The cellular level of nicotinamide adenine dinucleotide (NAD⁺), through its different functions, affects cellular metabolism and signalling^1,2,3. A decrease in the NAD⁺ content has been associated with various pathologies and physiological aging^4,5, while strategies to boost cellular NAD⁺ levels have been shown to be effective against age-related diseases in many animal models⁶. The link between decreased NAD⁺ levels and numerous pathologies and physiological aging has triggered the need for a simple quantification method for NAD⁺, ideally applicable at the point of care. Here, we introduce a bioluminescent biosensor for the rapid quantification of NAD⁺ levels in biological samples, which can be used either in laboratories or at the point of care. The biosensor is a semisynthetic, light-emitting sensor protein that changes the colour of emitted light from blue to red on binding of NAD⁺. This NAD⁺-dependent colour change enables the use of the biosensor in paper-based assays in which NAD⁺ is quantified by measuring the colour of the emitted light by using either a simple digital camera or a plate reader. We used the approach to quantify NAD⁺ levels in cell culture, tissue and blood samples, yielding results that agreed with those from standard testing methods. The same biosensor furthermore allows the quantification of NAD⁺-dependent enzymatic activities in blood samples, thus expanding its utility as a tool for point-of-care diagnostics.

细胞水平的烟酰胺腺嘌呤二核苷酸（NAD ⁺），通过其不同的功能，影响细胞的代谢和^1,2,3信号。NAD ⁺含量的下降与各种病理学和生理衰老[ ^4,5]有关，而在许多动物模型中，提高细胞NAD ⁺水平的策略已被证明可有效对抗与年龄有关的疾病⁶。降低的NAD ⁺水平与多种病理和生理衰老之间的联系引发了对NAD ⁺的简单定量方法的需求，最适合在护理时使用。在这里，我们介绍一种生物发光生物传感器，用于快速定量生物样品中NAD ^+的水平，可在实验室或护理现场使用。生物传感器是一种半合成的发光传感器蛋白，在结合NAD ^+时将发射光的颜色从蓝色变为红色。这种NAD ⁺依赖性的颜色变化使该生物传感器可用于基于纸张的测定，在该测定中，通过使用简单的数码相机或酶标仪测量发射光的颜色来定量NAD ⁺。我们使用这种方法来量化NAD ⁺细胞培养物，组织和血液样品中的水平，其结果与标准测试方法的结果一致。相同的生物传感器还允许量化血样中NAD ⁺依赖性酶的活性，从而扩展了其作为即时诊断的工具的效用。