Sensors, methods and kits for detecting nicotinamide adenine dinucleotides

The invention relates to the in vitro and in cellulo detection of the cofactors nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP). Provided is a sensor molecule for fluorescence or luminescence-based detection of a nicotinamide adenine dinucleotide analyte, in particular for detecting the concentrations of NAD+, NADP' and/or the ratios of the concentrations of NAD '/NAD H and NADP '/NADPH, the sensor comprising (i) a binding protein (BP) for the nicotinamide adenine dinucleotide analyte, the BP being derived from sepiapterin reductase (SPR; EC 1.1.1.153) (ii) a SPR ligand (SPR-L) capable of intramolecular binding to said BP in the presence of the oxidized form of said analyte; and (iii) at least one fluorophore.