Characterization of KB cell alkaline phosphatase. Evidence of similarity to placental alkaline phosphatase

The alkaline phosphatase from KB cells was purified, characterized, and compared to placental alkaline phosphatase, which it resembles immunologically. Two nonidentical nonomeric subunits of the KB phosphatase were found. The two subunits, which have apparent molecular weights of 64,000 and 72,000, can be separated on polyacrylamide gels containing sodium dodecyl sulfate. The Mr = 64,000 KB subunit appears to be identical in protein structure to the monomer of placental alkaline phosphatase. The Mr = 72,000 KB subunit, while differing in the NH2-terminal amino acid, appears also to be very similar to the placental alkaline phosphatase monomer. Both KB phosphatase subunits bind (32P)phosphate, and bind to Sepharose-bound anti-placental alkaline phosphatase. Native KB phosphatase is identical to the placental isozyme in isoelectric point, pH optimum, and inhibition by amino acids, and has a very similar peptide map. The data presented support the hypothesis that the Mr = 64,000 KB phosphatase subunit may the the same gene product as the monomer of placental alkaline phosphatase. This paper strengthens the evidence that the gene for this fetal protein, normally repressed in all cells but placenta, is derepressed in the KB cell line. In addition, this paper presents the first structural evidence that there are two different subunit proteins comprising the placental-like alkaline phosphatase from a human tumor cell line.