- Email: [email protected]
Reply to Drs. Awad, Morrow, and Roberts
945
Letters to the Editors
prostane ring containing compounds from unsaturated fatty acids as a result of free radical-induced peroxidation was described many years ago.’ Prostaglandin FZlike compounds resulting from free radical-induced peroxidation of arachidonic acid in vivo have been described more recently.3 F,-isoprostanes have the same molecular weight and chemical functional groups as cyclooxygenase-derived PGF,,; thus they have similar chromatographic characteristics. In fact, some F2isoprostanes cochromatograph with authentic PGF,, in our GC/NICI-MS assay for F,-isoprostanes. The effect of cyclooxygenase inhibitors in the experiments described by Liu and Li would be informative. The lack of an effect of these agents on the putative PGF,, peak would suggest that this peak represents F2-isoprostanes instead of PGF2,. The inhibitory effect of methylprednisolone on “PGF,, release” cited from a manuscript submitted for publication does not provide specific evidence that PGF,, is the prostanoid being measured. The effect of methylprednisolone on phopholipases might also inhibit release of F,-isoprostanes from phospholipids on which they are formed.4 In addition, steroids may scavenge free radicals. In summary, the formation of prostanoids by free radical-catalyzed peroxi-
REPLY
TO DRS. AWAD,
To the Editors: I have carefully read Dr. Awad et al’s letter. Although Drs. Yergey and Heyes demonstrated that geometrical isomers of 9a, 1 lp and 9p, 1 lcr forms of PGF2 could be separated from the 9a, 1 la form by gas chromatography (GC),’ it has not been demonstrated that prostaglandin F2 isomers can be separated by HPLC. The peak measured by HPLC may include other geometrical isomers of PGF,. Therefore, using PGF,-like compounds instead of PGF2, is more appropriate. However, the presence of PGF2, in the peak cannot be ruled out until all of the PGF, isomers are separated, and the fact that PGFzu is not produced by oxidation is demonstrated. PGF,, production following CNS injury has been demonstrated. It is generally generated enzymatically from arachidonate, and it may also be a product of membrane lipid peroxidation. Therefore, the suggested use of cyclooxygenase inhibitors to further clarify the pathways of PGF,, production is also valid and important. This work* is part of our effort to reveal the interactions between hydrolytic and peroxidative pathways in membrane degradation in the CNS in in vivo experiments. The PGF2-like substance was produced by HO
dation of arachidonic acid in vivo makes it important that the chemical identity of prostanoids measured in the context of oxidant stress be adequately confirmed. Joseph Awad Jason Morrow L. Jackson Roberts
II
Division of Clinical Pharmacology Vanderbilt University Nashville, TN 37232, USA REFERENCES Liu, D.; Liping, L. Prostaglandin Fz, rises in response to hydroxyl radical generated in vivo. Free Radic. Biol. Med. 1857 l576; 1995. Pryor, W. A.; Stanley, .I. P.; Blair, E. Autooxidation of polyunsaturated fatty acids: II. A suggested mechanism for the formation of TBA-reactive materials from prostaglandin-like endoperoxides. Lipids 85:370-379; 1976. Morrow, J. D.; Hill, K. E.; Burk, R. F.; Nammour, T. M.; Badr, K. F.; Roberts, L. J., II. A series of prostaglandin F,-like compounds am produced in vivo in humans by a noncyclooxygenase, free radical-catalyzed mechanism. Proc. Natl. Acad. Sci. USA 87:9383-9387; 1990. Morrow, J. D.; Awad, J. A.; Boss, H. J.; Blair, I. A.; Roberts, L. J., II. Formation of noncyclooxygenase derived prostanoids occurs in situ on phospholipids. Proc. Natl. Acad. Sci. USA 89:10721-10725; 1992.
MORROW,
AND
ROBERTS
generated in rat spinal cord. Recently, we measured HO’ formation” and malondialdehyde (MDA) release4 in response to administering PGF,, in rat spinal cord by microdialysis. These data correlated the presence of PGF,, to membrane phospholipid peroxidation. Our finding that methylprednisolone (MP) blocks HO’ formation’ and MDA release4 confirms the inhibitory effect of MP on membrane lipid peroxidation. This makes it clear that MP blocks the pathway PGF, + HO’ and subsequent MDA production. It is unlikely that MP directly “scavenges free radicals” because MP does not block MDA production induced by HO generated by administering Fenton reagents into rat spinal cord.’ Danxia
Liu
Marine Biomedical Institute The University of Texas Medical Branch at Galveston Galveston, TX 77555-1143, USA REFERENCES 1. Yergev, _ _ J. A.; Heves, M. P. Brain eicosanoid formation following acute penetration injury as studied by in vivo microdialysis. J. Cerebr. Blood Flow Metab. 10:143-146; 1990.
946
Letters to the Editors
2. Liu, D.; Li, L. Prostaglandin FZ, rises in response to hydroxyl radical generated in vivo. Free Radic. Biol. Med. l&571 -576; 1995. 3. Liu, D.; Li, L.; Augustus, L. In vivo evidence for a secondary injury cascade: Prostaglandin F,,, induces and methylprednisolone blocks hydroxyl radical generation in the rat spinal cord. J. Neurotrauma 12:130; 1995.
