Size characterization of manganese species from human serum and cerebrospinal fluid using size exclusion chromatography coupled to inductively coupled plasma mass spectrometry

Abstract

Manganese, at excess concentrations, can have adverse effects on health. Mn is a neurotoxicant. The health impacts of Mn are known mostly from occupational health studies, but the exact mechanisms by which Mn can enter the brain without transferrin-receptor mediated transport is unknown. Mn speciation at the neural barriers can help to obtain more information about the pathways and carriers. This paper therefore investigates the size distribution of Mn carriers (e.g., proteins, peptides, carbonic acids) in serum before the neural barriers and in cerebrospinal fluid behind them, as a first step in characterization of the Mn carriers involved in moving Mn across the neural barriers. Serum samples showed Mn concentrations between 1.5–2.5 μg L⁻¹ (mean 1.7 ± 0.8 μg L⁻¹), the lower figure being fully in the physiological range, the higher being slightly elevated. The size distribution of Mn corresponding to the carriers showed a main peak in the transferrin/albumin size fitting to the known physiological ligands. However, preliminary results also indicated an increasing Mn peak at 700 Da with increasing total Mn concentration. This finding could point to excess Mn being transported by a low molecular mass (LMM) carrier. Samples of cerebrospinal fluid (CSF) showed Mn concentrations between 1.8–6.7 μg L⁻¹ (mean 2.66 μg L⁻¹, = 48 nM). Only sparse values have been published with which our results could be compared. The lower concentrations seem to be in the physiological range, whilst the higher ones are clearly elevated. In cerebrospinal fluid Mn was found to be nearly exclusively bound to LMM Mn carriers. The major fraction is in the range of 640–680 Da, which is similar to the LMM compound in serum and to Mn–citrate complexes suggested to be present in body fluids. Therefore, citrate was additionally determined in CSF. The concentration ranged from 420–713 μM (mean 573 μM), proving that citrate is in huge excess compared with Mn (ratio 1:12000). This finding makes a Mn–citrate complex in CSF likely.