A sensitive fluorometric assay to detect aldo-keto reductase and carbonyl reductase activity based on a naphthaldehyde derivative

We have developed a fluorometric assay for detecting reductase activity in biological samples through 4-methoxy-1-naphthalenemethanol (MONOL-41) formation. The enzyme carbonyl reductase 1 (CBR1) and four members of the aldo-keto reductase (AKR) 1 family (AKR1A1, AKR1B1, AKR1B10, AKR1C3) were evaluated for their ability to reduce 4-methoxy-1-naphthaldehyde (MONAL-41). AKR1B1 and CBR1 followed Michaelis-Menten kinetics, whereas AKR1B10, AKR1A1, and AKR1C3 showed substrate inhibition above 10 {micro}M (70 {micro}M for AKR1C3). Among the tested enzymes, AKR1B10 displayed the highest catalytic efficiency in the absence of substrate inhibition. The MONOL-41 assay was compared with the standard NADPH-based method, showing improved sensitivity, robustness, and lower detection limits (0.77 {micro}g/mL vs. 1.49 {micro}g/mL). These results confirm its suitability for monitoring AKR1B10 activity. The assay was then applied to A549 cell extracts, which express multiple reductases. Activity decreased at substrate concentrations above 10 {micro}M, suggesting a predominant role of AKR1B10. Inhibition studies using tolrestat and high MONAL-41 concentrations indicated a limited contribution of CBR1 ([~]7-8%). Considering both catalytic efficiency and expression levels, AKR1B10 appears to be the main contributor to reductase activity in this model. In A549 living cells, MONAL-41 showed no cytotoxicity up to 50 {micro}M and enabled real-time monitoring due to its membrane permeability. However, oxidation by aldehyde dehydrogenases can generate MONOIC-41, which has similar spectral properties but a lower quantum yield, potentially affecting signal interpretation. Overall, this assay represents a sensitive and cost-effective tool for detecting reductase activity and screening inhibitors.