4. Liu, D.; Li, L.; Augustus L. In vivo evidence for the free radical mechanism of peroxidative damage to membrane lipids in the rat spinal cord. J. Neurotrauma 12:442; 1995. 5. Liu, D.; Li, L. Methylprednisolone oxidation but does not scavenge Sot. for Neurosci., Abst., 1995.
blocks membrane lipid perfree radicals. 25th Meeting,
Letters to the Editors
prostane ring containing compounds from unsaturated fatty acids as a result of free radical-induced peroxidation was described many years ago.’ Prostaglandin FZlike compounds resulting from free radical-induced peroxidation of arachidonic acid in vivo have been described more recently.3 F,-isoprostanes have the same molecular weight and chemical functional groups as cyclooxygenase-derived PGF,,; thus they have similar chromatographic characteristics. In fact, some F2isoprostanes cochromatograph with authentic PGF,, in our GC/NICI-MS assay for F,-isoprostanes. The effect of cyclooxygenase inhibitors in the experiments described by Liu and Li would be informative. The lack of an effect of these agents on the putative PGF,, peak would suggest that this peak represents F2-isoprostanes instead of PGF2,. The inhibitory effect of methylprednisolone on “PGF,, release” cited from a manuscript submitted for publication does not provide specific evidence that PGF,, is the prostanoid being measured. The effect of methylprednisolone on phopholipases might also inhibit release of F,-isoprostanes from phospholipids on which they are formed.4 In addition, steroids may scavenge free radicals. In summary, the formation of prostanoids by free radical-catalyzed peroxi-
REPLY
TO DRS. AWAD,
To the Editors: I have carefully read Dr. Awad et al’s letter. Although Drs. Yergey and Heyes demonstrated that geometrical isomers of 9a, 1 lp and 9p, 1 lcr forms of PGF2 could be separated from the 9a, 1 la form by gas chromatography (GC),’ it has not been demonstrated that prostaglandin F2 isomers can be separated by HPLC. The peak measured by HPLC may include other geometrical isomers of PGF,. Therefore, using PGF,-like compounds instead of PGF2, is more appropriate. However, the presence of PGF2, in the peak cannot be ruled out until all of the PGF, isomers are separated, and the fact that PGFzu is not produced by oxidation is demonstrated. PGF,, production following CNS injury has been demonstrated. It is generally generated enzymatically from arachidonate, and it may also be a product of membrane lipid peroxidation. Therefore, the suggested use of cyclooxygenase inhibitors to further clarify the pathways of PGF,, production is also valid and important. This work* is part of our effort to reveal the interactions between hydrolytic and peroxidative pathways in membrane degradation in the CNS in in vivo experiments. The PGF2-like substance was produced by HO
dation of arachidonic acid in vivo makes it important that the chemical identity of prostanoids measured in the context of oxidant stress be adequately confirmed. Joseph Awad Jason Morrow L. Jackson Roberts
II
Division of Clinical Pharmacology Vanderbilt University Nashville, TN 37232, USA REFERENCES Liu, D.; Liping, L. Prostaglandin Fz, rises in response to hydroxyl radical generated in vivo. Free Radic. Biol. Med. 1857 l576; 1995. Pryor, W. A.; Stanley, .I. P.; Blair, E. Autooxidation of polyunsaturated fatty acids: II. A suggested mechanism for the formation of TBA-reactive materials from prostaglandin-like endoperoxides. Lipids 85:370-379; 1976. Morrow, J. D.; Hill, K. E.; Burk, R. F.; Nammour, T. M.; Badr, K. F.; Roberts, L. J., II. A series of prostaglandin F,-like compounds am produced in vivo in humans by a noncyclooxygenase, free radical-catalyzed mechanism. Proc. Natl. Acad. Sci. USA 87:9383-9387; 1990. Morrow, J. D.; Awad, J. A.; Boss, H. J.; Blair, I. A.; Roberts, L. J., II. Formation of noncyclooxygenase derived prostanoids occurs in situ on phospholipids. Proc. Natl. Acad. Sci. USA 89:10721-10725; 1992.
MORROW,
AND
ROBERTS
generated in rat spinal cord. Recently, we measured HO’ formation” and malondialdehyde (MDA) release4 in response to administering PGF,, in rat spinal cord by microdialysis. These data correlated the presence of PGF,, to membrane phospholipid peroxidation. Our finding that methylprednisolone (MP) blocks HO’ formation’ and MDA release4 confirms the inhibitory effect of MP on membrane lipid peroxidation. This makes it clear that MP blocks the pathway PGF, + HO’ and subsequent MDA production. It is unlikely that MP directly “scavenges free radicals” because MP does not block MDA production induced by HO generated by administering Fenton reagents into rat spinal cord.’ Danxia
Liu
Marine Biomedical Institute The University of Texas Medical Branch at Galveston Galveston, TX 77555-1143, USA REFERENCES 1. Yergev, _ _ J. A.; Heves, M. P. Brain eicosanoid formation following acute penetration injury as studied by in vivo microdialysis. J. Cerebr. Blood Flow Metab. 10:143-146; 1990.
946
Letters to the Editors
2. Liu, D.; Li, L. Prostaglandin FZ, rises in response to hydroxyl radical generated in vivo. Free Radic. Biol. Med. l&571 -576; 1995. 3. Liu, D.; Li, L.; Augustus, L. In vivo evidence for a secondary injury cascade: Prostaglandin F,,, induces and methylprednisolone blocks hydroxyl radical generation in the rat spinal cord. J. Neurotrauma 12:130; 1995.
4. Liu, D.; Li, L.; Augustus L. In vivo evidence for the free radical mechanism of peroxidative damage to membrane lipids in the rat spinal cord. J. Neurotrauma 12:442; 1995. 5. Liu, D.; Li, L. Methylprednisolone oxidation but does not scavenge Sot. for Neurosci., Abst., 1995.
blocks membrane lipid perfree radicals. 25th Meeting,