Properties of a human liver ribonuclease. Inhibition by polynucleotides and specificity for phosphodiester bond cleavage to yield purine nucleosides at the 5' termini.

A ribonuclease, purified 2500-fold from human liver, was found to be inactive against synthetic homopolynucleotides, whereas synthetic co-polymers containing adenylic acid were rapidly degraded. The specificity of the RNase is unique in that only purine residues, in a 5:4 ratio of guanylic to adenylic acid, are found at the 5' termini of the degradation products of yeast RNA. No specificity was observed at the 3' termini of the fragments. When analyzed by DEAE-cellulose chromatography, approximately 80% of the oligonucletoides were 4 to 11 residues in length. The hydrolysis of RNA by the liver enzyme, when examined in low ionic strength buffer, could be increased severalfold over control levels by the addition of polyamines. The enzyme was found to exist as two distinct species on sucrose gradients, with molecular weights of 128,000 and 14,000. However, the addition of spermidine to the gradients resulted in the recovery of all the enzyme activity as the smaller species. The polyamines were also shown to reverse the inhibition of the enzyme by the ordered polynucleotides, polyguanylic acid and polyadenylic acid. Inhibition of enzyme activity by the polyadenylic acid segment of various mammalian mRNAs was also demonstrated.