Aggregation and inhibition of rat intestinal alkaline phosphatase by high concentrations of calcium. Reversibility of the processes

Intestinal alkaline phosphatase (IAP) is an enzyme of the brush border of the enterocyte. The activity of IAP biphasically depends on calcium. Although calcium increases IAP activity, when calcium is higher than 20 mmole/L, IAP activity decreases and the amount of an aggregated form increases. The reversibility of the effect of calcium and the aggregation process are unknown. The isoelectric point of the enzyme was higher in the presence of calcium, but was the same for the enzyme and the aggregated form. The treatment with EGTA after calcium addition did not restore the enzymatic activity but produced desaggregation of the enzyme and increase in the monomeric subunits of IAP. It is concluded that the binding of calcium does not produce important modifications on the structure of the protein, that the inhibitory effect is not reversible and that calcium could be involved in the stability of the structure of the enzyme.