S-Adenosylhomocysteine hydrolase from the Thermophilic Archeon Sulfolobus solfataricus

S-Adenosylhomocysteine hydrolase from Sulfolobus solfataricus, a thermoacidophilic archaeon optimally growing at 87°C, has been purified to homogeneity. The specific activity of the homogeneous enzyme is 161 nmol of S-adenosylhomocysteine formed per min per mg of protein, and the overall yield, by immunoaffinity purification, is 51%. The enzyme has a molecular mass of 190 kDa, is composed of four identical subunits (subunit mass 47 kDa), and contains four molecules of tightly-bound NAD+ per tetramer of which about 40% is in the reduced form. Physico-chemical features, including amino-acid composition and secondary structure, are reported. The pure protein, used to raise specific rabbit antisera, shows immunological properties different from other S-adenosylhomocysteine-metabolizing enzymes. The enzyme is thermophilic with an optimum temperature of 75°C, and shows an apparent melting temperature of 95°C by measuring its residual activity after 10 min incubation at increasing temperatures.