The effects of polyamines on a residue-specific human plasma ribonuclease.

A ribonuclease, purified some 2700-fold from human plasma, exhibited a strong predilection for the hydrolysis of internucleotide bonds containing cytidylic acid. Analysis of [3'-32P]- and [5'-32P]phosphoryl-terminal fragments obtained after enzymic digestion of rabbit liver and yeast RNA indicated that the nucleotide found at the 3' terminus of the fragments was invariably cytidylic acid. The nucleotide at the 5' terminus varied between cytidylic and uridylic acids in a ratio of 9:1. When characterized by DEAE-cellulose chromatography, approximately 70 per cent of the digest consisted of oligonucleotides from 4 to 8 nucleotides in length. Enzyme activity, when measured in low ionic strength buffer, could be increased severalfold above control levels by the addition of either of the polyamines, spermidine or spermine. These substances also restored nucleolytic activity to preparations inhibited by such ordered synthetic polyribonucleotides as polyguanylic acid. Estimations of the molecular weight of the enzyme, both by Sephadex gel filtration and sucrose density centrifugation, indicate that the weight may vary, depending on the presence or absence of certain cations. Of the cations examined, spermidine and spermine appear to have the greatest effect, causing an alteration in molecular weight from greater than 150,000 to approximately 32,000.