Phenotyping of flavin-containing monooxygenase using caffeine metabolism and genotyping of FMO3 gene in a Korean population

Flavin-containing monooxygenase (FMO) activity was determined in 82 Korean volunteers by taking molar concentration ratio of theobromine and caffeine present in the 1 h urine (between 4 and 5 h) samples collected after admini stration of a cup of coffee containing 110 mg of caffeine. Among 82 volunte ers, there were 1.9 women and 63 men (30 smokers and 52 non-smokers), Volun teers were divided into two groups comprising low (0.53-2.99) and high (3.1 8-11.95) FMO activities separated by an antimode of 3.18. peripheral bloods were sampled from these volunteers and their genomic DNAs were amplified b y polymerase chain reaction with oligonucleotides designed from intronic se quences of human FMO3 gene. Comparing nucleotide sequences of the amplified FMO3 gene originating from randomly selected individuals with low and high FMO activities, nine point mutations were identified in the open reading f rame sequences. Among these nine mutations, three FMO3 mutant types (FMO3/S top(148), Lys(158) and Gly(308)) were selected and correlated with FMO acti vities observed in our Korean population. A rare FMO3/Stop(148) mutant alle le originating from FMO3/Gly(148) occurred by substitution of G442T in exon 4 and yielded a premature TGA stop codon. The stop codon was detected in o ne individual having the second lowest FMO activity and he had the mutation in heterozygous state. In a pedigree study, he was found to have inherited the mutation from his mother who also had a heterozygous stop codon and eq ually low FMO activity. In our volunteers, two other common mutations were detected in exons 4 and 7. The one in exon 4 resulted from a G472A change e liminating a HinfI restriction site and produced an amino acid substitution from Glu(158) to Lys The other mutation in exon 7 resulted from an A923G c hange generating a DraII restriction site and produced a non-conservative r eplacement of Glu(308) to Gly. Based on the secondary structure maps of FMO 3 enzyme proteins for these two mutant types, FMO3/Gly(308) mutation transf ormed the helix structure into a sheet shape and indicated that dysfunction al FMO3 may be produced. FMO3/Lys(158) mutation did not alter the secondary structure. Approximately 80% of volunteers with homozygous and/or heterozy gous mutations on either one or two of these mutations had low FMO activiti es. Thus, individuals with these FMO3 gene mutations may have defective met abolic activity for many clinically used drugs and dietary plant alkaloids which are oxidized primarily by hepatic FMO3. Pharmacogenetics 9:155-164 (C ) 1999 Lippincott Williams & Wilkins.