Isolation and characterization of a ribonuclease from human leukemic blood cells specific for ribonucleic acid of ribonucleic acid deoxyribonucleic acid hybrid molecules

A ribonuclease that specifically hydrolyzes RNA in RNA DNA hybrids has been purified more than 100 fold from human acute leukemic white blood cells. The mol wt of this enzyme has been estimated as 80,000 by glycerol gradient centrifugation. It requires Mg²⁺ for activity and is inhibited by N ethylmaleimide. The optimum activity is observed at pH 8 (37°). It is a heat labile protein, t(1/2) at 50° being 2 min. Among the substrates examined, (A)(n) x (dT)(m), (I)(n) x (dC)(m), and phi chi 174 DNA x RNA were hydrolyzed efficiently. (U)(n) x (dA)(m) showed a slight substrate activity, while (C)(n) x (dG)(m) and (G)(n) x (dC)(m) were not significantly hydrolyzed. The enzyme is an endonuclease and does not require RNA ends in the substrate molecule. It is capable of converting more than 95% of the RNA portions in hybrid substrates into acid soluble products which are mono and oligonucleotides terminated in 3' OH and 5' phosphate.