COX Isoforms Are Bifunctional Enzymes Biology Essay

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Both COX isoforms are bifunctional enzymes, having two distinct catalytic activities: the first of them, dioxygenase step incorporates two molecule of oxygen into the arachidonic chain at C-11 and C-15 giving rise to the highly unstable endoperoxides intermediate PGG2 with a hydroperoxidase group at C-15.Second peroxidase function of the enzyme converts this to PGH2 with a hydroxyl group at C-15 which can transformed in a cell specific manner by separate isomerase, reductase, or synthase enzymes into other prostanoids. Both Cox-1 and COX-2 are hemi-containing enzymes which probably exist as homodimers attached to intracellular membranes.

Aspirin, Ibuprofen, celecoxib, fenoprofen and paracetamol etc. all the NSAIDs have actions very similar to aspirin. But aspirin keep out to reach of children which are old less than 12 years. Therefore, Ibuprofen is suitable for children. The anti inflammatory properties of aspirin can contribute to its antithrombotic effects. The greatest prevention benefit for aspirin vs. placebo was observed in patients who were satisfied to the highest quartile of C-reactive protein. Other potential antithrombotic effects of aspirin have been documented; all of the current techniques used to measure aspirin's effect are directed toward blockade of COX-1.Main symptoms of aspirin include headache, tinnitus, GI distress and respiratory stimulation (i.e., increased respiratory rate secondary to stimulation in the central nervous system .Patients may occur appear drowsy or confused. General adverse effects related to aspirin are the result of prostaglandin inhibition. Prostaglandins are found in most body tissues and organs and have opposing effects on the body. Aspirin may cause hypersensitivity reactions such as rashes, hives or bronchoconstriction and respiratory distress and anaphylaxis. This analgesic, antipyretic drug aspirin has no anti-inflammatory action. Its mechanism of action remains fully elucidated. It is inhibitor of COX, particularly COX-2 (30% maximal inhibition). There does not appear to be any difference in the anti-pyretic efficacy between NSAID and paracetamol.

Non-Steroidal Anti-Inflammatory Drugs

1.0. INTRODUCTION:-

The approach of drug discovery commences with the collection of natural material from their community. This is consisting of leaves, shoot, bark and roots of plants such as non-steroidal anti-inflammatory drug. Non-steroidal drug name derived from the class of compounds created to act as an anti-inflammatory and painkiller. Steroid is form of compound which may occurs in two forms by natural and as well as synthesis (Duarmuid Jeffreys, 2004). Rick Ng (2004) describe that Steroid is derived chemically as substances which consist of 17 carbons atoms arrange in series of four rings .. NSAIDs are provided for pain relief rheumatoid arthritis and also in most of acute inflammatory conditions such as fractures, injuries soft tissues injuries. They are suitable for the postoperative, dental and menstrual pain, from the pain of headaches and migraines. Most of NSAIDS has taken without the prescription for other types of aches and pains. (H.P Rang and M.M Dale's, J.M Ritter, R.J. Flower, pg. 226).

Sostres C, Gargalo C J, Arroyo MT, Lanas A (2010) argued that the main benefit derives from the anti-inflammatory and analgesic effect, but the use of these agents is not innocuous since they mainly increase the risk of gastrointestinal (GI) and cardiovascular complications compared with non-NSAID users.

The well-known of the NSAID class of medications is aspirin, or acetylsalicylic acid, first synthesized in 1893. Aspirin became from bark of a tree. Dr .A.M. Helmenstine expressed that Aspirin is the widely used over-the-counter drug in the world. Aspirin was used to reduce inflammation, pain relieve swelling muscle and lower fever. Salicylic acid is neutralised with sodium to produce sodium salicylate which is irritating for stomach. Salicylic acid could be modified to produce phenylsalicylate, for better tasting and less irritating, but it released the toxic substance phenol when metabolized.

