The metabolites of NAD synthesis are a valuable predictor of acute kidney injury.

Background: Acute kidney injury (AKI) is often iatrogenic and potentially preventable. Reduced renal nicotinamide adenine dinucleotide (NAD) is reported to increase the susceptibility of AKI. The present study explored the predictive value of urinary NAD synthetic metabolites for AKI using two independent cohorts.

Methods: The expression of NAD synthetic enzymes in human kidney was examined by immunohistochemistry and single-cell transcriptomes. Urine samples were collected from two independent cohorts: the methotrexate (MTX) cohort with high-dose MTX treatment for lymphoma ( = 189) and the liver transplantation cohort with orthotopic liver transplantation ( = 49). Urinary metabolomics study of NAD synthesis was performed by liquid chromatography with mass spectrometry, screening for AKI predictive biomarkers. Nephroseq database and immunohistochemistry were used to analyze kidney NAD synthetic enzymes expression in AKI-susceptible conditions.

Results: Human proximal tubule was the main structure in the kidney that expressed the necessary enzymes for NAD synthesis. In the MTX cohort, the urinary quinolinic acid (QA)/3-hydroxyanthranilic acid (3-OH AA) ratio before chemotherapy was significantly lower in those who developed AKI after chemotherapy compared with those who did not. This finding was consistent in the liver transplantation cohort. The area under the receiver-operating characteristic curve (AUC) of urinary QA/3-OH AA for AKI prediction was 0.749 and 0.729 in two cohorts, respectively. 3-Hydroxyanthranilic acid dioxygenase (HAAO), the enzyme catalyzing QA synthesis from 3-OH AA, decreased in AKI-susceptible diabetic kidneys.

Conclusions: The human proximal tubules were important source of NAD from the pathway. Reduced urinary QA/3-OH AA ratio, which possibly suggested decreased HAAO activity, could be a potential AKI predictive biomarker.