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The enzyme malate dehydrogenase (EC 1.1.1.37) from an extreme thermophile B. Caldolyticus was purified to about 91% homogeneity. The molar mass of the enzyme was determined as 73 000 daltons and it is composed of two subunits, each with a molar mass of 37 000. Initial velocity studies with oxaloacetic acid and NADH as substrates at pH 8.1, over a range of temperatures, indicates that the enzyme operates via a sequential type mechanism. Van't Hoff plots of the kinetic parameters displayed sharp changes in slope at characteristic temperatures, whereas the Arrhenius plot exhibited no such breaks over the temperature interval investigated. The enzyme was found to be stable at 41 degrees C and lower temperatures. At 51 degrees C and 59 degrees C an almost immediate 20% reduction in activity was obtained, but no further inactivation occurred during the 60 min of incubation. At 59 degrees C the enzyme lost 50% of its initial activity in about 38 s. High concentration of NADH was observed to greatly stabilize the enzyme at that temperature. It is suggested that the slope changes in the Van't Hoff plots and the stability profiles at 51 degrees C and 59 degrees C are representative of a temperature induced conformational change in the enzyme.

Membranes prepared from various members of the genus Halobacterium contained a Triton X‐100 activated adenosine triphosphatase. The enzyme from Halobacterium saccharovorum was unstable in solutions of low ionic strength (< 3 M NaCl) and maximally active in the presence of 3.5 M NaCl. A variety of nucleotide triphosphates was hydrolyzed. MgADP, the product of ATP hydrolysis, was not hydrolyzed and was a competitive inhibitor with respect to MgATP. The enzyme from H. saccharovorum was composed of at least 2 and possibly 4 subunits. The 83‐kDa and 60‐kDa subunits represented about 90% of total protein. The 60‐kDa subunit reacted with dicyclohexylcarbodiimide (DCCD) when inhibition was carried out in an acidic medium. The significance of the two minor components (28 kDa and 12 kDa) is not established. The enzyme from H. saccharovorum, which differs from previously described halobacterial ATPases, possesses properties of an F1F0 as well as an E1E2 ATPase.