According to Charles L. Millwood the history of aspirin according to medically, aspirin was born in 1897. Hippocrates Kos prescribed a juice extracted from the bark of the willow tree for pain and fever. This juice contained the active substance which removed the pain, it was salicylic acid. Its name gives a clue as to its origin, being derived from the Latin word willow: Salix. In the 19th century, Felix Hoffmann synthesized salicylic acid. In 1953, Lawrence craven published his manuscript suggesting that aspirin therapy reduced myocardial infarction. Basically, no one accepted the results of this study. In 1992, "international study of infract survival" conformed the cardioprotective effects of aspirin. So, the main usage of aspirin is a cardiovascular usage. Mostly aspirin is given to heart failure patient.

2.0. SIGNALLING PATHWAYS OF NSAIDs:-

David E. Golan (pg.275) stated that NSAIDs affect pain pathways in at least three different ways. In first pathway, prostaglandins reduce the activation threshold at the peripheral terminals of primary afferent nociceptor neurons. During the reduction of prostaglandin synthesis, NSAIDs decrease inflammatory hyperalgesia and allodynia. In second pathway, NSAIDs decrease the recruitment of leukocytes and production of leukocyte derived inflammatory mediators. In third pathway, NSAIDs that cross the blood brain barrier prevent the generation of prostaglandins that act as pain producing neuromodulators in the spinal cord dorsal horn. NSAIDs and COX-2 inhibitors act both peripherally and centrally. According to preclinical data the acute action of NSAIDs is peripheral; much of their analgesic effect derives from their central action to prevent a PGE-2 induced reduction in glycinergic inhibition. Non selective COX inhibiting NSAIDs have some deleterious side effects, particularly injury to the gastric mucosa and the kidneys.

NSAIDs differ in the extent in which they inhibit COX-1 and COX-2.This is expressed as the COX-2: COX-1 selectivity ratio, which shows significant variation according to the source enzyme or type of cells used for the assays. It is also not clear how these ratios translate into clinical practice. The older traditional NSAIDs inhibit COX-1 and COX-2 to a similar degree. But in contrast, etodolac and meloxicam inhibit COX-2 up to 50 times more than COX-1.)

2.1. MECHANISM OF ACTION -NSAIDS

The mechanism of action of NSAIDs is the inhibition of the enzyme cyclooxygenase (COX).COX converts the fatty acid arachidonic acid into endoperoxides, prostaglandins and thromboxane in a cell specific manner. These prostanoids have different physiological functions like protection of the gastric testinal tract, renal homeostasis, platelet aggregation, contraction of uterine smooth muscle and are widely implicated in pathological states associated with inflammation. There are two isoforms of COX which are COX-1 and COX-2. COX-1 function as a physiological enzyme producing the prostaglandins critical for maintaining normal renal function, gastric mucosal integrity and haemostasis.COX-2 is undetectable in most tissues under physiological conditions. NSAIDs act by direct inhibition of COX-1 and COX-2 by blockade of the COX enzyme site. The inhibition of prostaglandins reduces inflammation but result in extra activities on platelet aggregation, renal homeostasis and gastric mucosal integrity. NSAIDs interfere with a variety of other processes which may contribute to their effects. These include leukotriene synthesis, superoxide generation, lysosomal enzyme release, neutrophil function, and lymphocyte function and cartilage metabolism. (Roger Walker and Cate Whittlesea, pg.763).

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Devillier P argued in his article that NSAID have a common effect, inhibiting the production of prostanoids via reduced activity of two cyclo-oxygenases (COX-1 and COX-2). COX-2 is an isoform predominantly expressed during the inflammatory process. Excepting two compounds recently marketed (celecoxib, rofecoxib) selective for COX-2, all other NSAID have few or no selective properties. COX-1 is implicated in the regulation of many physiological functions. Inhibition of COX-1 explains most of the classical side effects of non-selective NSAID.

