Fumonisins are carcinogenic mycotoxins that are frequently found as natural contaminants in maize from warm climate regions around the world. The aminotransferase FumI is encoded as part of a gene cluster of Sphingopyxis sp. MTA144, which enables this bacterial strain to degrade fumonisin B-1 and related fumonisins. FumI catalyzes the deamination of the first intermediate of the catabolic pathway, hydrolyzed fumonisin B-1. We used a preparation of purified, His-tagged FumI, produced recombinantly in Escherichia coli in soluble form, for enzyme characterization. The structure of the reaction product was studied by NMR and identified as 2-keto hydrolyzed fumonisin B-1. Pyruvate was found to be the preferred co-substrate and amino group receptor (K (M) = 490 mu M at 10 mu M hydrolyzed fumonisin B-1) of FumI, but other alpha-keto acids were also accepted as co-substrates. Addition of the co-enzyme pyridoxal phosphate to the enzyme preparation enhanced activity, and saturation was already reached at the lowest tested concentration of 10 mu M. The enzyme showed activity in the range of pH 6 to 10 with an optimum at pH 8.5, and in the range of 6A degrees C to 50A degrees C with an optimum at 35A degrees C. The aminotransferase worked best at low salt concentration. FumI activity could be recovered after preincubation at pH 4.0 or higher, but not lower. The aminotransferase was denatured after preincubation at 60A degrees C for 1 h, and the residual activity was also reduced after preincubation at lower temperatures. At optimum conditions, the kinetic parameters K (M) = 1.1 mu M and k (cat) = 104/min were determined with 5 mM pyruvate as co-substrate. Based on the enzyme characteristics, a technological application of FumI, in combination with the fumonisin carboxylesterase FumD for hydrolysis of fumonisins, for deamination and detoxification of hydrolyzed fumonisins seems possible, if the enzyme properties are considered.