Sensors, methods and kits for detecting nicotinamide adenine dinucleotides

A sensor molecule for fluorescence or luminescence-based detection of a nicotinamide adenine dinucleotide analyte, in particular for detecting the concentration of NADP+ or the ratio of the concentrations of NADP+ and NADPH, NADP+/NADPH, the sensor comprising

• (i) a binding protein (BP) for the nicotinamide adenine dinucleotide analyte, the BP being sepiapterin reductase (SPR), wherein the SPR is human sepiapterin reductase (hSPR) according to SEQ ID NO:4, rat sepiapterin reductase according to SEQ ID NO:3, mouse sepiapterin reductase according to SEQ ID NO:2, or a variant thereof having at least 95% sequence identity with SEQ ID NO:4, SEQ ID NO:
• 3 or SEQ ID NO:2, provided that the variant comprises the residues corresponding to Ser157, Tyr170, Lys174, and Asp257 in hSPR;
• (ii) an SPR ligand (SPR-L) capable of intramolecular binding to said BP in the presence of the oxidized form of said analyte, wherein said SPR-L is based on a benzenesulfonamide;
• and
• (iii) at least one fluorophore.