Bone Abstracts

Background: Alkaline phosphatase (ALP) has an essential role in bone mineralization. ALP is attached to the surface of matrix vesicles (MVs) in which hydroxyapatite crystals are initially formed. Formation of a tentative collagen–ALP complex may be an early step in the calcification process. The present study was designed to investigate the binding properties of different ALPs to collagen present in bone and cartilage, i.e. types 1 and 2 collagen.

Methods: The binding properties between ALP and collagen were studied by surface plasmon resonance using a BIAcore system. Human type 1 collagen was coupled to a sensor chip and human bone ALP was introduced in solution. The quantity of ALP bound to collagen was monitored in real time and in relation to the response from a reference surface where no collagen was present. Collagen (human type 1 and bovine types 1 and 2) was separated on a gel, blotted to a membrane and incubated with different human ALPs (bone, liver, kidney, and intestinal) and E. coli ALP. The binding of ALP to a collagen type 1 coupled column was studied with an ÄKTA liquid chromatography system.

Results: Surface plasmon resonance studies indicated significant binding between bone ALP and collagen. The various ALPs bound with different affinities to collagen blotted to the membrane. ALP eluted as two fractions from the collagen type 1 coupled column indicating some electrostatic binding.

Conclusion: Our results, from applying various methods, suggest that ALP binds to collagen, which could be part of the process that occurs at the surface of and between collagen fibrils during mineralization. Some types of ALP show higher affinity for collagen, possibly due to posttranslational glycosylation differences, or differences in molecular structures among various ALPs.