Isoforms of bone alkaline phosphatase: Characterization and origin in human trabecular and cortical bone

Alkaline phosphatase (ALP) is a glycoprotein and functions as an ectoenzyme attached to the cell membrane by a hydrophobic glycosyl-phosphatidylinosit ol (GPI) anchor. Three bone ALP (BALP) isoforms in human serum were separat ed and quantitated by high-performance liquid chromatography. B/I, a minor fraction, is composed on average of bone (70%) and intestinal (30%) ALP, an d two major isoforms, B1 and B2, Treatment with GPI-specific phospholipase C (GPI-PLC) did not influence the activities or retention times for B1 and B2, indicating that the biochemical differences between B1 and B2 are likel y to be due to different glycosylation patterns. The B/I fraction in serum, on average 4% of total ALP, was found to be composed of B1 and B2 isoforms , each with an intact hydrophobic GPI cell membrane anchor. We investigated the origin of these three BALP isoforms and osteocalcin in human femora fr om five healthy individuals (four males), mean age 51 years, obtained from a tissue bank. Bone was sampled from three sites: cortical bone, trabecular bone from the diaphysis, and trabecular bone from the greater trochanter. Trabecular bone, from both sites, had higher BALP activities compared with cortical bone. Conversely, the osteocalcin content of cortical bone was mor e than 3-fold greater than that of trabecular bone. Cortical bone had appro ximately 2-fold higher activity of B1 compared with B2, whereas trabecular bone had similar to 2-fold higher activity of B2 compared with B1, We obser ved a previously undescribed BALP isoform (B1x) in all bone samples. B1x wa s also observed in sera from some patients (60%) with severe renal insuffic iency and on chronic dialysis therapy (n = 20), The isoforms of BALP may pr ovide information relating to bone metabolism within specific bone compartm ents.