Pattern of Altered Plasma Elemental Phosphorus, Calcium, Zinc, and Iron in Alzheimer's Disease

Abstract

Metal/mineral dyshomeostasis has been implicated in the development of Alzheimer's disease (AD). The aim of the study was to investigate the difference in absolute and percentage levels of plasma phosphorus, calcium, iron, zinc, copper, selenium in cognitively normal (CN) and AD subjects. Total reflection X-ray fluorescence (TXRF) spectroscopy was used to detect plasma metals/minerals in CN and AD subjects (n = 44 per group). TXRF detected significantly increased plasma levels of phosphorus (p = 1.33 × 10^-12) and calcium (p = 0.025) in AD compared to CN subjects, with higher phosphorus/calcium (p = 2.55 × 10^-14) ratio in the former. Percentage concentrations calculated for phosphorus, calcium, iron, zinc, copper, selenium by dividing the concentration of each element by the total concentration of these elements and multiplying by 100%, demonstrated phosphorus was higher in AD compared to CN subjects, while calcium, iron, zinc, copper and selenium were lower in AD subjects, with area under the curves as high as 0.937 (p = 6 × 10^-5) computed from receiver operating curves. With exclusion of high levels of phosphorus and calcium from percentage calculations, iron levels remained low in AD whereas zinc was higher in AD, and copper and selenium levels were similar. We demonstrate altered distribution of elements in the plasma of AD subjects with high interdependencies between elemental levels and propose the potential of TXRF measurements for disease monitoring.

Figure 1

Dot plot of Total Reflectance X-Ray Fluorescence (TXRF) measurements of elements in plasma from cognitively normal (CN) and Alzheimer’s Disease (AD) subjects. P-values for each element and the phosphorus/calcium ratio are shown following normalisation of data and ANCOVA testing (see Table 1).

Figure 2

Heat map of partial correlations between the phosphorus, calcium, iron, copper, selenium and zinc (of the transformed data), controlling for age, sex, APOE status and diagnosis. (The correlation coefficient, r, is the top value in the matrix cell with the p-value below.) Phosphorus, copper and selenium have been square-root transformed, while calcium, iron and zinc were log-transformed.

Figure 3

Receiver operating curves (ROC) of binary logistic regression modelling of cognitively normal (CN) and Alzheimer’s Disease (AD) subjects. Base model includes age, sex, APOEε4 genotype (A) and addition of percentage phosphorus (B); calcium (C); iron (D), copper (E), selenium (F) and zinc (G), and phosphorus/calcium (H). AUC – Area under the curve.