Lipoamide dehydrogenase and diaphorase catalyzed conversion of some NO donors to NO and reduction of NO scavenger 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO).

One of the key functions of nitric oxide (NO) in human is to dilate blood vessels. We tested glycerol trinitrate (GTN) and other well-known NO donors together with those bearing a >C=N-OH group for possible conversion to NO (or nitrites, respectively) by diaphorase (DP) and lipoamide dehydrogenase (LAD). Both, DP and LAD were unable to convert formamidoxime (FAM), acetone oxime (AC), acetohydroxamic acid (AHA) and Nomega-hydroxy-L-arginine (L-NOHA). On the other hand, we observed good conversion of GTN without the requirement of superoxide anion. However, superoxide anion participated to a varying extent in the conversion of other donors (formaldoxime (FAL), acetaldoxime (AO), nitroprusside (NP), S-nitrosoglutathione (SNOG), S-nitroso-N-acetylpenicillamine (SNAP) and hydroxylamine (HA)). All DP- and LAD-mediated reactions were inhibited by diphenyleneiodonium chloride (DPI), (an inhibitor of flavine enzymes), in a concentration-dependent manner. For these inhibition reactions we determined Ki and IC50 values. In addition, we found that conversion of SNOG was significantly accelerated by glutathione reductase (GTR). Like with DP, 2-phenyl- 4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO) was reduced also by LAD and thioredoxin reductase (TRR). In summary, we found that LAD significantly accelerates the conversion of a defined subset of NO donors to NO, especially GTN, and eliminates the NO scavenging effect of PTIO.

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