According to Stefan Laufer, Steffen Gay and Kay Brune (2003,pg.70), NSAIDs act by inhibiting the synthesis of prostaglandins and thromboxane, a group of lipid mediators formed from arachidonic acid. Arachidonic acid is released by phopholipase A2 from cell membrane phospholipids and is converted by prostaglandin -H2-(PGH2) synthase to prostaglandins.PGH2 synthase is a membrane bound enzyme that occurs ubiquitously in the human body. For the sake of simplicity PGH2 synthase is equated with cyclooxygenase. It is bifunctional enzyme combining the activity of a cyclooxygenase (COX) and that of a hydroperoxidase (HOX).In the first enzymatic step prostaglandin G2 which is immediately converted to prostaglandin H2. The main inflammatory mediator formed from arachidonic acid via the COX metabolic pathways prostaglandin E2 (PGE2).A second enzyme, 5-lipoxygenase, and converts arachidonic acid to leukotriene and HPETEs (hydroperoxyeicosatetraenoic acids).

3.0. ASPIRIN:-

the most common NSAIDs is aspirin. H P Rang (2007) suggested that aspirin (acetylsalicylic acid) was among the earlier drugs synthesised, and is still one of the most commonly consumed drugs worldwide. It is relatively insoluble, but is sodium and calcium salts are readily soluble. According to R.S.Satoskar, S.D.Bhandarkar & Nirmala N.Rege (2009) aspirin introduced as an analgesic -antipyretic in 1899.It is still an important drug in the therapy of pain and inflammation. Salicylates, unlike the opioid analgesics produce relief of pain without hypnosis or impairment of mental activity. Their analgesic actions are predominantly peripheral but a central component cannot be ruled out. Therapeutically it may be rational to combine aspirin with opioid analgesic like codeine for a synergistic effect.

3.1. MECHANISM OF ASPIRIN:-

Acetylsalicylic acid (aspirin) works by covalently acetylating the cyclooxygenase active site in both COX-1 and COX-2. Aspirin is rapidly absorbed and distributed throughout the body. Chronic aspirin use can produce gastric irritation and erosion, haemorrhage; vomiting and renal tubular necrosis. Gregory Makowski (Vol.42, pg.83) described that the mechanism of aspirin inhibition of platelet aggregation is via inhibition of COX-1.Aspirin covalently acetylates serine at position 530 resulting in permanent enzyme inhibition. In platelets, inhibition of COX-1 blocks the metabolism of AA to thromboxane A2 (TXA2)-a potent platelet agonist and vasoconstrictor. Aspirin produces a mild inhibition of platelet function believed to reduce the rate of acclusive atherogenic events, Aspirin has other antithrombotic effects that may contribute to its utility as an agent that decrease the rate of atherogenic. Aspirin acetylates fibrinogen which is less effective in supporting adenosine diphosphate (ADP)-stimulated platelet aggregation as measured by light aggregometer. Acetylated fibrinogen is more easily lysed by tissue plasminogen activator. Inflammatory endothelial dysfunction produced in healthy volunteers by salmonella typhi vaccination is prevented by aspirin.

According to Charles L. Millwood (2007, pg.155) as a result of activation by various stimulators (physical, inflammatory, hormonal, etc), phospholipases synthesizes arachidonic acid from phospholipids of the cellular membrane. Prostaglandin synthase has cyclooxygenase (COX) and hydroperoxidase (HOX) activity produces prostaglandin H2 from arachidonic acid. The most important product of COX which led platelets to aggregate is thromboxane A2 (TxA2). Platelets are enucleated cells and can not regenerate COX once inhibited. The recovery of TxA2 biosynthesis and normal platelet function depend on new platelet formation. Aspirin's antiplatelet effect continues for the life of the platelet. This level of platelet inhibition could be achieved with 80 to 160 mg aspirin therapy once per day. Aspirin deactivates especially the COX-1 Enzyme. The second Isoforms of COX (COX-2) could be found in monocytes, macrophages and newly formed platelets. At anti-platelet doses aspirin inhibits COX-1 up to 170 fold more than COX-2.To achieves a complete inhibition of COX-2 pathway there is need for higher aspirin doses.

