S-Adenosylhomocysteinase from mouse liver. Inactivation of the enzyme in the presence of metabolites.

1. S-Adenosylhomocysteinase (S-adenosylhomocysteine hydrolase, EC 3.3.1.1) was slowly inactivated in the presence of adenine and adenine nucleotides (Ueland & Saebø, 1979b). 2. The enzyme was stabilized by 2-mercaptoethanol and dithiothreitol, and was slowly inactivated at 37 degrees C in the absence of reducing agents and rapidly inactivated in the presence of 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB). The inactivation (both in the absence and presence of DTNB) was partly prevented by adenine, AMP and ADP. 3. A slow decrease in enzyme activity was observed in the presence of AMP, ADP and ATP, and this process was enhanced by sulfhydryl compounds like L-homocysteine, L-cysteine, 2-mercapthoethanol and dithiothreitol. 4. Inactivation of the enzyme by adenine was independent of sulfhydryl compounds, and was characterized by an initial phase showing first-order kinetics and saturability with respect to adenine. 5. Inorganic phosphate nearly abolished the inactivation of S-adenosylhomocysteinase induced by both adenine nucleotides and adenine. 6. The enzyme activity was recovered when adenine was removed by dilution or gel filtration. Attempts to reverse the effect of adenine nucleotides on the enzyme were not successful. 7. The effect of adenine nucleotides was a Vmax-effect, and the inactivation was not associated with dissociation or polymerization of the enzyme or dissociation of enzyme-bound NAD.