Purification and properties of a thiol protease from rat liver nuclei

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Abstract
A thiol protease was purified about 800-fold from the chromatin fraction of rat liver by employing Sepharose 6B gel filtration, chromatofocusing and Sephadex G-100 gel filtration. It was nearly homogeneous on sodium dodecyl sulfate/polyacrylamide gel electrophoresis and its MW was about 29,000. The isoelectric point of the enzyme was 7.1. The pH optimum for degradation of 3H-labeled ribosomal proteins was 4.5. The maximal activity was shifted to pH 5.5 by DNA, and 30-40% of the maximal activity was observed at neutral pH in the presence of DNA. The activity was increased about twice by 2-4mM dithiothreitol. The protease may be specific for the nuclei because it is different from all lysosomal thiol proteases ever known.