Effect of calcium on rat intestinal alkaline phosphatase activity and molecular aggregation

Two fractions of rat intestinal alkaline phosphatase (IAP) were detected by Western blot: 168 ± 6 and 475 ± 45 kDa. The low molecular weight fraction constitutes 43% of the isolated proteins exhibiting 82% of the enzymatic activity, and a heavier fraction constitutes 57% of the isolated proteins and has 18% of the enzymatic activity. Calcium produced an increase of the 475-kDa form to the detriment of the 168-kDa form. This work also describes the kinetic and structural changes of IAP as a function of calcium concentration. With [Ca²⁺] < 10 mmole/L, the Ca²⁺-IAP interaction fitted a binding model with 7.8 ± 4.4 moles of Ca²⁺ /mole of protein, affinity constant = 19.1 ± 8.4 L/mmole, and enzymatic activity increased as a linear function of [Ca²⁺] (r = 0.946 p < 0.01). On the other hand, with [Ca²⁺] >10 mmole/L the data did not fit this model and, the enzymatic activity decreased as a function of [Ca²⁺] (r = ? 0.703 p < 0.05).