The synthesis of the mixture of the two diastereomers of guanosine 2 ',3 '-cyclophosphorothioate is described. Their chemical shifts in the 31P nuclear magnetic resonance (nmr) are identical with those of the corresponding isomers of uridine 2',3'-cyclophosphorothioate ( -75.22 ppm for the endo, -76.75 ppm for the exo isomer). From the unseparable mixture of isomers of guanosine 2 ',3 '-cyclophosphorothioate only the endo isomer is hydrolyzed by ribonuclease Ti to guanosine 3 '-phosphorothioate, without loss of sulfur.The resistant exo isomer is a competitive inhibitor ( = 2.5 X 10-4 m) for this enzyme. On enzymatic methanolysis of the mixture of isomers only the endo isomer reacts to r> X Vábonuclease Ti hydrolyzes guanosine 3 '-phosphate esters in a two-step mechanism to guanosine 3'-phosphate with guanosine 2',3'-cyclophosphate as intermediate. Apart from the specificity for guanosine instead of pyrimidine nucleosides it resembles pancreatic ribonuclease A in the overall mechanism. In contrast to pancreatic ribonuclease A, how-PHA-630 T, a glass electrode G-2222c, a calomel electrode K-401 and an autoburet ABU-la. The consumption of base was recorded on a titrigraph SBR-2c. The temperature was held at 30°. The enzyme was used as a 9.6 X 10-4 m solution in 0.2 m NaCl adjusted to pH 7.0. Substrate and inhibitor were also dissolved in 0.2 m NaCl. Microliter quantities of substrate and, where needed, inhibitor solutions were added to 2 ml of 0.2 m NaCl solution (pH 7.0), and the pH was kept at 7.0 with 0.01 n KOH for 1 hr. The reaction was then started by addibiochemistry, vol.
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