Evidence that β-Galactosidase of <i>Sulfolobus solfataricus</i> Is Only One of Several Activities of a Thermostable β- <scp>d</scp> -Glycosidase

<jats:p> A survey of <jats:italic>Sulfolobus</jats:italic> isolates showed all to contain thermostable enzyme activities hydrolyzing various glycosidic compounds. Of those not previously reported, the β-glucosidase activity of <jats:italic>Sulfolobus solfataricus</jats:italic> isolate P2 was chosen for further study and found to have the same kinetics of inactivation, apparent molecular weight, and many (though not all) other biochemical properties of the β-galactosidase also present in this strain. The two activities copurified approximately 850-fold to apparent homogeneity. The enzyme, whose subunit <jats:italic>M</jats:italic> <jats:sub>r</jats:sub> was estimated to be 60,000 to 65,000 by gel permeation chromatography of the active enzyme and 70,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the denatured form, hydrolyzed a variety of low-molecular-weight, β-linked glycosides and could account for most of the corresponding activities found in crude extract. Kinetic analyses indicated that chromogenic β- <jats:sc>d</jats:sc> -galactosides and β- <jats:sc>d</jats:sc> -glucosides are hydrolyzed at a common active site and that β-glucosides and β-fucosides represent the preferred substrates. The liberation of aglycone from aryl β- <jats:sc>d</jats:sc> -glucosides was stimulated by alcohols in a manner suggesting specific interaction between alcohol and enzyme. </jats:p>