LAUSR

Metabolites have been recognized as biomarkers for diagnostic testing and monitoring of many diseases (1). However, in comparison to many clinical tests that target nucleic acids and proteins, detection and quantification of metabolites, including those applied in point-of-care (POC)2 settings, have been less developed, despite the proven success of glucometers widely used by diabetics. A major contributing factor is the lack of methods that, once developed, can be generalized and applied to many other metabolites. Without such generalizable methods, the costs of development become higher and the resulting tests less affordable for home users. Although nucleic acid hybridization and the antibody generation can be generalized for detection of nucleic acids and proteins, respectively, metabolites have been difficult to detect with a generalized method. Furthermore, although a colorimetric sensor is convenient and thus suitable for POC detection, it is vulnerable to interferences from colors in samples such as in blood. Fluorescent sensors, in contrast, have many advantages in sensitivity, accuracy, and time response, but the requirement of an excitation light source increases the size of the detector and costs of detection. Recently, Yu et al. have taken a major step toward addressing these issues (2), by developing a paper-based enzymatic assay method in which many metabolites can be quantified through oxidation by nicotinamide adenine dinucleotide phosphate (NADPH) (Fig. 1A), resulting in ratiometric changes in bioluminescence that can be quantified with a digital camera without a need for an excitation source. Fig. 1. Scheme of metabolite quantification by an engineered semisynthetic NADPH-dependent protein. A specific enzymatic reaction is performed in a reaction buffer to oxidize the analyte of interest by NADP+ (A). Design of a BRET sensor for NADPH (B). A protein chimera between a receptor and a luciferase (NanoLuc) is tethered via a SNAP-tag to a ligand derivatized with a fluorophore. NADPH triggers ligand binding to the receptor, thereby increasing …