Placental alkaline phosphatase in nonmalignant human cervix.

At least three loci determine human alkaline phosphatases [orthophosphoric-monoester phosphohydrolase (alkaline optimum), EC 3.1.3.1]: one coding for the placental form of the enzyme; at least one coding for the intestinal forms; and at least one for the liver, bone, and kidney forms. It is generally believed that the locus determining the placental form is, in the normal individual, expressed only in placenta. However, ectopic or aberrant expression of this locus occurs in certain malignancies of other tissues and in cell lines, such as HeLa, derived from malignancies. We have examined by thermostability, inhibition, and immunologic studies the alkaline phosphatases in endocervix, cervical mucus, and endometrium from nonpregnant women with no evidence of malignancy. It was found that on the average about 18% of the alkaline phosphatase activity in endocervix and in cervical mucus is placental in type, the remainder being of the liver/bone/kidney type. The quantity of the placental enzyme is, however, low and amounts to only about 0.5% of the amount in normal term placenta. In endometrium all the alkaline phosphatase activity was of the liver/bone/kidney type. Thus the placental alkaline phosphatase locus is expressed in nonmalignant endocervix. The result is of some significance in connection with the widely held view that the expression of placental alkaline phosphatase in certain malignancies (including cervical malignancy) is due to derepression of a locus for a fetal enzyme protein normally repressed in adult tissues.