of Nicotine 
1 - Absorption of nicotine
Absorption of nicotine through cellular membranes depends on the pH. If the pH is acidic, nicotine is ionized and does not easily pass through membranes. At physiologic pH (pH = 7.4), 31% of nicotine is not ionized and easily passes through membranes.
The tobacco smoke pH is acidic, and this acidity only allows a little absorption in the mouth. Inhalation is therefore necessary to allow nicotine to be absorbed by the huge area of alveolar epithelium. In the lungs, nicotine is quicklyt absorbed by the systemic circulation. This absorption is easy because the blood flow is high in the lung capillaries : a volume equal to the blood volume of the body passes each minute. So, the rate of nicotine qickly rises when a cigarette is smoked. Absorbed nicotine is rapidly distributed among all the organs, and it reaches the brain within only  ten seconds.  
2 - Action on nicotinic receptors
The active form of nicotine is a cation whose charge is located on the nitrogen of the pyrrole cycle. This active form is very close to acetylcholine. It has been demonstrated that nicotine interferes with acetylcholine, which is the major neurotransmitter of the brain. Acetylcholine can bind to two different kinds of receptors: nicotinic receptors, which are activated by nicotine, and muscarinic receptors, which are activated by muscarine. Nicotine and muscarine are thus specific agonists of one kind of cholinergic receptors (an agonist is a molecule that activates a receptor by reproducing the effect of the neurotransmitter.)
Nicotine competitively binds to nicotinic cholinergic receptors. The binding of the agonist to the nicotinic receptor triggers off a conformation change of the architecture of the receptor, which opens the ionic channel during a few milliseconds. This channel is selective for cations (especially sodium). Its opening thus leads to a brief depolarization. Then, the channel closes and the receptor transitionally becomes refractory to agonists. This is the state of  desensitization. Then, the receptor  usually goes back to a state of rest, which means that it is closed and sensitive to the agonists. In case of continuous exposure to agonists (even in small doses), this state of desensitization will last long (long-term inactivation).
Operating cycle of a nicotinic receptor:  
Physiological normal conditions: After the opening of the canal by binding to acethylcholine, the receptor becomes desensitized before it goes back to the state of rest or it is regenerated.
Continuous exposure to tobacco: Nicotine substitutes for acetylcholine and over stimulates the nicotinic receptor. Then, the receptor is long-term inactivated and its regeneration is prevented by nicotine.
3 - Tolerance and dependence on nicotine
Nicotine rises the stimulation of nicotinic receptors. The excessive and chronic activation of these receptors is balanced by a down-regulation in the number of active receptors. The reduction of the number of active receptors reduces the psychotropic effect of nicotine. Due to the phenomenon of tolerance, the smoker needs to smoke more and more cigarettes to keep a constant effect.
Nicotine activates dopamine systems within the brain. Dopamine is a neurotransmitter which is directly responsible for mediating the pleasure response. Nicotine triggers off the production of dopamine in the nucleus accumbens. A prolonged exposure of these receptors to nicotine reduces the efficiency of dopamine by cutting down the number of available receptors. Consequently, more and more nicotine is needed to give the same pleasurable effect.
After a brief period of abstinence (overnight for instance), the brain concentration of nicotine lowers and allows a part of the receptors to recover their sensibility. The return to an active state rises the neurotransmission to an abnormal rate. The smoker feels uncomfortable, which induces him to smoke again. The first cigarette of the day is the most pleasant because the sensibility of the dopamine receptors is maximal. Then, the receptors are soon desensitized and the pleasure wears off. This is the vicious circle of smoking.
  4 - Chemical transformations undergone by nicotine
Nicotine is mainly transformed in the liver, but also in the lungs and the kidneys. The primary metabolites of nicotine are cotinine and  nicotine N-oxide, which are some products of the hepatic oxidation of nicotine by  P-450 cytochrome.
How can nicotine be dangerous for the body?
Nicotine and its metabolites may be dangerous for the body. Actually, nicotine is a strong carcinogen. In fact, nicotine can undergo several kinds of transformation like a pyrrole cycle opening. The methyl group on this cycle can become a very powerful alkylating agent when removed from the cycle.
The amine function of nicotine may react with nitrogen monoxide or with nitrous acid in order to form a "nitrosonium" type molecule. This compound may then be transformed by the body, which means oxidized and opened. This opening leads to two isomers, two "nitrosamino" type molecules (R2N-N=O) where one of the two R group is a methyl. This reaction occurs as follows:
A = 4 (N-methyl-N-nitrosamino)-1-(3-pyridyl)-butan-1-one          B = 4 (N-methyl-N-nitrosamino)-4-(3-pyridyl)-butanal
In acidic medium, the oxygen of the "nitrosamino" group is protonated and the double bond moves to the central nitrogen, which becomes positively charged. This new molecule is a methyl source. The "nitrosamino" group can then react with another amine, which removes the positive charge from the nitrogen. If the amine that reacts is a part of the structure of the DNA, an irreversible alkylation of the DNA occurs:
This alkylation is really  noxious and may help in the development of cancer as it prevents the normal development of the cell.
Sumber : http://www.chm.bris.ac.uk