This is the first report describing the purification and enzymatic properties of a native invertase (beta-D-fructosidase) in Thermotogales. The invertase of the hydrogen-producing thermophilic bacterium Thermotoga neapolitana DSM 4359 (hereby named Tni) was a monomer of about 47 kDa having an amino acid sequence quite different from other invertases studied up to now. Its properties and substrates specificity let us classify this protein as a solute-binding protein with invertase activity. Tni was specific for the fructose moiety and the enzyme released fructose from sucrose and raffinose and the fructose polymer inulin was hydrolyzed in an endo-type fashion. Tni had an optimum temperature of 85 degrees C at pH 6.0. At temperatures of 80-85 degrees C, the enzyme retained at least 50% of its initial activity during a 6 h preincubation period. Tni had a K(m) and k(cat)/K(m) values (at 85 degrees C and pH 6.0) of about 14 mM and 5.2 x 10(8) M(-1) s(-1), respectively.