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 degrees 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 microM and 20 microM for L-methionine and of 50 microM and 150 microM for ATP respectively. Adenosylmethionine, a product of the reaction, acts as a powerful non-competitive inhibitor (Ki = 50 microM) 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 microM 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 degrees C and no significant loss of activity is observable after incubation of the two isoforms at 100 degrees C in the presence of ATP. The Arrhenius plots observed for both isozymes are biphasic, indicating different activation energies below and above 75 degrees C. The cytoplasmic levels of ATP, methionine and adenosylmethionine are evaluated.