Sesamin is one of the major lignans found in sesame oil. Although some microbial metabolites of sesamin have been identified, sesamin-metabolic pathways remain uncharacterized at both the enzyme and gene levels. Here, we isolated microorganisms growing on sesamin as a sole-carbon source. One microorganism showing significant sesamin-degrading activity was identified as Sinomonas sp. no. 22. A sesamin-metabolizing enzyme named SesA was purified from this strain and characterized. SesA catalyzed methylene group transfer from sesamin or sesamin monocatechol to tetrahydrofolate (THF) with ring cleavage, yielding sesamin mono-or di-catechol and 5,10-methylenetetrahydrofolate. The kinetic parameters of SesA were determined to be as follows: K m for sesamin = 0.032 ± 0.005 mM, V max = 9.3 ± 0.4 (μmol·min), and k cat = 7.9 ± 0.3 s −1. Next, we investigated the substrate specificity. SesA also showed enzymatic activity toward (+)-episesamin, (−)-asarinin, sesaminol, (+)-sesamolin, and piperine. Growth studies with strain no. 22, and Western blot analysis revealed that SesA formation is inducible by sesamin. The deduced amino acid sequence of sesA exhibited weak overall sequence similarity to that of the protein family of glycine cleavage T-proteins (GcvTs), which catalyze glycine degradation in most bacteria, archaea, and all eukaryotes. Only SesA catalyzes C1 transfer to THF with ring cleavage reaction among GcvT family proteins. Moreover, SesA homolog genes are found in both Gram-positive and Gram-negative bacteria. Our findings provide new insights into microbial sesamin metabolism and the function of GcvT family proteins.sesamin | metabolism | lignan | tetrahydrofolate W e have been involved in studies of not only microbial metabolism of man-made compounds (1-3) but also biologically active natural compounds, such as curcumin (4). In this study, we characterize the microbial metabolism of the lignan sesamin.Lignans (5) are plant-derived compounds consisting of dimers of phenylpropane units (6). They are found in a wide variety of plant-based foods. Whole-grain products, vegetables, fruits, nuts, seeds, and beverages such as tea, coffee, and wine are dietary sources of lignans. In Asian countries, sesame, which contains lignans, is used traditionally as a food. A major lignan is sesamin, which is a biologically active compound with antioxidative (7), cholesterol-lowering (8), lipid-lowering (9), antihypertensive (10), and antiinflammatory (11) properties.In humans, sesamin is metabolized by CYP450 enzymes into sesamin mono-and di-catechol in liver microsomes (12). Sesamin monocatechol is metabolized further by UDP-glucuronosyltransferase (UGT) and O-methyl transferase (COMT) (13), and the resulting glucuronides of sesamin metabolites are excreted in the bile and urine (14).In microorganisms, on the other hand, metabolism of sesamin has been reported in few species. Aspergillus oryzae converts sesamin to sesamin mono-and di-catechol (15) whereas intestinal bacteria convert sesamin to the so-called "mammalian lignans" e...