An Exonuclease from Brain, Purification and Properties

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Abstract
1. pH Dependence of RNA degrading activity of brain homogenate revealed a peak near pH6. 2. Starting from acetone powder of beef brain, the activity at pH 6 was purified by extraction with 0.15 M sodium chloride, ammonium sulfate fractionation, pH 4 treatment, acetone treatment, and DEAEcellulose column chromatography. Further purification by zone electrophoresis was also performed when desirable. 3. The purified enzyme had a pH optimum at 6.2, required no metal ions, and was relatively unstable to heat and acid treatments. 4. The purified enzyne hydrolyzed RNA completely to acid-soluble material. The products were 3′-adenylic acid, 3′-guanylic acid, and pyrimidine nucleotides, probably bearing their phosphate group in the 3′ position. 5. Thymidylyl-3′: 5′-thymidine was hydrolyzed by the enzyme and formed 3′-thymidylic acid and thymidine. 3′-Thymidylic acid was identified enzyrnatically by venom 5′-nucleotidase [EC 3. 1.3.5]. Denatured DNA, ribonucleoside-2′, 3′ cyclic phosphates, and di(p-nitrophenyl) phosphate were not attacked.

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