The functional site of placental anticoagulant protein: essential histidine residue of placental anticoagulant protein.

Placental anticoagulant protein (PAP) rapidly lost its anticoagulant effect due to photooxidation in the presence of methylene blue at pH 7.9 and 8 degrees C. Photooxidized PAP failed to bind the phospholipid vesicle. It seemed unlikely that the protein underwent a change in molecular size during the photooxidation on the basis of its behavior in electrophoresis and gel filtration. Photooxidized PAP had significantly decreased histidine contents, whereas the contents of other amino acids remained essentially unchanged. The peptide, SHLRKV, was included in the functional site of PAP and still showed an anticoagulant activity. On the other hand, the peptide which substituted histidine by alanine, SALRKV, no longer showed the activity. It was shown that the histidine residue is involved in Ca2+ or the phospholipid binding site of the protein.