Separation of six uremic middle molecular compounds by high performance liquid chromatography and analysis by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Plasma protein characteristics of long-term hemodialysis survivors

Hemodialysis (HD) patients are under recurrent circulatory stress, and hemodialysis has a high mortality rate. The characteristics of plasma proteomes in patients surviving long-term HD remain obscure, as well as the potential biomarkers in predicting prognoses. This study reports the proteome analyses of patient plasma from non-diabetic long-term HD (LHD, dialysis vintage 14.9±4.1 years, n = 6) and the age/sex/uremic etiology-comparable short-term HD (SHD, dialysis vintage 5.3±2.9 years, n = 6) using 2-DE and mass spectrometry. In addition, a 4-year longitudinal follow-up of 60 non-diabetic HD patients was subsequently conducted to analyze the baseline plasma proteins by ELISA in predicting prognosis. Compared to the SHD, the LHD survivors had increased plasma vitamin D binding proteins (DBP) and decreased clusterin, apolipoprotein A-IV, haptoglobin, hemopexin, complement factors B and H, and altered isoforms of α1-antitrypsin and fibrinogen gamma. During the 45.7±15 months for follow-up of the 60 HD patient cases, 16 patients died. Kaplan-Meier analysis demonstrated that HD patients with the lowest tertile of the baseline plasma DBP level have a significantly higher mortality rate. Multivariate Cox regression analysis further indicated that DBP is an independent predictor of mortality. In summary, the altered plasma proteins in LHD implicated accelerated atherosclerosis, defective antioxidative activity, increased inflammation/infection, and organ dysfunction. Furthermore, lower baseline plasma DBP in HD patients is related to mortality. The results suggest that the proteomic approach could help discover the potential biomarker in HD prognoses.

Update of uremic toxin research by mass spectrometry

Mass spectrometry (MS) has been successfully applied for the identification and quantification of uremic toxins and uremia-associated modified proteins. This review focuses on the recent progress in the MS analysis of uremic toxins. Uremic toxins include low-molecular weight solutes, protein-bound low-molecular weight solutes, and middle molecules (peptides and proteins). Based on MS analysis of these uremic toxins, the pathogenesis of the uremic symptoms will be elucidated to prevent and manage the symptoms. Notably, protein-bound uremic toxins such as indoxyl sulfate, p-cresyl sulfate, and 3-carboxy-4-methyl-5-propyl-2-furanpropionic acid have emerged as important targets of therapeutic removal. Hemodialysis even with a high-flux membrane cannot efficiently remove the protein-bound uremic toxins because of their high albumin-binding property. The accumulation of these protein-bound uremic toxins in the blood of dialysis patients might play an important role in the development of uremic complications such as cardiovascular disease. Indoxyl sulfate is the most promising protein-bound uremic toxin as a biomarker of progress in chronic kidney disease. Novel dialysis techniques or membranes should be developed to efficiently remove these protein-bound uremic toxins for the prevention and management of uremic complications.

Recent progress in the analysis of uremic toxins by mass spectrometry

Mass spectrometry (MS) has been successfully applied for the identification and quantification of uremic toxins and uremia-associated modified proteins. This review focuses on recent progress in the analysis of uremic toxins by using MS. Uremic toxins include low-molecular-weight compounds (e.g., indoxyl sulfate, p-cresol sulfate, 3-carboxy-4-methyl-5-propyl-2-furanpropionic acid, asymmetric dimethylarginine), middle-molecular-weight peptides, and proteins modified with advanced glycation and oxidation. These uremic toxins are considered to be involved in a variety of symptoms which may appear in patients with stage 5 chronic kidney disease. Based on MS analysis of these uremic toxins, the pathogenesis of the uremic symptoms will be elucidated to prevent and manage the symptoms.

Separation, identification of uremic middle molecules, and preliminary study on their toxicity

BACKGROUND

Compounds accumulating in uremic serum with molecular mass from 300 to 5000 Da are called uremic middle molecules (UMMs). In our previous work, two UMM fractions A and B were obtained from uremic sera, urine, and normal urine by gel permeation chromatography (GPC), and six UMMs from subfraction A3 of uremic plasma and normal urine were purified and characterized.

METHODS

Urine and serum samples from uremic patients and healthy subjects were isolated by GPC, ion exchange chromatography (IEC), and reversed-phase high-performance liquid chromatography (RP-HPLC). Moreover, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and liquid chromatography/electrospray ionization tandem mass spectrometry (LC/ESI-MS/MS) were used to characterize the compounds. The effects of subfraction A3 on renal function were studied in rabbit models with chronic renal failure (CRF).

RESULTS

A compound with molecular weight 1007.94 in subfraction A3 was determined to be an octapeptide by mass spectrometry, with an amino acid sequence of Val-Val-Arg-Gly-Cys-Thr-Trp-Trp. Two CRF rabbits injected with A3 died in 5 days, while the other two CRF rabbits (no injection) survived a few days. By multistep chromatography and MALDI-TOF MS, another 11 endogenous compounds were found not only in the subfraction B9 of uremic sera but also in that of normal urine.

CONCLUSION

Seventeen endogenous middle molecular compounds were found in fractions A and B of uremic plasma and normal urine, among them an octapeptide with M(W) 1007.94 in subfraction A3. Preliminary experimental results on rabbits indicate that subfraction A3 could accelerate the death of rabbits with CRF.