Differences between acylcarnitine profiles in plasma and bloodspots.

UNLABELLED: Quantification of acylcarnitines is used for screening and diagnosis of inborn error of metabolism (IEM). While newborn screening is performed in dried blood spots (DBSs), general metabolic investigation is often performed in plasma. Information on the correlation between plasma and DBS acylcarnitine profiles is scarce. In this study, we directly compared acylcarnitine concentrations measured in DBS with those in the corresponding plasma sample. Additionally, we tested whether ratios of acylcarnitines in both matrices are helpful for diagnostic purpose when primary markers fail. STUDY DESIGN: DBS and plasma were obtained from controls and patients with a known IEM. (Acyl)carnitines were converted to their corresponding butyl esters and analyzed using HPLC/MS/MS. RESULTS: Free carnitine concentrations were 36% higher in plasma compared to DBS. In contrast, in patients with carnitine palmitoyltransferase 1 (CPT-1) deficiency free carnitine concentration in DBS was 4 times the concentration measured in plasma. In carnitine palmitoyltransferase 2 (CPT-2) deficiency, primary diagnostic markers were abnormal in plasma but could also be normal in DBS. The calculated ratios for CPT-1 (C0/(C16+C18)) and CPT-2 ((C16+C18:1)/C2) revealed abnormal values in plasma. However, normal ratios were found in DBS of two (out of five) samples obtained from patients diagnosed with CPT-2. CONCLUSIONS: Relying on primary acylcarnitine markers, CPT-1 deficiency can be missed when analysis is performed in plasma, whereas CPT-2 deficiency can be missed when analysis is performed in DBS. Ratios of the primary markers to other acylcarnitines restore diagnostic recognition completely for CPT-1 and CPT-2 in plasma, while CPT-2 can still be missed in DBS.

Fatal outcome due to deficiency of subunit 6 of the conserved oligomeric Golgi complex leading to a new type of congenital disorders of glycosylation.

Deficiency of subunit 6 of the conserved oligomeric Golgi (COG6) complex causes a new combined N- and O-glycosylation deficiency of the congenital disorders of glycosylation, designated as CDG-IIL (COG6-CDG). The index patient presented with a severe neurologic disease characterized by vitamin K deficiency, vomiting, intractable focal seizures, intracranial bleedings and fatal outcome in early infancy. Analysis of oligosaccharides from serum transferrin by HPLC and mass spectrometry revealed the loss of galactose and sialic acid residues, whereas import and transfer of these sugar residues into Golgi-enriched vesicles or onto proteins, respectively, were normal to slightly reduced. Western blot examinations combined with gel filtration chromatography studies in patient-derived skin fibroblasts showed a severely reduced expression of the mentioned subunit and the occurrence of COG complex fragments at the expense of the integral COG complex. Sequencing of COG6-cDNA and COG6 gene resulted in a homozygous mutation (c.G1646T), leading to amino acid exchange p.G549V in the COG6 protein. Retroviral complementation of the patients' fibroblasts with the wild-type COG6-cDNA led to normalization of the COG complex-depending retrograde protein transport after Brefeldin A treatment, demonstrated by immunofluorescence analysis.

Quantification of free and total sialic acid excretion by LC-MS/MS.

BACKGROUND: The main purpose for measuring urinary free sialic acid (FSA) is to diagnose sialic acid (SA) storage diseases. Elevated amounts of conjugated sialic acid (CSA) are observed in several diseases indicating the need to quantify CSA as well. A LC-MS/MS method for quantification of FSA and total sialic acid (TSA) in urine is developed and validated. METHODS: FSA is analyzed directly after filtration of urine samples. For determination of TSA an enzymatic (neuraminidase) and a chemical (acid) hydrolysis were compared. 13C3-sialic acid was used as internal standard. LC-MS/MS was performed in negative electrospray ionisation mode with multiple reaction monitoring of transitions m/z 308.2-->87.0 (SA) and m/z 311.2-->90.0 (13C3-SA). CSA was calculated by subtracting FSA from TSA. RESULTS: Limit of detection for FSA and TSA was 0.3 and 1.7 micromol/L, respectively. Limit of quantification for FSA and TSA was 1.0 and 5.0 micromol/L. Intra- and inter-assay variations of FSA were 4.6% and 6.6% (n=10) for FSA and 6.5% and 3.6% (n=10) for TSA. Linearity was tested till 7800 micromol/L (r2=0.9998). Values of SA analyzed after neuraminidase- or acid hydrolysis treatment were comparable. Urine samples from patients with inborn errors of SA (related) metabolism were analyzed and compared with age-related reference values. CONCLUSION: A method has been developed for routine determination of urinary FSA and TSA. The method is rapid, specific, robust and sensitive. Age-related reference values for FSA, TSA and CSA were determined and improved diagnostic efficacy.