Aspirin is effective in the secondary prevention of important vascular events; the effectiveness of traditional NSAIDs in this respect is unknown. Prospective, controlled trials have been limited, and initial case control analysis suggests that NSAIDs do not reduce the risk of a first myocardial infarction. NSAIDs, unlike aspirin bind reversibly at the active site of the enzyme; usually depressing platelet function is impaired for only a portion of the dosing interval. (www.nejm.org)

3.2. EFFECTS OF ASPIRIN:-

Aspirin, being a weak acid, is protonated in the acid environment of the stomach, thus facilitating its passages across the mucosa. Most absorption, however, occurs in the ileum, because of extensive surface area of the microvilli. Aspirin is rapidly (probably 30 minutes) hydrolysed by esterase in the plasma and the tissues practically the tissues - particularly the liver yielding salicylate. This compound itself has anti inflammatory action. (Shilyn B. Mckenzie, 1996, PG.624-625)

In addition to its anti -inflammatory action, aspirin inhibits platelet aggregation, and its main clinical importance now is in the therapy of myocardial infarction now is in the therapy of myocardial infarction.

It is given orally and is rapidly absorbed: 75% is metabolised in liver

Elimination follows first order kinetics with high doses (half - life over 15 hours).

3.3. ADVERSE EFFECTS OF ASPIRIN:-

According to Shilyn B. Mckenzie (1996,PG.624-625) to be a true reflection of in vivo platelet function, the patient should avoid the use of aspirin - like drugs for 7 days before the bleeding time since the ingestion of aspirin and many aspirin - like drugs will result in a prolonged bleeding time. Likewise a platelet count should be performed before the bleeding time since a platelet count less than 100.0 * 10 9/ L will result in a prolonged bleeding time. If the above conditions are met, a prolonged bleeding time will indicate platelet dysfunction. Possible causes of platelet dysfunctions, von willebrand disease, afibrinogenemia, severe hypofibrinogenemia, and some vascular bleeding disorders, and some vascular bleeding disorders.

Salicylates may produce both local and systemic toxic effects. Aspirin shares many of the general unwanted effects of NSAIDs outline above In addition there are certain specific unwanted effects that occur with aspirin and other Salicylates.

Salicylism, characterised by tinnitus, vertigo, decreased hearing, and sometimes also nausea and vomiting, occurs with over dosage of any salicylate.

Reye's syndrome a rare disorder of children that is characterised by hepatic encephalopathy following an acute viral illness and 20 - 40 % mortality. Since the withdrawal of the aspirin for paediatric use in the UK, incidence of Reye's syndrome has fallen dramatically.

Diane S. Aschenbrenner, Samantha J.Venable (2008, pg.405) argued that there are two adverse effects specific to aspirin therapy are salicylism and salicylate poisoning. Salicylism is mild aspirin toxicity, which may occur with long term or high dose Aspirin therapy.

3.4. SYSTEM ACTIONS:-

ANALGESIA

Salicylates relieve pain of mild to moderate intensity such as headache, muscular pain, toothache, joint pain and dysmenorrhoea. They do not relieve visceral pain except menstrual pain. They relieve pain by inhibiting prostaglandin synthesis and raising the threshold to pain reception. Other sensations, like touch are not effected. They don't produce sedation, tolerance and drug abuse.

ANTIPYRETIC

Salicylate reduce the body temperature by inhibiting brain PGE synthesis in febrile patients but don't lower the normal body temperature or the temperature increase due to exercise or heat, over normal body temperature. They act by resetting the hypothalamic body thermostat for normal temperature and also by more heat loss by sweating and cutaneous vasodilatation. They don't decrease heat production.

