The control and regulation of the eicosanoids is ultimately determined by the amount of the two essential fatty acids, alpha-linolenic acid and linoleic acid, that are consumed and metabolized. They are the precursors to the arachidonic acid cascade and it is from arachidonic acid that the eicosanoids are derived. The release of phospholipids from the cell membrane is initiated by signals such as hormones. Bradykinin and epinephrine are known for initiating the cascade and initiating phospholipase action at the cell membrane. Norepinephrine is known to change distribution profiles of eicosanoids.

The cell type is an important regulator of eicosanoid production because the cell type defines the ratios and amounts of different enzymes that are active within the cell. Different enzymes lead to different eicosanoid products and, therefore, to different physiological processes such as vaso-constriction, smooth muscle contraction, and inflammatory disorders.

The limited availability of the eicosanoids in the body at any one time allows for a large degree of control. The half-life of eicosanoids is generally less than five minutes long and they are readily produced by liberating phospholipids from the cell membrane. This allows the body to decrease or increase the amount of eicosanoids very quickly.

The inhibition of synthesis is performed by nonsteroidal anti-inflammatory drugs (NSAIDs) such as aspirin or ibuprofen, and by negative feedback loops. In contrast, positive feedback loops exist that allow for increased synthesis of the eicosanoids. Immunological responses can also enhance eicosanoid synthesis. In addition, ionic concentrations have been linked to eicosanoid synthesis.

Control and Regulation

Membrane phospholipids are attacked by phospholipases to liberate arachidonate, the anion of arachidonic acid [1]. Metabolism via different enzymes creates what is known as the arachidonic cascade. It is by way of this cascade that all the eicosanoids are synthesized. The phospholipase action is initiated by hormones such as bradykinin or epinephrine, by proteases, by immunological stimuli, and by other stimuli; the full range of stimuli is unknown at this time. The factors involved in causing the release of the precursors of the eicosanoids are targeted to specific tissues. [2]

Control and regulation of the eicosanoids is dependent on many factors, the dominant of which is the amount and ratio of the essential fatty acids, linolenic acid and linoleic acid, that are ingested and metabolized by the body. Changes in the course of artherothrombotic, hypertensive, and inflammatory disorders are recognized effects of changing availability of the eicosanoid precursors [3]. Norepinephrine has been shown to change the distribution profiles of arachidonic and docosahexaenoic acids in rat pineal cells. [6] Other major factors include pathophysiological reactions, physiological reactions, and the ratio and amount of specific enzymes within a tissue cell. The ratio and amount of enzymes within a cell depend on the type of cell. Some eicosanoids stimulate or inhibit their own production (or that of other eicosanoids) via feedback loops with enzymes.

Additional regulatory factors are the "life span" of the eicosanoids and their limited availability in the system at any one time. The ratio of body mass to total eicosanoid mass is estimated to be one million to one. Experimental work has found some specific prostaglandins (PGs), PGE2 and PGF2-alpha, with half-lives of less than one minute [3]. Because of these factors, the inhibition or stimulation of eicosanoid production can cause significant effects within the body in a short time.

Tissue injury often causes disruption of the arachidonate system particular to a tissue. Inflammatory cells such as monocytes are able to activate the arachidonate cascade to produce eicosanoids. When these cells move into a tissue and interact with the tissue's cells, then a change in arachidonate metabolism, and thus the eicosanoid production, is made.

A specific example of a tissue that is disrupted by a pathophysiologic state is the tissue in a hydronephrotic kidney. In particular, the rate of metabolism is increased due to a 20-fold increase in the V-max of cortical cyclooxygenase. This rate increase appears to be due to the interaction of the macrophages (from the monocytes) with the interstitial cells of the kidney. The macrophages release at least one factor (which may be interleukin-1) that causes fibroblast-like interstitial cells to proliferate, to increase in cyclooxygenase activity, and to increase PGE2 release. In addition, thromboxane synthetase and thromboxane A2 most likely come from the macrophage; there is no production of either when the tissue is healthy. A similar mechanism is possible for other inflammatory disorders such as myocardial infarction, pulmonary fibrosis, rheumatoid arthritis, and ulcerative colitis [4].

Assuming the availability of arachidonate precursors, enzymes control and direct which eicosanoid will be produced. The initial enzyme that ultimately leads to prostaglandins and thromboxanes is cyclooxygenase. Thromboxane production is initiated via lipoxygenase. Additional enzymes are involved in these pathways. These three eicosanoids are the most commonly known; however, discovery of metabolically active eicosanoids is not uncommon due to the relative youth of this area of research--leukotrienes were not discovered until 1979. Cytochrome P-450 mediates the production of many other eicosanoids. [3]

Thromboxanes are a product of the cyclooxygenase mediated pathway. The activation of platelets initiates the formation of considerable amount of thromboxane-A2 (TXA2) and is thought to be involved in maintenance of normal hemostasis. Recently, the presence of large amounts of TXA2 has been confirmed in human mast cell-1. Therefore, distribution and control of this form of thromboxane is more completely accounted for--the full amount of TXA2 could not be accounted for by the platelets' production of TXA2 alone. [7]

