Reconsidering the role of albumin towards amorphous calcium phosphate-based calciprotein particles formation and stability from a physico-chemical perspective

The formation of calciprotein particles (CPPs) in serum is a physiological phenomenon fundamental to prevent the rise of ectopic calcifications. CPPs are colloidal hybrid particles made of amorphous calcium phosphate stabilized by a protein, fetuin-A. Since albumin is the most abundant protein present in serum, we aimed at understanding if it plays a synergic action together with fetuin-A towards CPPs formation and stability. CPPs were prepared using a constant fetuin-A concentration (5 µM) and different concentrations of albumin (0-606 µM). The stability of CPPs, their crystallization and sedimentation were followed in situ by combining turbidimetry, precipitation analysis and dynamic light scattering. The morphology was investigated by scanning electron microscopy and cryo-transmission electron microscopy, while crystallinity was inspected by infrared spectroscopy. The effect of albumin on the amount of formed CPPs was also studied, as well as the amount of protein adsorbed on CPPs. We found that albumin is not able to prolong the lifetime of the amorphous phase, but it is very effective in delaying the sedimentation of CPPs after crystallization. Albumin also significantly decreases the amount and size of CPPs when present in their synthetic medium, likely playing a fundamental role in our organism together with fetuin-A towards the stabilization of CPPs.