The Transformation of Glyceryl Trinitrate and Other Nitrates by Glutathione-Organic Nitrate Reductase

The major route for glyceryl trinitrate transformation appears to be denitration in the presence of reduced glutathione (GSH). One molecule of glyceryl trinitrate reacts with two GSH to release one inorganic nitrite ion from either the 2- or 3-position to form 1,3- or 1,2-glyceryl dinitrate. With liver enzyme the glyceryl dinitrates and inorganic nitrite produced account for about 90% of the glyceryl trinitrate transformed. There is no product inhibition of the glyceryl trinitrate degradation reaction. The denitration of glyceryl dinitrate proceeds at only 2-5% of the rate for glyceryl trinitrate. Glyceryl mononitrate is practically unaltered by the liver enzyme. Thus, the denitration of glyceryl trinitrate does not lead to glycerol formation. A rapid, sensitive enzymic assay based on the reaction of organic nitrates with GSH to yield GSSG was developed. The spectrophotometric or fluorometric method measures TPNH disappearance as the GSSG is catalytically reduced by glutathione reductase. Mannitol hexanitrate, erythrityl tetranitrate, and glyceryl trinitrate are rapidly transformed by the liver soluble enzyme in the presence of GSH, whereas isosorbide dinitrate and pentaerythritol tetranitrate are degraded at a slow rate by this system. The denitration of these compounds leads to a number of products with a lower number of nitrate ester groups. However, complete degradation of the parent molecule does not occur. The in vivo mechanism of organic nitrate detoxication appears to be similar to the enzymic denitration observed with liver glutathione-organic nitrate reductase. ACKNOWLEDGMENT The authors acknowledge the assistance of Miss Lois Schwartz in some of these experiments. This research was supported by General Pharmacology Training Grant No. Tl-GM-0096, The National Institutes of Health, U. S. Public Health Service.