Alcohol dehydrogenase 4 and aldo-keto reductase 1A1 catalyze the oxidation of 4-hydroxytolbutamide, a metabolite of tolbutamide, in the human liver.

Tolbutamide is metabolized by cytochrome P450 2C9 into 4-hydroxytolbutamide (4-OH TB), which retains pharmacological activity. 4-OH TB is further oxidized to the inactive metabolite 4-carboxytolbutamide, likely via the intermediate tolbutamide aldehyde (4-CHO TB). Because the conversion of 4-OH TB to 4-CHO TB is considered the rate-limiting step, the enzyme(s) catalyzing this reaction may play a crucial role in determining drug efficacy. We aimed to identify the enzyme(s) responsible for this process in the human liver. 4-CHO TB was formed from 4-OH TB in human liver cytosol (HLC) in the presence of nicotinamide-adenine dinucleotide (NAD+). The relative activity factor approach revealed that this reaction was primarily attributed to alcohol dehydrogenase 4 (ADH4). Interestingly, 4-CHO TB was also formed in HLC in the presence of nicotinamide-adenine dinucleotide phosphate (NADP+), with a 1.6-fold higher intrinsic clearance than that of NAD+. Untargeted proteomic analysis revealed a significant correlation between aldo-keto reductase 1A1 (AKR1A1) protein levels and NADP+-dependent 4-CHO TB formation in 15 HLC samples (r = 0.627, P < .05). Recombinant AKR1A1 effectively catalyzed this reaction, contributing 92% of NADP+-dependent 4-CHO TB formation in HLC. Based on hepatic NAD+ and NADP+ concentrations and the expression levels of ADH4 and AKR1A1, AKR1A1 was estimated to contribute one-third of ADH4 to 4-CHO TB formation in the human liver. In conclusion, we demonstrated that ADH4 and AKR1A1 jointly mediate the oxidation of 4-OH TB to 4-CHO TB in the human liver, highlighting the novel role of AKR1A1 as an oxidase in drug metabolism. SIGNIFICANCE STATEMENT: This study identifies aldo-keto reductase 1A1 as a novel enzyme involved in the oxidation of 4-hydroxytolbutamide in the human liver. Alongside alcohol dehydrogenase 4, aldo-keto reductase 1A1 contributes to NADP+-dependent aldehyde formation, suggesting a previously unrecognized role in drug metabolism and variability in tolbutamide clearance.