HUFA Balance & Prostaglandins in Inflammatory Conditions
Essential 20-carbon highly unsaturated fatty acids (HUFA) form prostaglandins which are potent hormone-like mediators. The more vigorous formation and action of n-6 compared to n-3 prostaglandins gives benefit and also harm as n-6 over-reactions shift healthy physiology toward inflammatory pathophysiology. (1.3 min.video). Conversion of HUFA to prostaglandins requires peroxide tone and is inhibited by non-steroidal anti-inflammatory drugs (NSAID).
PEROXIDE TONE AND ANTI-INFLAMMATORY DRUGS AFFECT PROSTAGLANDIN SYNTHESIS
The prostaglandin synthase has two actions when forming prostanoids: a peroxidase and a cyclooxygenase. The cyclooxygenase produces a hydroperoxide, prostaglandin G2, that is both a substrate for the peroxidase and an activator of the cyclooxygenase. A mechanistic model for synthase activities was set into an interactive computer program to make quantitative predictions of the reaction kinetics. The model fit experimental observations under a wide variety of conditions, including inhibition by hydroperoxide scavenger, stimulation by hydroperoxide, and low activity of eicosapentaenoic acid, 20:5n-3, oxygenation under conditions where n-6 arachidonic acid, 20:4n-6, reacts rapidly. Kulmacz RJ, Pendleton RB, Lands WE. Interaction between peroxidase and cyclooxygenase activities in prostaglandin-endoperoxide synthase. Interpretation of reaction kinetics. J Biol Chem. 1994 Feb 25;269(8):5527-36. PMID: 8119886
Hydroperoxides were measured in arterial (radial artery) and mixed venous (pulmonary artery) plasma from twelve patients with sepsis syndrome using a sensitive assay based on activating prostaglandin H synthase. Increased hydroperoxides in blood draining septic foci are markers of oxyradical release associated with severe infection. Keen RR, Stella L, Flanigan DP, Lands WE. Differential detection of plasma hydroperoxides in sepsis. Crit Care Med. 1991 Sep;19(9):1114-9. PMID: 1884610
Hydroperoxides in the plasma of patients during normal postoperative periods following major thoracic or abdominal operations were measured with a sensitive assay based upon activating prostaglandin H synthase. Standard surgical procedures generated hydroperoxides from pulmonary and intraabdominal tissues. Following major thoracic operations, the mean difference between the arterial (0.49) and mixed venous (-0.09) level of hydroperoxide was 0.58 microM. In contrast, major abdominal operations led to a mean difference between the arterial (-0.19 microM) and mixed venous (0.46 microM) hydroperoxide levels of -0.65 microM. The A-MV differences suggest that blood-borne hydroperoxides are rapidly cleared from the circulation by tissue capillary beds. Keen RR, Stella LA, Flanigan DP, Lands WE. Differences between arterial and mixed venous levels of plasma hydroperoxides following major thoracic and abdominal operations. Free Radic Biol Med. 1990;9(6):485-94. PMID: 2127763
Formation of prostaglandins by prostaglandin H synthase is limited by availability of the fatty acid substrate or the hydroperoxide activator and also by a self-catalyzed inactivation associated with the oxygenation reaction. Each pmol of synthase appeared able to form only about 1300 pmol of prostaglandin from arachidonate before it was inactivated. This extent of synthesis was not diminished when substrate fatty acid was complexed with cytosolic proteins even though the velocity of the oxygenation reaction was greatly decreased by the lower availability of substrate acid. Decreased availability of hydroperoxide activator by added glutathione peroxidase decreased the extent of oxygenation per mol of synthase irrespective of the amount of cytosolic protein present. Approximately 65% of the total prostaglandin synthesis in homogenates was suppressed by a glutathione peroxidase to prostaglandin H synthase ratio of about 90. The ratio of peroxide-removing capacity to peroxide-forming capacity in selected tissues ranged from over 1800 in rat liver to less than 30 in leukocytes. Marshall PJ, Kulmacz RJ, Lands WE. Constraints on prostaglandin biosynthesis in tissues. J Biol Chem. 1987 Mar 15;262(8):3510-7. PMID: 3102488
A sensitive and selective assay for lipid hydroperoxides was developed using activation of the cyclooxygenase activity of prostaglandin H synthase by hydroperoxides. The response was approximately linear in the range 10 to 150 pmol of added lipid hydroperoxide. Assaying lipid peroxides has about 50-fold greater sensitivity than that of the thiobarbiturate assay with fluorescence detection. When applied to samples of human plasma, the assay indicated a concentration of hydroperoxides near 0.5 microM in normal subjects, more than 50-fold lower than the 30-50 microM estimated by the thiobarbiturate assay. A circulating concentration of 0.5 microM approaches that reported to have deleterious effects upon vascular prostacyclin synthase. Marshall PJ, Warso MA, Lands WE. Selective microdetermination of lipid hydroperoxides. Anal Biochem. 1985 Feb 15;145(1):192-9. PMID: 3923861
