Nicotinamide adenine dinucleotide (NAD+) is a pivotal coenzyme, essential for cellular reactions, metabolism, and mitochondrial function. Depletion of kidney NAD+ levels and reduced de novo NAD+ synthesis through the tryptophan-kynurenine pathway are linked to acute kidney injury (AKI), while augmenting NAD+ shows promise in reducing AKI. We investigated de novo NAD+ biosynthesis using in vitro, ex vivo and in vivo models to understand its role in AKI. 2D cultures of human primary renal proximal tubule epithelial cells (RPTECs) and HK-2 cells showed limited de novo NAD+ synthesis, likely due to low pathway enzyme gene expression. Employing 3D spheroid culture model improved the expression of tubular-specific markers and enzymes involved in de novo NAD+ synthesis. However, de novo NAD+ synthesis remained elusive in the 3D spheroid culture, regardless of injury conditions. Further investigation revealed that 3D cultured cells couldn't metabolize tryptophan (Trp) beyond kynurenine (KYN). Intriguingly, supplementation of 3-hydroxyanthrilinic acid into RPTEC spheroids was readily incorporated into NAD+. In a human precision-cut kidney slice (PCKS) ex vivo model, de novo NAD+ synthesis was limited due to substantially downregulated kynurenine 3-monooxygenase (KMO), which is responsible for KYN to 3-hydroxykynurenine conversion. KMO overexpression in RPTEC 3D spheroids successfully reinstated de novo NAD+ synthesis from Trp. Additionally, in vivo study demonstrated that de novo NAD+ synthesis is intact in the kidney of the healthy adult mice. Our findings highlight disrupted tryptophan-kynurenine NAD+ synthesis in in vitro cellular models and an ex vivo kidney model, primarily attributed to KMO downregulation.