RESPIRATION

Salicylates increase the rate and depth of respiration by direct and indirect action. They enhance the consumption of oxygen by the skeletal muscles and this leads to increased production of CO2; a high level of CO2 stimulate respiration. They increase respiration rare by direct action on the medulla also. This lead to respiratory alkalosis, but after high doses or prolonged exposure, the medulla is depressed; there is decreased rate and depth of respiration leading to respiratory acidosis.

GIT

Salicylate may cause nausea and vomiting due to gastric irritation and stimulation of chemore captor trigger Zone (CTZ). Locally these reduce Cox-L leading to decrease PGE2 synthesis, causing loss of the protective effect of PGE2. Thus they also induce gastric bleeding. Therefore, salicylate should not be used if a patient has peptic ulcer. The bleeding effect is increased if they are taken with ethanol.

BLOOD

Aspirin prolongs bleeding time by irreversibility inhibiting thromboxane A2 synthesis and thereby interfering with platelet aggression. Aspirin irreversibly inhibits platelet aggression for 5-10 days (life time of the platelets) after a single dose. Thus it is useful in the treatment of the coronary arterial thrombosis.

KIDNEYS

PGE and PG1 cause vasodilatation. Therefore chronic use of aspirin and other

NSAIDs like paracetamol cause rental damage by blocking PG synthesis.

METABOLIC EFFECTS

Salicylates enhance utilisation of glucose by the tissues, which produces a

hypoglycaemic effect.

ADME

Salicylate is absorbed from the stomach and small intestine. They bind to plasma proteins and are readily distributed in most of tissues. They readily cross the blood brain barrier and placenta. They are mainly metabolised in liver and excreted in urine in conjugated from with glucuronic acid or sulphate. Alkalinisation of urine with sodium bicarbonate hastens excretion of Salicylates because Salicylates are ionised and not reabsorbed. This principle is utilised in their poisoning to hasten their elimination. (H P Rang, M.M. Dale, J.M. Ritter, R.J. Flower, 2007)

4.0. ISCHAEMIA:-

Russell J Greene and Norman D Harris described that NSAIDs may cause Ischaemia, which can affect conducting tissue as well as cardiac muscle, either quietly, or chronically, leading to arrhythmias. Myocardial Ischaemia may cause the cardiac abnormalities. Ventricular fibrillation can account for many cases of sudden cardiac death. Most of ischaemic symptoms can occur unassimilated with any coronary obstruction, not even vasospasm. For example excessive cardiac oxygen demand, thyrotoxicosis. But during the less supply of oxygen severe anaemia can cause. Ischaemic pain is certainly related to the accumulation of the products of anaerobic metabolism, for example acid or lactate.

5.0. PARACETAMOL:-

Paracetamol is a form of NSAIDS. The well known drug such as paracetamol is not a part of traditional medical armoury. This drug is chemically synthesized. There are recently 90 products containing paracetamol. According to Micahel J Neal (2009, pg.71) paracetamol acts at least partly by reducing cytoplasmic peroxide tone: peroxide is necessary to activate the COX enzyme to the ferryl form. In areas of acute inflammation, paracetamol is not very effective because neutrophils and monocytes produce high levels of H2O2 and lipid peroxide, which overcome the actions of the drug. Paracetamol is an effective analgesic in conditions in which leucocyte infiltration is absent or low.

6.0. IBUPROFEN:-

Ibuprofen is commonly used and frequently prescribed NSAID. Rabia Bushra and Nousheen Aslam argued that it is supplied as tablets with a potency of 200 to 800 mg. Ibuprofen is good absorbed orally; peak serum concentrations are attained in 1 to 2 hours after oral administration. It is immediately bio transformed with a serum half life of 1.8 to 2 hours. The drug is eliminated in 24 hours after the last dose and eliminated through metabolism. The drug is more than 99% protein bound, extensively metabolised in the liver and little is excreted unchanged. Ibuprofen has been rated as the safest conventional NSAID by spontaneous adverse drug reaction reporting systems in the UK.

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