Inhibition of embryonic retinoic acid synthesis by aldehydes of lipid peroxidation and prevention of inhibition by reduced glutathione and glutathione S-transferases.
Inhibition of conceptal biosynthesis of all-trans-retinoic acid (t-RA) by aldehydes generated from lipid peroxidation was investigated. Oxidative conversion of all-trans-retinal (t-RAL, 18 microM) to t-RA catalyzed by rat conceptal cytosol (RCC) was sensitive to inhibition by trans-2-nonenal (tNE), nonyl aldehyde (NA), 4-hydroxy-2-nonenal (4HNE), and hexanal. With an initial molar ratio of aldehyde/t-RAL of 2:1, tNE, NA, and 4HNE caused 70, 65, and 40% reductions of t-RA synthesis, respectively. Hexanal reduced generation of t-RA by approximately 50% as the ratio of aldehyde/t-RAL was raised to 20:1. tNE significantly increased the Km of the reaction and kinetic analyses indicated a mixed competitive/noncompetitive inhibition. By contrast, analogous reactions catalyzed by adult rat hepatic cytosol (ARHC) were highly resistant to inhibition by the same aldehydes. Significant inhibition (> 40% reduction of t-RA generation) by 4HNE, NA, and tNE were achieved at high molar ratios of aldehyde/t-RAL (> 175:1). Hexanal did not inhibit the reaction significantly even at very high ratios of aldehyde/t-RAL (> 2,000:1). Interestingly, when reduced glutathione (GSH, 10 mM) alone or GSH plus glutathione S-transferase (GST) were added to RCC-catalyzed reactions, additions of tNE or 4HNE showed either no significant inhibition or a partial lack of inhibition. Results suggested that GSH-dependent conjugation with 4HNE proceeded slowly compared to conjugation with tNE. To test the hypothesis that GST-catalyzed GSH conjugation can effectively prevent inhibition of t-RA synthesis by aldehydic products of lipid peroxidation, triethyltin bromide (TEB, a potent inhibitor of GST, 20 microM) was added to ARHC-catalyzed reactions when hexanal or tNE were present in the incubations. Eighty and 60% of hexanal and tNE inhibition, respectively, were observed. This was apparently due to TEB blockage of GST-catalyzed GSH conjugation reactions and thus strongly supported the stated hypothesis.
Chen
H
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Department of Pharmacology, School of Medicine, University of Washington, Seattle 98195, USA.
Juchau
M R
MR
D000447:Aldehydes / Q000494:pharmacology*; D000818:Animals; D004622:Embryo, Mammalian / Q000187:drug effects / Q000378:metabolism*; D005260:Female; D005978:Glutathione / Q000378:metabolism / Q000494:pharmacology*; D005982:Glutathione Transferase / Q000378:metabolism / Q000494:pharmacology*; D007700:Kinetics; D015227:Lipid Peroxidation; D011247:Pregnancy; D051381:Rats; D017207:Rats, Sprague-Dawley; D014212:Tretinoin / Q000037:antagonists & inhibitors* / Q000378:metabolism
D000447:Aldehydes; C013166:2-nonenal; D014212:Tretinoin; C010463:n-hexanal; D005982:Glutathione Transferase; D005978:Glutathione; C027576:4-hydroxy-2-nonenal