Lipoyl dehydrogenase catalyzes reduction of nitrated DNA and protein adducts using dihydrolipoic acid or ubiquinol as the cofactor

Inflamed tissues generate reactive nitrogen oxide species (RNO_x), such as peroxynitrite (ONO₂⁻)and nitryl chloride (NO₂Cl), which lead to formation of nitrated DNA and protein adducts, including 8-nitroguanine (8NG), 8-nitroxanthine (8NX), and 3-nitrotyrosine (3NT). Once formed, the two nitrated DNA adducts are not stable in DNA and undergo spontaneous depurination. Nitration of protein tyrosine leads to inactivation of protein functions and 3NT has been detected in various disease states. We herein report that reduction of these nitro adducts to their corresponding amino analogues can be catalyzed by lipoyl dehydrogenases (EC 1.8.1.4) from Clostridium kluyveri (ck) and from porcine heart (ph) using NAD(P)H as the cofactor. We also found that dihydrolipoic acid (DHLA) and ubiquinol can be used as effective cofactors for reduction of 8NG, 8NX, and 3NT by these lipoyl dehydrogenases. The reduction efficiency of the mammalian enzyme is higher than the bacterial isozyme. The preference of cofactors by both lipoyl dehydrogenases is DHLA>NAD(P)H>ubiquinol. In all the systems examined, the nitrated purines are reduced to a greater extent than 3NT under the same conditions. We also demonstrate that this lipoyl dehydrogenase/antioxidant system is effective in reducing nitrated purine on NO₂Cl-treated double stranded calf thymus DNA, and thus decreases apurinic site formation. The nitroreductase activity for lipoyl dehydrogenase might represent a possible metabolic pathway to reverse the process of biological nitration.