High levels of NaCN (20 to 250 mM) inhibit cyclooxygenase catalysis and cause extended lag periods (up to 1.6 min). Endogenous or exogenous hydroperoxides can eliminate the lag period. In addition, glutathione peroxidase inhibition showed that hydroperoxides were essential in the early phases and throughout catalysis. Phenolic antioxidants slightly extended lag periods while inhibiting oxygenation rates more than 50%. When combined with agents which interfere with peroxide activation (NaCN or glutathione peroxidase), low levels of phenol (which is normally stimulatory) gave potent inhibition with long lag times. A mechanism consistent with all of the above properties of cyclooxygenase has been proposed. The reaction-catalyzed self-inactivation of cyclooxygenase appears to involve a destructive reaction intermediate intrinsic to the cyclooxygenase mechanism. Hemler ME, Lands WE. Evidence for a peroxide-initiated free radical mechanism of prostaglandin biosynthesis. J Biol Chem. 1980 Jul 10;255(13):6253-61. PMID: 6771268
Eicosapentaenoic acid 20:5n-3 is not oxidized by cyclooxygenase in the conditions of low peroxide tone in which arachidonate 20:4n-6 is rapidly oxygenated. In this condition, 20:5n-3 reversibly interferes with 20:4n-6 forming prostaglandins. When the level of peroxide in incubations rises, the cyclooxygenase reacts with 20:5n-3 at one-half the rate and one-third the extent observed with 20:4n-6. When peroxides were low but not totally absent, 20:5n-3 irreversibly inactivated the cyclooxygenase with little overall oxygenation occurring. The low peroxide levels expected in vivo allow the n-3 fatty acid to inhibit n-6 prostaglandin formation by both reversible and irreversible actions. Culp BR, Titus BG, Lands WE. Inhibition of prostaglandin biosynthesis by eicosapentaenoic acid. Prostaglandins Med. 1979 Nov;3(5):269-78. PMID: 121610
A GSH-dependent peroxidase in many cells could suppress prostaglandin (PG) biosynthesis by destroying the essential hydroperoxide activator as it is formed, leaving some cyclooxygenase not activated and not producing PG. A decrease in cellular GSH could increase production of PG. Cook HW, Lands WE. Mechanism for suppression of cellular biosynthesis of prostaglandins. Nature. 1976 Apr 15;260(5552):630-2. PMID: 817209
NONSTEROIDAL ANTIINFLAMMATORY DRUGS (NSAID) – – MECHANISMS OF ACTION
When the steady-state concentration of hydroperoxides in prostaglandin H synthase systems was lowered by added glutathione peroxidase, several NSAID (meclofenamic acid, mefenamic acid, acetamidophenol and phenylbutazone) became more potent inhibitors of the synthase. Paradoxically, these agents stimulated oxygen incorporation in the absence of added peroxidase. On the other hand, dithiothreitol, ibuprofen, flurbiprofen and indomethacin all inhibited prostaglandin formation in a dose-dependent manner, and their inhibitory potencies were unaffected by added glutathione peroxidase. Aspirin, dl-gamma-tocopherol and salicylic acid were not directly inhibitory (i.e., without preincubation) under the assay conditions employed in this study. The potencies of some anti-inflammatory agents may be impaired by high local peroxide concentrations. Hanel AM, Lands WE. Modification of anti-inflammatory drug effectiveness by ambient lipid peroxides. Biochem Pharmacol. 1982 Oct 15;31(20):3307-11. PMID: 6816243
Increasing the rate of peroxide removal with higher amounts of glutathione peroxidase increases the inhibitory potency of phenolic agents. When the concentration of peroxide activators is decreased in vitro by glutathione peroxidase, the non-steroidal antiinflammatory drug, MK 447 inhibits cyclooxygenase activity in a dose-dependent manner. The results support a vital role of tissue peroxides in facilitating prostaglandin biosynthesis in vivo. They also support the corollary concept that general hyperalgesia involves lower levels of hydroperoxide than are found in inflammatory conditions. Thus, hyperalgesia may be more amenable than chronic inflammation to therapy with phenolic agents like MK 447 and acetamidophenol. Lands WE, Hanel AM. Phenolic anticyclooxygenase agents in antiinflammatory and analgesic therapy. Prostaglandins. 1982 Aug;24(2):271-7. PMID: 6815721
Several anti-inflammatory drugs cause a time-dependent inactivation of the cyclooxygenase that produces prostaglandins. Time-dependent inhibitors studied contained carboxylic acid moieties, and their time-dependent property was totally eliminated after methylation of the carboxylate group. Methylation did not, however, alter the ability of the inhibitors to competitively inhibit the oxygenase. Rome LH, Lands WE. Structural requirements for time-dependent inhibition of prostaglandin biosynthesis by anti-inflammatory drugs. Proc Natl Acad Sci U S A. 1975 Dec;72(12):4863-5. PMID: 1061075
updated November, 2017