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Protein-Bound Calcium Phosphate in Uremic Rat Serum: A Quantitative Study

preprint
revised on 04.08.2020 and posted on 05.08.2020 by Hong-Xing Fan, Bao-Di Gou, Yu-Xi Gao, Gang Wu, Shu-Hu Liu, Fan Li, Tian-Lan Zhang
Protein-bound calcium (prCa) constitutes about 40% of serum total calcium, in which albumin is the most dominant protein. Given the chemical interaction between calcium and phosphate (Pi), the increased serum Pi in chronic kidney disease may cause changes in the composition and structure of the prCa fraction. Here, we report the phosphate binding on the protein-bound calcium in uremic rat serum. Using adenine-fed rats as a uremic model, we determined the levels of calcium and phosphate fractions in rat serum by ultrafiltration, and found that the level of protein-bound phosphate (prPi) in the uremic serum was markedly higher than in control. The elevated prPi level was comparable to the prCa level, consistent with the presence of protein-bound calcium phosphate pr(Ca)j-m(CaPi)m. We then confirmed its presence by ex vivo X-ray absorption near-edge structure spectroscopy, revealing the discrete state of the calcium phosphate clusters associated with protein. Finally, in a quantitative investigation using Ca- and Pi-boosted serum, we discovered the threshold concentration for the Pi binding on prCa, and determined the binding constant. The threshold, while preventing Pi from binding to prCa in normal condition, allows excess Pi to do so. The protein-bound calcium phosphate could act as a link between the metabolism of serum proteins and the homeostasis of phosphate and calcium, and it deserves further investigation whether the molar ratio of (prPi/prCa)×100% may serve as a serum index of the vascular calcification status in chronic kidney disease.

Funding

National Natural Science Foundation of China and the Chinese Academy of Sciences (No. U1632105)

History

Email Address of Submitting Author

tlzhang@bjmu.edu.cn

Institution

Peking University School of Pharmaceutical Sciences

Country

China

ORCID For Submitting Author

0000-0003-4589-0118

Declaration of Conflict of Interest

The authors declare that they have no conflict of interest.

Version Notes

2020manu-revised

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in JBIC Journal of Biological Inorganic Chemistry

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