The effects of serum and human albumin on calcium hydroxyapatite crystal growth

Abstract

The effects of potential serum inhibitors upon the growth of calcium hydroxyapatite (HAP) crystals were studied in vivo using a pH-stat system. Whole serum caused a marked decrease in crystal growth in a dose-dependent manner. At a protein concentration of 13 micrograms/ml, whole serum reduced the initial rate of crystal growth from 84 mumol of KOH/h to 48 mumol of KOH/h. Serum components were separated by ultrafiltration (10,000 Da cut-off). The high-molecular-mass fraction containing serum proteins gave an initial rate of crystal growth of 48 mumol of KOH/h compared with 64 mumol of KOH/h given by the low-molecular-mass components. Thus, two-thirds of the inhibitory activity was associated with proteins and other serum macromolecules, whilst the remainder of the activity was associated with the low-molecular-mass components. Albumin-depleted serum showed an initial rate of crystal growth of 59 mumol of KOH/h, whilst albumin purified by affinity chromatography gave an initial rate of crystal growth of 56 mumol of KOH/h at the same protein concentration. Albumin, therefore, not only accounts for half of the protein concentration in serum, but also contributes half of the inhibitory activity of the high-molecular-mass fraction. Heat denaturation of albumin dramatically enhanced the inhibition of HAP seeded growth with the initial rate of crystal growth falling to 27 mumol of KOH/h after treatment compared with 62 mumol of KOH/h before denaturation. Isoelectric focusing indicated that the tertiary and secondary structure, and hence the distribution of surface charge of albumin, are altered by heat denaturation. Gels showed a mixture of species with isoelectric points ranging from 6.0 to 5.0 compared with the native protein value of 4.7. These data suggest that adsorption of serum proteins to the growing HAP crystals is one mechanism of growth inhibition. It is also clear that the most abundant serum protein, albumin, is an important mediator of this process.