The fate of glyoxal and methylglyoxal in peritoneal dialysis

Abstract

Many data in the literature suggest that patients on peritoneal dialysis (PD) show a gradual loss of peritoneal membrane function, leading to the failure of PD.1 Peritoneal membrane dysfunction is probably due to enhanced dissipation of the glucose-dependent osmotic gradient across the peritoneal membrane and loss of ultrafiltration capacity.2 One of the well-known causes of the deterioration of peritoneal membrane function is the cyto-toxicity of PD fluids, all containing glucose; its presence in the peritoneal solution as osmotic agent leads not only to the formation of glucose degradation products3,4 but also of reactive carbonyl compounds, such as glyoxal,methylglyoxal and 3-deoxyglucosone, deriving from heat-sterilization of PD fluids.5,6 These compounds lead to ‘carbonyl stress’, and play a pivotal role in peritoneal membrane dysfunction, not only modifying peritoneal matrix proteins and altering their structure but also reacting with mesothelial and endothelial surface proteins and, finally, initiating a range of inflammatory responses.3,7,8 In PD, reactive dicarbonyl compounds, precursors of advanced glycation end-products (AGEs), may derive from two sources: the peritoneal solution and the circulation, due to the condition of carbonyl stress present in uremia.9 AGEs are typical of diabetes.10 They accumulate on serum and tissue proteins in diabetic subjects, and their role in the pathogenesis of long-termdiabetic complications has been proved.11–13 However, recent studies have also shown that AGEs accumulate in the serum and tissues of patients with chronic renal failure, independently of the presence of diabetes.12,14 ‘Carbonyl stress’ has been invoked to explain the long-term complications associated with chronic renal failure and dialysis, such as atherosclerosis and dialysis-related amyloidosis.15 In this panorama, the development of specific analyticalmethods capable of evaluating glyoxal and methylglyoxal levels either in the PD solutions or in the dialysis fluids are certainly of great interest, allowing physicians to establish the role of carbonyl stress in the general pathological picture and to undertake possible therapies. Thirty-six 600-l plasma samples – 20 from healthy subjects and 16 from uremic patients before and after 12 h of peritoneal dialysis – were analyzed. In order to deproteinize plasma and dialysate samples, the approach employed by Tsukushi et al.22 was used, which is based on ultrafiltration on a Centricon membrane. Glyoxal and methylglyoxal levels were expressed as g/l. Values were expressed as means+- SD (standard deviation) or means +- SEM (standard error of mean). To compare mean values between quantitative variables, Student’s t-test for paired and unpaired data was applied. A statistically significant difference was accepted at p < 0.05 (two-tailed test). As an example of the results so obtained, the SIM chromatograms related to PD solution at t D 0 and to PD fluid drawn after 12 h of PD are reported in Fig. 1. As can be seen, a clear decrease of glyoxal and methylglyoxal levels is observed in the case of PD solution before and after dialysis (Fig. 1), while a smaller decrease is observed in the case of plasma samples. Themean values of glyoxal and methylglyoxal levels obtained by GC/MS for plasma and dialysate samples at time t D 0 and after 12 h of dwell time are summarized in Fig. 2. Glyoxal levels showed a significant decrease in plasma, from 22.6 +- 3.0 to 18.1+- 3.9 g/l (p < 0.005, t-test for paired data), and these values were always higher than those observed in normal controls (12.5 š 0.5 g/l, p < 0.001). An analogous trend was observed for methylglyoxal levels, decreasing from 17.5 +- 6.9 to 15.3 +- 6.0 g/l (p < 0.001) – significantly higher than those in normal controls (8.5 +- 0.5 g/l, p < 0.001). The presence of both glyoxal and methylglyoxal at t D 0 in the dialysis medium confirmed the previous data5,6,23 indicating that they originate from the heat treatment required to sterilize the dialysis solution. In our study, the decreases in glyoxal and methylglyoxal concentrations in the dialysate after a 12-h dwell time was not accompanied by relative increases in their plasmatic concentrations; on the contrary, plasma concentrations of glyoxal and methylglyoxal decreased after a 12-h dialytic exchange. Moreover, our data show that the concentrations of pentosidine and free pentosidine, after 12 h of PD, decrease in plasma and increase in dialysate. These data do not support the hypothesis of a diffusion of glyoxal and methylglyoxal from dialysate to systemic circulation. On the contrary, they suggest their transformation into AGEs in the peritoneal cavity, or possible interaction with structural proteins of the peritoneal membrane; these mechanisms may modify peritoneal matrix proteins and alter their structure, reacting with mesothelial and endothelial surface proteins. Our data indicate the pivotal role played by glyoxal and methylglyoxal in mediating carbonyl stress in the peritoneal membrane in PD patients. These mechanisms start a range of inflammatory responses leading to chronic dysfunction of the peritoneal membrane.3,7,8,24 It is to be emphasized that comparable results have recently been obtained by Agalou et al. by LC-MS/MS.25 The results obtained by the GC/MSmethod showtrends analogous to those observed by LC measurement, confirming the validity of this approach for evaluation of glyoxal and methylglyoxal levels in biological fluids