Metabolite profiling identifies markers of uremia

Abstract

ESRD is a state of small-molecule disarray. We applied liquid chromatography/tandem mass spectrometry-based metabolite profiling to survey>350 small molecules in 44 fasting subjects with ESRD, before and after hemodialysis, and in 10 age-matched, at-risk fasting control subjects. At baseline, increased levels of polar analytes and decreased levels of lipid analytes characterized uremic plasma. In addition to confirming the elevation of numerous previously identified uremic toxins, we identified several additional markers of ESRD, including dicarboxylic acids (adipate, malonate, methylmalonate, and maleate), biogenic amines, nucleotide derivatives, phenols, and sphingomyelins. The pattern of lipids was notable for a universal decrease in lower-molecular-weight triacylglycerols, and an increase in several intermediate-molecular-weight triacylglycerols in ESRD compared with controls; standard measurement of total triglycerides obscured this heterogeneity. These observations suggest disturbed triglyceride catabolism and/or beta-oxidation in ESRD. As expected, the hemodialysis procedure was associated with significant decreases in most polar analytes. Unexpected increases in several metabolites, however, indicated activation of a broad catabolic program, including glycolysis, lipolysis, ketosis, and nucleotide breakdown. In summary, this study demonstrates the application of metabolite profiling to identify markers of ESRD, provide perspective on uremic dyslipidemia, and broaden our understanding of the biochemical effects of hemodialysis.

Figure 1.

Differential effects of ESRD and hemodialysis on polar versus lipid metabolites. (A) Baseline ESRD versus controls. Each data point represents one metabolite. The y-axis shows the ratio of the median metabolite level in ESRD versus the median metabolite level in controls on a logarithmic scale. Metabolites are ordered along the x-axis from highest to lowest ratio. Red data points signify metabolites in which the difference between ESRD and control reached statistical significance (P < 0.0005). (B) Percent change with hemodialysis. Each data point represents one metabolite. The y-axis shows the median percent change in metabolite level with hemodialysis, and metabolites are ordered along the x-axis from lowest to highest percent change. Red data points signify metabolites in which the difference between pre- and postdialysis metabolite level reached statistical significance (P < 0.0005).

Figure 2.

Medications monitored by the platform. (A) Box and whisker plots indicating pantothenate, 4-pyridoxate, and thiamine levels in ESRD subjects receiving Nephrocaps (n = 31) versus ESRD subjects not receiving Nephrocaps (n = 13). The lines in the boxes indicate the median peak area for each metabolite; the lower and upper boundaries of the box represent the 25th and 75th percentiles, respectively; the lower and upper whiskers represent the minimum and maximum values. (B) Box and whisker plots indicating the percent change in sucrose in six patients who received IV iron sucrose during hemodialysis versus 27 patients who did not. (C) Box and whisker plots indicating the percent change in lisinopril, metoprolol, diltiazem, and atorvastatin with hemodialysis.

Figure 3.

Correlation matrix for metabolites elevated in ESRD versus control. Shades of red and green represent positive and negative correlation, respectively. Correlation values represent Pearson correlation coefficients of log-transformed metabolite levels.

Figure 4.

Depletion of lower-molecular-weight TAGs in ESRD. (A) Ratio of median TAG levels in ESRD versus controls. Each data point represents a distinct TAG organized along the x-axis on the basis of total acyl chain carbon content; TAGs with the same carbon content but different saturation align vertically. (B) Box and whisker plots indicating total triglycerides (mg/dl) in ESRD versus controls. The lines in the boxes indicate the median triglyceride concentration; the lower and upper boundaries of the box represent the 25th and 75th percentiles, respectively; the lower and upper whiskers represent the minimum and maximum values. (C) Ratio of median TAG levels in a subset of ESRD patients (n = 10) versus controls; ESRD subjects were matched to controls on the basis of total triglycerides. (D) Box and whisker plots indicating total triglycerides in matched ESRD subjects versus controls. (E) Percent change of TAGs with hemodialysis.

Figure 5.

ESRD TAG pattern is exaggerated in patients with high adipate levels. The y-axis represents the ratio of median TAG level in ESRD versus controls. All 51 TAGs monitored by the platform are arrayed along the x-axis. The first number for each TAG denotes the total number of carbons in the three acyl chains of the TAG and the second number (after the colon) denotes the total number of double bonds across the three acyl chains. Black bars represent ESRD subjects with the lowest quartile of adipate values. White bars represent ESRD subjects with the highest quartile of adipate values.