Cyclic AMP-adenosine binding protein/S-adenosylhomocysteinase from mouse liver. A fraction of adenosine bound is converted to adenine.

1. Adenosine bound to the cyclic AMP-adenosine binding protein/S-adenosylhomocysteinase from mouse liver was partly converted to a product which was identified as adenine in four chromatographic systems. Ribose was formed in equivalent amounts. 2. The time course of the reaction was characterized by an initial burst phase lasting for less than one second followed by a slow progressive phase. The reaction was partly reversed by prolonged incubation, slow denaturation of the protein, dilution of the incubation mixture and removal of adenosine by converting it to inosine by the enzyme adenosine deaminase. 3. Both the ATP-treated (Ueland, P.M. and Døskeland, S.O. (1978) Arch. Biochem. Biophys. 185, 195--203) and the non-treated protein were subjected to polyacrylamide gel electrophoresis at pH 8.8. The adenosine-adenine, the cyclic AMP binding activities and the conversion activity comigrated with the main protein band, indicating that these properties reside on the same protein molecule. 4. Adenine generated by hydrolysis of adenosine was mainly bound to the protein as judged by nearly complete reversion of the conversion upon dilution in the presence of excess unlabelled adenine and by Sephadex G-25 chromatography. 5. The conversion of adenosine to inosine by the enzyme adenosine deaminase was decreased in the presence of the binding protein. 6. Adenine formation could also be demonstrated under condition of enzymic formation of S-adenosylhomocysteine, i.e. in the presence of hymocysteine.