Two different enzymatic activities hydrolyzing 2',3'-cyclic phosphates of purine and pyrimidine ribonucleosides to give 2'- or 3'-ribonucleotides have been detected in plasma membranes of the chick chorioalantois. The enzymes differ in their temperature optima: At lower temperatures, cleavage to the 2'-isomers predominates.

Ribonuclease activiy was found associated with plasma membranes isolated from chick embryo chorioallantoic cells. The enzyme was solubilized with buffered 1% Triton X-100 and purified 40-fold by 1-butanol extraction and gel-filtration on Sephadex G-100. The purified Rnase was found to be free of deoxyribonuclease, alkaline phosphatase and decyclizing 2',3'-phosphodiesterase activities. the enzyme is capable to degrade RNA and polycytidylic acid into acid-soluble oligonucleotides terminated in 3'-phosphate. No nucleoside monophosphates and 2',3'-cyclic nucleoside monophosphates were detected. The optimal pH of RNA and poly C hydrolysis were 7.2 and 7.8, the optimal temperatures 60 and 45°C, respectively. The purified enzyme is thermolabile, and it requires monovalent cations for the full enzyme activity.

The method of isolating plasma membranes from chick embryo chorioallantoic membranes is described along with some morphological and chemical properties of these membrane preparations. Besides 5'-nucleotidase, alkaline phosphodiesterase, alkaline phosphatase, ribonucleaseendonuclease and 2',3'-cyclic nucleotide 3'-phosphohydrolase activities, the plasma membranes possesses a high activity of 2',3'-cyclic nucleotide 2'-phosphohydrolase. They exert negligible activities for glucose 6-phosphatase and succinate dehydrogenase.