Thermostable β‐galactosidase from the archaebacteriumSulfolobus solfataricusPurification and properties

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Abstract
A thermophilic and thermostable β‐galactosidase activity was purified to homogeneity from crude extracts of the archaebacteriumSulfolobus solfataricus, by a procedure including ion‐exchange and affinity chromatography. The homogeneous enzyme had a specific activity of 116.4 units/mg at 75°C witho‐nitrophenyl β‐galactopyranoside as substrate. Molecular mass studies demonstrated that theS. solfataricusβ‐galactosidase was a tetramer of 240 ± 8 kDa composed of similar or identical subunits. Comparison of the amino acid composition of β‐galactosidase fromS. solfataricuswith that fromEscherichia colirevealed a lower cysteine content and a lower Arg/Lys ratio in the thermophilic enzyme. A rabbit serum, raised against the homogeneous enzyme did not crossreact with β‐galactosidase fromE. coli.The enzyme, characterized for its reaction requirements and kinetic properties, showed a thermostability and thermophilicity notably greater than those reported for β‐galactosidases from other mesophilic and thermophilic sources.