NPAS2 is a transcription factor that regulates mammalian circadian rhythms. It has been suggested that NPAS2 DNA-binding activity is regulated by the intracellular redox state of NAD(P)H, although the mechanism remains unclear. To investigate the NAD(P)H interaction site of murine NPAS2, we performed electrophoretic mobility shift assays using several truncation mutants of the NPAS2 bHLH domain. Among the mutants, NPAS2 containing the N-terminal 61 residues formed a heterodimer with BMAL1 to bind DNA, and NAD(P)H enhanced the binding activity, while NAD(P)H inhibited the DNA-binding activity of the BMAL1 homodimer in a dose-dependent manner. NAD(P)H derivatives such as 2',5'-ADP, nicotinamide, nicotinic acid and nicotinic acid adenine dinucleotide (NAAD) did not affect the DNA-binding activity. Interestingly, NAD(P)(+), previously reported as an inhibitor, did not affect NPAS2 binding activity in the presence or absence of NAD(P)H in our system. These results suggest that NPAS2 DNA-binding activity is specifically enhanced by NAD(P)H independently of NAD(P)(+) and that the N-terminal 1-61 amino acids of NPAS2 are sufficient to sense NAD(P)H. (C) 2013 Elsevier Inc. All rights reserved.