On the correlation between alkaline phosphatase and phosphate transport in rat renal brush border membrane vesicles

Alkaline phosphatase activity in renal cortex homogenates and in isolated brush border membrane vesicles was compared with the rates of sodium-dependent transport of inorganic phosphate (P_i) by isolated brush border membranes. Brush border membrane vesicles were isolated from renal cortical homogenates of rats adapted during a period of 5–7 weeks to diets with different dietary contents of P_i (low P_i diet=0.15 g%, high P_i diet=2.0 g%).Alkaline phosphatase activity was not increased in the low P_i diet group as compared to the standard P_i diet group but was reduced in the high P_i diet group. Sodium-dependent transport of P_i was increased 2–3-fold in the low P_i diet group as compared to the standard P_i diet group, whereas transport activity was only unsignificantly decreased in the high P_i diet group.Studying kinetik parameters in the two extreme dietary groups it has been found that these differences are due to alteredV_max of the transport activity as well as of alkaline phosphatase activity. TheK_m for both activities remained unaltered.Alkaline phosphatase activity and transport of P_i in brush border membrane vesicles were also compared in the presence of EDTA or Zn²⁺ at concentrations which inhibit alkaline phosphatase activity. Transport of P_i was not affected by the inhibitors even when alkaline phosphatase was inhibited by more than 70% (0.5 mmol/l Zn²⁺) or completely (0.5 mmol/l EDTA).The experiments suggest that no correlation between alkaline phosphatase activity and transport of P_i exists in isolated brush border membrane vesicles.