Macromolecular alkaline phosphatase and an immunoglobulin G that inhibited alkaline phosphatase in a patient's serum

Macromolecular alkaline phosphatase (EC 3.1.3.1) was found in the serum of a patient suffering from myasthenia gravis (adult type II) complicated with thymoma, and was shown by immunoelectrophoresis to be bound to immunoglobulins A and G (IgG). Placental alkaline phosphatase, complexed with either the patient's serum or IgG purified from the patient's serum, remained at the origin on electrophoresis, with significant loss of activity. Intestinal alkaline phosphatase, complexed with either the patient's serum or the patient's IgG, migrated to a position similar to that of the macromolecular alkaline phosphatase in the patient's serum on electrophoresis. About 50% of the placental alkaline phosphatase activity was inhibited with 0.1-0.2 g of the patient's IgG per liter, but 6.93 g of the IgG per liter was required for about 20% inhibition of the intestinal alkaline phosphatase activity. The complex of intestinal alkaline phosphatase with the patient's IgG was fairly heat stable. From these results, we concluded that the macromolecular alkaline phosphatase in the patient's serum consisted of intestinal alkaline phosphatase and IgG that was specific for placental alkaline phosphatase.