Heparin-binding histidine and lysine residues of rat selenoprotein P

Selenoprotein P is a plasma protein that has oxidant defense properties. It binds to heparin at pH 7.0, but most of it becomes unbound as the pH is ra ised to 8.5. This unusual heparin binding behavior was investigated by chem ical modification of the basic amino acids of the protein. Diethylpyrocarbo nate (DEPC) treatment of the protein abolished its binding to heparin. DEPC and [C-14]DEPC modification, coupled with amino acid sequencing and matrix -assisted laser desorption ionization-time of flight mass spectrometry of p eptides, identified several peptides in which histidine and lysine residues had been modified by DEPC. Two peptides from one region (residues 80-95) w ere identified by both methods. Moreover, the two peptides that constituted this sequence bound to heparin. Finally, when DEPC modification of the pro tein was carried out in the presence of heparin, these two peptides did not become modified by DEPC. Based on these results, the heparin-binding regio n of the protein sequence was identified as KHAHLKKQVS-DHIAVY. Two other pe ptides (residues 178-189 and 194-234) that contain histidine-rich sequences met some but not all of the criteria of heparin-binding sites, and it is p ossible that they and the histidine-rich sequence between them bind to hepa rin under some conditions. The present results indicate that histidine is a constituent of the heparin-binding site of selenoprotein P. The presence o f histidine, the pK(a) of which is 7.0, explains the release of selenoprote in P from heparin binding as pH rises above 7.0. It can be speculated that this property would lead to increased binding of selenoprotein P in tissue regions that have low pH.