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Aldehyde oxidase 1 (AOX1) is a molybdenum-containing enzyme that catalyzes the oxidation of a range of aldehyde compounds and clinical drugs, including azathioprine and methotrexate. The purpose of this study was to elucidate the effects of single-nucleotide polymorphisms (SNPs) in the coding regions of the human gene on protein dimer formation and metabolic activity. Six variants (Q314R [rs58185012], I598N [rs143935618], T755I [rs35217482], A1083G [rs139092129], N1135S [rs55754655], and H1297R [rs3731722]), with allele frequencies greater than 0.01 in 1 or more population, were obtained from the genome aggregation and 1000 Genomes project databases. Protein expression and dimer formation were evaluated using HEK293T cells expressing the wild-type (WT) or different SNP variants of AOX1. Kinetic analyses of phthalazine oxidation were performed using S9 fractions of HEK293T cells expressing WT or each the different mutant AOX1. Although we detected no significant differences among WT AOX1 and the different variants with respect to total protein expression, native PAGE analysis indicated that one of the SNP variants, T755I, found in East Asian populations, dimerizes less efficiently than the WT AOX1. Kinetic analysis, using phthalazine as a typical substrate, revealed that this mutation contributes to a reduction in the maximal rates of reaction without affecting enzyme affinity for phthalazine. Our observation thus indicates that the T755I variant has significantly negative effects on both the dimer formation and catalytic activity of AOX1. These findings may provide valuable insights into the mechanisms underlying the inter-individual differences in the therapeutic efficacy or toxicity of AOX1 substrate drugs. The T755l (rs35217482) SNP variant of the AOX1 protein, which is prominent in East Asian populations, suppresses protein dimer formation, resulting in a reduction in the reaction velocity of phthalazine oxidation to less than half of that of wild-type AOX1.