Ribonuclease

I.U.B.: 3.1.27.5
Ribonucleate 3'-pyrimidino-oligonucleotidohydrolase

Pancreatic ribonuclease (RNase) also called "Ribonuclease 1", catalyzes cleavage of the phosphodiester bond between the 5'-ribose of a nucleotide and the phosphate group attached to the 3'-ribose of an adjacent pyrimidine nucleotide forming a 2',3'-cyclic phosphate which may then be hydrolyzed to the corresponding 3'-nucleoside phosphate.

RNase is found in greatest quantity in ruminant pancreas (Barnard, 1969). A comprehensive review of RNase is that by Richards and Wyckoff (1971). The major component of the crystalline enzyme is RNase A; a minor component is RNase B which is a glycoprotein. See Beintema et al. (1976).

Characteristics of RNase A from Bovine Pancreas:

Molecular weight: 13,700 (Hirs et al. 1956).

Composition: The composition and structure of ribonuclease A has been extensively investigated. The tertiary structure was reported by Kartha et al. (1967). See also Lin (1970). Wang et al. (1975) indicate that flexibility in a three-dimensional structure is essential for optimal catalytic activity.

Optimum pH: 7.0 - 7.5 (Brown and Todd 1955).

Extinction coefficient: = 7.3 (Worthington RNase A).

Isoelectric point: pH 9.45 (Richard and Wyckoff 1971).

Specificity: The enzyme is specific for pyrimidine nucleoside linkages, i.e., those cleaving cyclic 2',3'-pyrimidine nucleotide residues either singly or at the termination of purine nucleotide chains (Volkin and Cohn 1953).

Inhibitors: Ribonuclease is inhibited by heavy metal ions and is competitively inhibited by DNA. The effect of denatured DNA is much greater than that of the native nucleic acid (Sekine et al. 1969).

Stability: The enzyme is stable for years stored as a refrigerated dry powder or frozen in solution; refrigerated solutions are stable for weeks. The enzyme aggregates upon lyophilization and storage (Fruchter and Crestfield 1965). Special care should be given to handling of the enzyme because of its affinity for glass surfaces (Hummel and Anderson 1965).

Characteristics of RNase B from Bovine Pancreas:

Molecular weight: 14,700 ± 300 (Plummer and Hirs 1963).

Composition: RNase B is a glycoprotein which possesses an amino acid composition indistinguishable from that of RNase A and which contains carbohydrate to the extent of 6 residues of mannose and 2 residues of N-acetylglucosamine per molecule. It is consequently considered to be a carbohydrate derivative of RNase A. (Tarentino et al. 1970).

Properties: RNase B has the same specificity as RNase A toward both cyclic cytidylate and yeast RNA (Plummer and Hirs 1963).