Leukotriene formation occurs via the 5-lipoxygenase pathway found in neutrophils, eosinophils, monocytes, mast cells, keratinocytes, and in lung, spleen, brain, and heart tissue. In addition, some leukotriene pathways of production show a positive relationship with concentration of available calcium. [4]

Prostaglandins are located in essentially all healthy tissues in the human body. Their synthesis is regulated by cyclooxygenase (and sometimes by cytochrome P-450). In peritoneal macrophages cyclooxygenase-2 (COX2)is enhanced by stimulation of arachidonic acid metabolism (which COX2 itself stimulates), and is amplified by prostaglandin-E2. As well, the arachidonic cascade stimulating agents calcium, magnesium, and zymosan amplify the expression of COX2. [5]

The most famous pathway in eicosanoid metabolism involves cyclooxygenase. (Please see image below. [1]) It is well-known due to the discovery that this is the pathway on which aspirin, ibuprofen, and other NSAIDs, work. The process begins when the phospholipid bilayer is attacked by a phospholipase in order to release arachidonate ions. The arachidonate is then acted on by prostaglandin endoperoxide synthase to add two diatomic oxygens to synthesize prostaglandin-G2. Aspirin, acetylsalicylate, acts on the prostaglandin endoperoxide synthase at an essential serine residue. The serine's hydroxyl group attacks the carbonyl carbon on the aspirin and forms a covalent bond, thereby irreversibly inhibiting the enzyme's action. Ibuprofen inhibits this same step; the inhibition is probably accomplished by acting as a false intermediate or substrate in the reaction [1]. Other NSAIDs are known to inhibit COX2's enzymatic activity or to inhibit COX2's expression. [5]

Eicosanoids and Cytochrome P-450

Cytochrome P-450 is the general term for the heme proteins that are characterized by strongly absorbing light at 450 nm. Unlike most cytochromes, which are found in the mitochondria as part of the electron transport chain (also known as the respiratory chain), cytochrome P-450 enzymes are located in the endoplasmic reticulum. Predominantly, they perform hydroxylation reactions, but they are also involved in epoxidation, dealkylations, deaminationsl, and dehalogenations. They bear a strong resemblance to mitochondrial cytochrome oxidase in that they bind diatomic oxygen and carbon monoxide. It is when they are bound to carbon monoxide that they show the absorption spectra that gives them their name. The synthesis of the cytochrome P-450 enzymes can be enhanced by the substrates they metabolize and by barbiturates such as phenobarbital. [2]

Cytochrome P-450 mediated pathways use arachidonate, and many of its derivatives, as substrates. The characteristic reactions involved include omega hydroxylation, epoxidation, and allylic oxidation. Cytochrome P-450 availability is a determining factor in the production of at least 14 types of eicosanoids including 20-hydroxy prostaglandins, the hydroxyeicosatetraenoic acids, glutathione adducts, and the diepoxides/epoxy-alcohols. The diversity of the structures and functionalities of the eicosanoids is greatly amplified by the cytochrome P-450 enzymes. [3]

The epoxygenase pathway catalyzed by cytochrome P-450 produces the epoxyeicosatrienoic acids. This pathway, unlike many other paths, is selective for arachidonic acid and only occurs in the presence of cytochrome P-450. Regulation of the production of the epoxyeicosatrienoic acids is the subject of interest because they are mainly found in the phospholipid bilayer (as phosphatidylcholine esters in cells in the liver). They therefore indicate a role for cytochrome P-450 as a regulator of the microenvironment and of the functionality of cell membranes. Fluctuations in the amount of cytochrome P-450 should therefore be effective in changing the concentration of the types of phospholipids in the membrane and, thus, they may be a controlling factor in physiochemical, structural, and functional properties of the membrane. [3]

References for Eicosanoid:

Lehninger, Albert L., Nelson, David L., and Cox, Michael M. (1993), Principles of biochemistry, pp. 656-657.

Mathews,Christopher K., and van Holde, K. E. (1990), Biochemistry, pp. 637-641.

Clissold, D. and Thickitt, C. (1994), Natural Product Reports, 11(6), 621-635.

Needleman, Philip; Turk, John; Jakschik, Barbara A.; Morrison, Aubrey R.; and Lefkowith, James B. (1986), Am. Rev. Biochem., 55, 69-102.

Tordjman, C., Coge, F., Andre, N., Rique, H., Spedding, M., and Bonnet, J. (1994), Biochimica et Biophysica Acta, 1256, 249-256.

Delton, Isabelle; Gharib, Abdallah; Moliere, Patrick; Lagarde, Michel; Sarda, Nicole; (1995), Biochimica et Biophysica Acta, 1254, 147-154.

Macchia, Luigi; Hamberg, Mats; Kumlin, Maria; Butterfield, Joseph H.; and Haeggstrom, Jesper Z.; (1995), Biochimica et Biophysica Acta, 1257, 58-74.