Protein-bound calcium phosphate in uremic rat serum: a quantitative study
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ORIGINAL PAPER
Protein‑bound calcium phosphate in uremic rat serum: a quantitative study Hong‑Xing Fan1,2 · Bao‑Di Gou3 · Yu‑Xi Gao2 · Gang Wu1 · Shu‑Hu Liu4 · Fan Li4 · Tian‑Lan Zhang3 Received: 14 April 2020 / Accepted: 4 August 2020 © Society for Biological Inorganic Chemistry (SBIC) 2020
Abstract Abstract 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 separated the 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 the reaction to take place in hyperphosphatemia conditions. 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. Graphic abstract
Keyword Calcium phosphate · Albumin · Serum · Hyperphosphatemia · Threshold concentration
* Gang Wu [email protected] * Tian‑Lan Zhang [email protected] Extended author information available on the last page of the article
Abbreviations ACP Amorphous calcium phosphate CPPs Calciprotein particles CKD Chronic kidney disease cCa Complexed calcium by small anions DLS Dynamic light scattering FMC Fetuin–mineral complex
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JBIC Journal of Biological Inorganic Chemistry
FTIR Fourier transform infrared spectroscopy fCa Free calcium HAP Hydroxyapatite prCa Protein-bound calcium prPi Protein-bound phosphate SAXS Small-angle X-ray scattering tCa Total calcium TEM Transmission electron microscopy uCa Ultrafilterable calcium XANES X-ray absorption near-edge structure
Introduction Albumin, as the dominant constituent of human serum proteins, markedly affects the distribution of serum calcium. Protein-bound calcium (prCa) makes up about 40% of serum total calcium (tCa), in which albumin accounts for 81% of the bound calcium ions [1]. Because the formation of prCa causes a drop in the lev
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