A protein designed to sense metabolites: Many diseases cause characteristic changes in blood metabolites. Yu et al. describe a paper-based assay in which a chosen metabolite can be oxidized to generate reduced nicotinamide adenine dinucleotide phosphate (NADPH). Color changes in a designed NADPH sensor protein are then quantified by a digital camera. The sensor system successfully generated point-of-care measurements of phenylalanine, glucose, and glutamate. Concentrations of phenylalanine in the blood of phenylketonuria patients were analyzed within minutes with only half a microliter of blood. Science , this issue p. 1122

We introduce a general method to transform antibodies into ratiometric, bioluminescent sensor proteins for the no‐wash quantification of analytes. Our approach is based on the genetic fusion of antibody fragments to NanoLuc luciferase and SNAP‐tag, the latter being labeled with a synthetic fluorescent competitor of the antigen. Binding of the antigen, here synthetic drugs, by the sensor displaces the tethered fluorescent competitor from the antibody and disrupts bioluminescent resonance energy transfer (BRET) between the luciferase and fluorophore. The semisynthetic sensors display a tunable response range (submicromolar to submillimolar) and large dynamic range (ΔR_max>500 %), and they permit the quantification of analytes through spotting of the samples onto paper followed by analysis with a digital camera.

Nicotinamide adenine dinucleotide (NAD⁺) level has been associated with various age‐related diseases and its pharmacological modulation emerges as a potential approach for aging intervention. But human NAD⁺ landscape exhibits large heterogeneity. The lack of rapid, low‐cost assays limits the establishment of whole‐blood NAD⁺ baseline and the development of personalized therapies, especially for those with poor responses towards conventional NAD⁺ supplementations. Here, we developed an automated NAD⁺ analyzer for the rapid measurement of NAD⁺ with 5 μL of capillary blood using recombinant bioluminescent sensor protein and automated optical reader. The minimal invasiveness of the assay allowed a frequent and decentralized mapping of real‐world NAD⁺ dynamics. We showed that aerobic sport and NMN supplementation increased whole‐blood NAD⁺ and that male on average has higher NAD⁺ than female before the age of 50. We further revealed the long‐term stability of human NAD⁺ baseline over 100 days and identified major real‐world NAD⁺‐modulating behaviors.