Acid ribonuclease (RNase) was extracted from mycelia of
Aspergillus niger 1617 and separated into two fractions designated acid RNases I (EC 3.1.27.1) and II (EC 3.1.27.3). Each fraction was partially purified and its enzymatic properties were examined. The molecular weight of acid RNase I was approximately 45, 000, and that of II was 13, 000. The optimal pH of acid RNase I was 3.5 and that of II, 4.5. Acid RNase I was inhibited markedly by Fe
3+, Cu
2+, Hg
2+ and 5′-ATP, whereas II was inhibited markedly by Fe
3+. Acid RNase I had no base specificity producing 3′-monophosphates of the four nucleosides and guanosine-2′, 3′-cyclic monophosphate. Acid RNase II was a guanine-specific RNase.
Changes in the activity of acid RNases I and II were studied during culture on agar medium. The activity of acid RNases I and II increased and reached their maxima during the late phase of active growth. After their maxima, the activity of acid RNase I remained unchanged, while that of II decreased and finally disappeared.
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