Some Observations on the Nature of Bone Mineral 



139 



id ion strength. We have 



^^7 



MacGregor and Nordin (1960, 1962) and MacGregor (1964 a, b) developed the 

 suggestion that bone mineral could be said to have a "solubility product" in terms 

 of {Ca""}3 {PO4"""}- -■'" at physiological pH, temperatu 

 never suggested that bone was "tri-calcium 

 phosphate", only that the behaviour of pow- /a 



dered bone going into solution in vitro could 

 best be described in terms of that ion product. ^ 



The studies were extended to rat bone ^ 



and also to child bone (MacGregor, 1962) 

 and data such as those shown in Fig. 1 were 

 obtained. It can be seen that at equilibrium 

 (see MacGregor and Nordin, 1960 for ex- ^ 

 perimental details), the calcium concentrations v 

 in all three series of experiments were similar r^ 

 and dependent only on the pH of the buffer -^ 

 employed. On the other hand, the concen- ^ 

 trations of inorganic phosphate are relatively 

 independent of pH but rise from about 0.2 mM 

 for human adult bone to about 0.4 mM for 

 powdered child bone and 1.0 niM for bone 

 from growing rats. Consequently, when the 

 empirical ion products were calculated they 

 were consistent for equilibrations with each 

 material, but very different for bone from 

 different sources. Nevertheless, it was clear 

 that the equilibrium concentrations were re- 

 lated to the blood levels of {Ca"""^} and jP } 

 in each case and it was concluded that if bone 

 mineral had an "equilibrium constant" then 

 either there were specific ion effects unallowed 

 for in both child and rat plasma and appropriate 



equilibrating fluids (rather unlikely), or there was a pH gradient between bone and 

 plasma from rat to child to human adult (possible), or thirdly that the bone salt of 

 rats and children was qualitatively different from that of adults (MacGregor, 

 1964 b). This last alternative was thought to be unlikely but very recently Mac- 

 Gregor and Brown (1965) presented evidence to support the view that bone mineral 

 is first laid down as octocalcium phosphate (OCP) which then slowly hydrolyses to 

 hydroxyapatlte (HA). The anatomical skeleton, therefore, would be composed of 

 "reactive" and "unreactive" bone having predominantly OCP and HA in the lattice 

 respectively — young actively growing bone having proportionally more of the 

 former. It is interesting that Ibsen and Urist (1964) have independently shown that 

 homogenised bone fragments labelled with tetracycline yield more pyrophosphate 

 than equivalent amounts of unlabelled moities from the same mix (see also McLean, 

 1965, p. 6) when heated to 325 °C for one hour. 



The presence of OCP stoichiometry in child bone and calf bone mineral was 

 demonstrated by calculation of the chemical potential relationships from equilibration 



'' The brackets { } are used to Imply "the concentration of". 



S.S 6.8 7.0 7.2 7.1^ 26 



pW 



Fig. 1. The concentrations of calcium and 

 phosphate at equilibrium in equilibration ex- 

 periments with human adult, child and rat 

 bone 



