Vol. 91, No. 3, 1979 BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS December 14,1979 Pages 982-990 OXYGEN POISONING OF NAD BIOSYNTHESIS: A PROPOSED SITE OF CELLULAR OXYGEN TOXICITY Olen R. Brown, Frederick Yein, Daniel Boehme, Laurie Foudin and Cheng Shu Song The John M. Dalton Research Center and Department of Veterinary Microbiology, University of Missouri-Columbia, Missouri 65211 Received October 22, 1979 SUMMARY: Quinolinate phosphoribosyl transferase was rapidly inactivated in Escherichia coli exposed to hyperbaric oxygen. The enzyme is essential for de novo biosynthesis of NAD in r. coli and man. Because of its sensitivity and essentiality, inactivation of this enzyme is proposed as a significant mechanism of cellular oxygen toxicity. Niacin which enters the NAD biosynthe- tic pathway below the oxygen-poisoned enzyme provided significant protection against the decrease in pyridine nucleotides and the growth inhibition from hyperoxia in g. coli and could be useful in cases of human oxygen poisoning. INTRODUCTION: Life forms which use oxygen must avert the potentially harmful effects of oxygen radicals which are formed by univalent reduction. Cells are protected from the toxic radicals by enzymes such as superoxide dismutases, catalases and peroxidases. The margin of safety is narrow, however, and aerobic life forms suffer damage in oxygen tensions elevated as little as 2 to 10 times that of air [l, 21. Using Escherichia coli as a model for some aspects of cellular oxygen toxicity, we obtained partial protection with specific amino acids [3]. Cer- tain intermediates of the biosynthetic pathways for these amino acids also protected, indicating that specific enzymes for synthesis of branched-chain and aromatic amino acids were inactivated rapidly upon exposure of E. coli to hyperbaric oxygen [3]. Subsequently, dihydroxyacid dehydratase was identified as the specific site of oxygen poisoning in biosynthesis of branched-chain amino acids [4]. These data accounted for the acute growth-cessation in mini- mal medium as the consequence of inhibition of protein synthesis and induction of stringency through deprivation of specific amino acids. However, the pro- tection afforded by these specific amino acids decreased on continued exposure to high oxygen tension [3]. This might be expected if oxygen inhibited synthe- 0006-291X/79/230982-09$01.00/0 Copyrighr @ 1979 by Academic Press, Inc. All rights of reproduction in anyform reserved. 982