Expression of a hybrid form of alkaline phosphatase isoenzyme in a newly established cell line (HuG-1) from a gastric cancer patient.

An unusual alkaline phosphatase (AP), named HuG-AP, was found in a newly established cell line (HuG-1) derived from a patient with stomach cancer. The enzyme was purified about 300-fold by affinity chromatography. On polyacrylamide gradient (4-30%) gel electrophoresis, the one band with the enzyme activity was observed. The enzymic properties of HuG-AP did not conform to those of liver, intestinal, placental, and germ cell AP isoenzymes which were recognized as homodimeric structure. Thermostability of the HuG-AP showed an intermediate value between intestinal and placental APs. Immunologically, the HuG-AP reacted with both anti-intestinal and anti-placental AP monoclonal antibodies. Dot blot analysis showed that both intestinal and placental AP mRNAs were expressed in HuG-1 cells concurrently. Therefore, we concluded that this novel AP had a hybrid form, namely heterodimeric structure, consisting of one subunit each of intestinal and placental APs. However, all of the properties of this hybrid AP did not conform to the intermediate enzymic properties between intestinal and placental APs which would be shown when they both coexist. Because an AP identical to HuG-AP had already been found in the metastatic lesion of the liver of the same patient, the expression of this novel AP seemed to occur in the patient's original cancer cells but did not result from spontaneous transformation of cultured cells.