Carbonyl reduction of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK)by cytosolic enzymes in human liver and lung

The tobacco specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-buta none (NNK) is a potent pulmonary carcinogen, independent of the route and t ype of administration. There are competing metabolic activation and detoxif ication pathways. NNK is activated by cu-hydroxylation at either the methyl or methylene carbonyl adjacent to the N-nitroso group to yield intermediat es that methylate and pyridyloxobutylate DNA. Detoxification of NNK in huma ns usually occurs via carbonyl reduction to its hydroxy product NNAL, which undergoes glucuronosylation and final excretion. In vitro studies on NNK m etabolism have usually been performed with tissue homogenates, microsomal f ractions and/or purified microsomal enzymes, but cytosolic metabolism of NN K has been ignored until today. The results of this study demonstrate that cytosolic fractions of human liver and lung also participate in NNK metabol ism. We provide evidence that a substantial degree of NNK carbonyl reductio n occurs by cytosolic enzymes and that these enzymes may contribute to NNK detoxification in human liver and lung. The relative contribution of cytoso lic vs, microsomal NNK carbonyl reduction is nearly identical in liver, whe reas it is more than 3-fold higher in lung microsomes compared to lung cyto sol. The inhibition profile suggested that mainly carbonyl reductase (EC 1. 1.1.184) was active in cytosol of both organs. The expression of carbonyl r eductase mRNA in liver and lung was proven by reverse transcription-(RT)-PC R. In conclusion, the results of this study provide the first data on cytos olic enzymes participating in NNK detoxification in human liver and lung. ( C) 2000 Elsevier Science Ireland Ltd. All rights reserved.