Abstract
A plasma membrane preparation derived from incisor pulps of 250-g rats was used as the source of alkaline phosphatase and an ATPase activated by either Ca2+ or Mg2+. Properties of the two enzymes were then compared under a variety of experimental conditions to determine if ATPase activity is clearly distinct from alkaline phosphatase activity. (a) The optimum pH for ATP hydrolysis was 8.0, compared with 10.2 forp-nitrophenylphosphate. (b) At the optimum pH, ATP hydrolysis required either Ca2+ or Mg2+ for activation, whereasp-nitrophenylphosphate hydrolysis did not require and was little influenced by the presence of divalent cations. (c) Alkaline phosphatase showed maximal activity at 40°C and ATPase at 50°C, with complete inactivation at 60°C and 70°C, respectively. (d) When the plasma membrane preparation was preincubated in the absence of substrate at varying temperatures of pH, alkaline phosphatase was less stable than ATPase at extreme pH and high temperatures. (e) Alkaline phosphatase activity was lost more rapidly than ATPase activity during storage at 4°C for up to 10 days. (f) Butanol extraction of the plasma membrane preparation to remove phospholipid destroyed ATPase activity and enhanced alkaline phosphatase activity approximately fivefold. On the basis of these comparisons it is concluded that ATP hydrolysis andp-nitrophenyl-phosphate hydrolysis are the result of separate enzyme activities.
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