Structure, Function and Regulation of Lung-Specific Carbonyl Reductase

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Carbonyl reductase(EC 1.1.1.184)is one of several NADPH-dependent oxidoreductases having wide specificity for carbonyl compounds. Lungs of pig, mouse and guinea-pig specifically contain an oligomeric carbonyl reductase different from the monomeric enzymes in other mammalian tissues. In this study, we purified the oligomeric carbonyl reductase from these animal lungs, and characterized its structural and functional properties, localization in lung, and regulation of the enzyme activity by effectors, in order to elucidate its role in pulmonary metabolism of xenobiotic and endogenous carbonyl compounds.1. Structure. The pulmonary carbonyl reductase was a tetramer with a Mr of 103, 000, consisting of apparent identical subunits of Mr 24, 000. A CDNA clone for carbonyl reductase was isolated from a pig lung CDNA library by use of a specific antibody to the pulmonary enzyme, and the nucleotide sequence of the clone was determined. The predicted amino acid sequence was confirmed by partial se … More quence analysis of the enzyme protdin, and also localized a putative coenzyme binding region similar to that of many other pyridine nucleotide-dependent dehydrogenases. Chemical modification study suggested the presence of essential lysyl and histidyl residues at or near the active center of the enzyme.2. Function. The enzyme reduced various aliphatic and aromatic carbonyl compounds including 3-ketosteroids and fatty aldehydes, and oxidized aldehydes to carboxylic acids. Immunohistochemical study revealed that the enzyme was localized in the Clara cells, ciliated cells, and Type II alveolar cells in these animal lungs. Thus, the enzyme may play roles in dismutation of carbonyl compounds derived from lipid peroxidation, steroid metabolism, and biosynthesis of ether lipids, as well as drug metabolism.3. Regulation. The enzyme exhibited negative cooperativity with respect to the carbonyl substrates. Fatty acids acted as allosteric effectors abolishing the negative interaction. Especially, cis-unsaturated fatty acids such as oleic acid and arachidonic acid were most effective activators showing Ka values of 2.2-14 muM. The fatty acid effect may be important in the regulation of the synthesis of ether lipids from fafiy acids by the enzyme, and the low Ka values for the fatty acids suggests an additional function that the enzyme acts as a lung-specific binding protein for fatty acids. Less