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The chemical components of ribonuclease L (RNase L) from Aspergillus sp. were tested in order to clarify the structure of the enzyme. The molecular weight and the number of the total anuno acid residues of the enzyme were about 4.5 × 104 and 393, respectively. The amino- and carboxyl-terminal amino acids of the enzyme were determined to be glycine and threonine, respectively. RNase L contained about 6% of neutral sugars and an glycopeptide was obtained by digestion with pronase. The isoelectric point of the enzyme was 3.47.

Inhibitory study of RNase L with ferric ion was performed to clarify the function of the enzyme. The maximal inhibitory pH of ferric ion for the enzyme is in 3.5 ~ 5.0 and the type of inhibition is found to be competitive with the substrate. The Km for RNA and Ki of ferric ion are 1.54 × 10−1 mg/ml and 22.5 µM, respectively.As the recovery of activity from iron-inactivated RNase L is almost completely realized by dialysis against EDTA solution, the inhibition of RNase L with ferric ion is considered to be a reversible reaction. Moreover, the enzyme is rapidly inactivated by methylene blue- or rose bengal-sensitized photooxidation, but the photoinactivation of RNase L is protected in the presence of the competitive inhibitor, ferric ion. It may reasonably be taken as an evidence that ferric ion attacks the active site of the enzyme.