ID 4.6.1.23 DE ribotoxin. AN alpha-sarcin. AN rRNA endonuclease. CA a 28S rRNA containing guanosine-adenosine pair + H2O = an [RNA fragment]- CA 3'-adenosine-3'-phosphate + a 5'-a hydroxy-guanosine-3'-[RNA fragment]. CC -!- Ribotoxins are rRNA endonucleases that catalyze the cleavage of the CC phosphodiester bond between guanosine and adenosine residues at one CC specific position in 28S rRNA. CC -!- The enzyme secreted by Aspergillus giganteus specifically cleaves rat CC 28S rRNA between G(4325) and A(4326) and displays cytotoxic activity CC toward animal cells. CC -!- It can also act on bacterial rRNAs. CC -!- The enzyme catalyzes a two-stage endonucleolytic cleavage. CC -!- The first reaction produces 5'-hydroxy-phosphooligonucletides and CC 3'-phosphooligonucleotides ending with 2',3'-cyclic phosphodiester, CC which are released from the enzyme. CC -!- The enzyme then hydrolyzes these cyclic compounds in a second CC reaction that takes place only when all the susceptible CC 3',5'-phosphodiester bonds have been cyclised. CC -!- The second reaction is a reversal of the first reaction using the CC hydroxyl group of water instead of the 5'-hydroxyl group of ribose. CC -!- The overall process is that of a phosphorus-oxygen lyase followed by CC hydrolysis to form the 3'-nucleotides. CC -!- Formerly EC 3.1.27.10. DR C0HLG3, RNAG_CYCAE ; P00655, RNAS_ASPGI ; //