KINETIC MECHANISM OF HALOBACTERIUM-HALOBIUM MN2-ACTIVATED ALKALINE-PHOSPHATASE()

The extreme halophilic archaebacterium Halobacterium halobium contains an atypical alkaline phosphatase which is selectively activated by Mn 2+ ions (Bonet et al., 1991: Int. J. Biochem. 23, 1445-1451). Enzyme k inetic mechanism in the presence of Mn2+ with p-nitrophenylphosphate a s substrate was analysed by initial rate and product and competitive i nhibition studies. The results indicate that there is an ordered addit ion of activator and substrate, Mn2+ being first in binding to the pho sphatase, and that inorganic phosphate is the last product in leaving the enzyme active site. Strong inhibition by vanadate suggests that a phosphoenzyme intermediate is formed during enzymatic phosphohydrolysi s of substrate.