S-Adenosylmethionine synthetase in the thermophilic archaebacterium Sulfolobus solfataricus.

Two isoforms of methionine adenosyltransferase (S-adenosylmethionine synthetase), A and B, have been partially purified from Sulfolobus solfataricus, a thermophilic archaebacterium optimally growing at 87°C. The chromatographic procedure, involving hydrophobic chromatography on a phenyl-Sepharose column as a major step. results in 330-fold and 150-fold purification of adenosylmethionine synthetase A and B respectively. The apparent molecular masses, estimated by gel filtration, are 180 kDa for A and 75 kDa for B. The A and B isoforms follow Michaelis-Menten kinetics with apparent Km values of 10 μM nd 20 μM for L-methionine and of 50 μM and 150 μM for ATP respectively. Adenosylmethionine, a product of the reaction, acts as a powerful non-competitive inhibitor (Ki= 50 μM) of the A isoform while it inhibits only slightly the B isoform. Both isozymes exhibit tripolyphosphatase activity but only that associated with the form A is stimulated by 5 μM adenosylmethionine concentration. The two enzymes absolutely require a divalent cation for the activity, but are not affected by monovalent ions and reducing agents. The optimum temperature is 90°C and no significant loss of activity is observable after incubation of the two isoforms at 100°C in the presence of ATP. The Arrhenius plots observed for both isozymes are biphasic, indicating different activation energies below and above 75°C. The cytoplasmic levels of ATP, methionine and adenosylmethionine are evaluated.