ROLE OF CHELATION IN DECALCIFICATION SYSTEMS 643 



Hypothesis 3. There is good evidence that citrate can exchange 

 with tlie phosphate in h\ droxyapatite and that this is associated with 

 an increased sohibihtv of the crystals. 



Schartnni and Nichols ( 1961 ) concluded that much of the effect 

 of pretreatment with PTE on the dissolving of bone in vitro could 

 be accounted for by changes in passive solubility rather than by 

 effects on cell metabolism, since it could be detected in bone inac- 

 tivated by heat. The\' suggest that the increased citrate concentra- 

 tion in bone found bv others after parathyroid treatment might 

 account for this change in solubility. In a later paper Schartum and 

 Nichols (1962) showed that if the solubilities of living bone and 

 bone killed bv heating were compared in media over a range of pH 

 values, the living bone dissolved more readily above pH 6.5, but 

 below this pH both samples dissolved to the same extent. This could 

 mean that living bone maintains the fluid bathing the crystal surface 

 at a pH of about 6.5, so that if the medium was at that pH already 

 the living bone had no means of increasing solubility; this would 

 favor the idea that acid is the solubilizing agent. These results were 

 in good agreement with the conclusion of Nordin ( 1957 ) that plasma 

 is in equilibrium with bone if the pH on the surface of the crystal is 

 about 6.6 to 6.8. Schartum and Nichols were reluctant to conclude 

 that their results supported the acid theory, since, as they pointed 

 out, at a low pH value some mechanisms normally at work, such as 

 citrate production, might be impaired. 



Hartles (1961) has summarized the results from his own labora- 

 tory on the factors influencing the concentration of citrate in bone. 

 Briefly, he reports that diets deficient in vitamin D and calcium led 

 to the formation of bone low in calcium and citrate, and that the 

 animals developed tetany. With the same low calcium diet to which 

 normal or high levels of vitamin D were added, tetany did not occur 

 despite the fact that the concentration of calcium in the bone was 

 not significantly altered, although the bone citrate was much higher 

 ( Table I ) . Hartles and Leaver ( 1961 ) point out that the bone with 

 the higher citrate appeared to be able to supply calcium to the blood 

 (presumably in response to the increased parathyroid stimulus as 

 a result of the low calcium diet ) and thus avoid tetany, whereas the 