Enhanced conversion of triglyceride-rich lipoproteins and increased low-density lipoprotein removal in LPLS447X carriers.

OBJECTIVE: Lipoprotein lipase (LPL) exerts 2 principal actions, comprising enzymatic hydrolysis of triglyceride-rich lipoproteins (TRLs) and nonenzymatic ligand capacity for enhancing lipoprotein removal. The common LPLS447X variant has been associated with cardiovascular protection, for which the mechanism is unknown. We therefore evaluated enzymatic and nonenzymatic consequences of this LPL variant on TRL metabolism. METHODS AND RESULTS: TRL apolipoprotein B100 (apoB100) metabolism was determined in 5 homozygous LPLS447X carriers and 5 controls. Subjects were continuously fed and received infusion of stable isotope l-[1-(13C)]-valine. Results were analyzed by SAAMII modeling. Also, preheparin and postheparin LPL concentration and activity were measured. Compared with controls, carriers presented increased very low-density lipoprotein 1 (VLDL1) to VLDL2 apoB100 flux (P=0.04), increased VLDL2 to intermediate-density lipoprotein (IDL) apoB100 flux (P=0.02), increased IDL to low-density lipoprotein (LDL) apoB100 flux (P=0.049), as well as an increased LDL clearance (P=0.04). Additionally, IDL apoB100 synthesis was attenuated (P=0.05). Preheparin LPL concentration was 4-fold higher compared with controls (P=0.01), and a correlation was observed between preheparin LPL concentration and LDL clearance (r2=0.92; P=0.01). CONCLUSIONS: Enhanced TRL conversion and enhanced LDL removal combined with increased preheparin LPL concentration suggest increased enzymatic consequences as well as increased nonenzymatic consequences of LPL in LPLS447X carriers, which might both contribute to the cardiovascular benefit of this LPL variant.

Hypertriglyceridemia in patients with chronic renal failure: possible mechanisms.

Cardiovascular disease (CVD) is a major cause of mortality in patients with chronic renal failure (CRF) caused by numerous factors defined as traditional and uremia-related risk factors. One of these risk factors, dyslipidemia, is often observed in patients with CRF, resulting in abnormal concentrations and composition of plasma lipoproteins. The prominent features of uremic dyslipidemia are an increase in plasma triglycerides and cholesterol in nearly all lipoproteins, and a reduction in high-density lipoprotein (HDL) cholesterol. Because of its direct contact with the circulating blood, the endothelium is preferentially subjected to the modulatory effects of these altered lipoproteins. Little is known about the mechanisms for hypertriglyceridemia in CRF. This review highlights several studies over the past years that have contributed to knowledge of hypertriglyceridemia, especially in combination with renal diseases and their dialysis treatment. The underlying mechanisms behind hypertriglyceridemia have not been fully clarified and may indeed be multifactorial. Hypertriglyceridemia may contribute to the progression of atherosclerosis. Therefore, it is essential to study the putative mechanisms for uremic dyslipidemia, since optimal treatment is essential for the prevention or delay of cardiovascular complications in patients with CRF.

Idiopathic hypoalbuminemia explained by reduced synthesis rate and an increased catabolic rate.

OBJECTIVE: To determine the contribution of albumin synthetic and catabolic rates to steady state levels in a patient with idiopathic hypoalbuminemia. METHODS: Using L-[1-(13)C] valine, both FSR (fractional synthesis rate) as well as FCR (fractional catabolic rate) were studied. Human albumin cDNA analysis and determination of the exact albumin mass by electrospray mass spectrometry were performed. RESULTS: Compared with controls, plasma albumin concentration in the patient was reduced (6.7 vs. 37.0 +/- 2.6 g/L). Albumin FSR (= FCR in steady state) was increased compared to controls. The ASR (absolute synthesis rate) of albumin was decreased based on the enrichment in plasma valine and KIV, but estimated to be normal based on VLDL apoB100 at plateau compared to controls. Direct estimation of albumin FCR rejected the latter. No mutation was found in the transcribed region of albumin gene. The exact mass of albumin (66.493 Da) was not different from controls. CONCLUSION: The hypoalbuminemia was a result of accelerated clearance of albumin from plasma in addition to defective albumin synthesis. This study also shows that the chosen method of the precursor pool could lead to misinterpretation of data in hepatic protein synthesis.

Albumin turnover: experimental approach and its application in health and renal diseases.

Plasma albumin is an important protein in the human body and is responsible for transport and binding of many molecules. Furthermore, it is involved in mediating blood volume and colloid osmotic pressure (COP). As hypoalbuminemia occurs, as is the case in a number of clinical disorders, adaptation mechanisms may be involved. Serum albumin concentration is the net result of physiological processes like synthesis and catabolism. Measurement of one of these processes can provide therefore a more dynamic insight into the adaptation mechanism of albumin metabolism in relation to an underlying disease than would be obtained by changes in albumin concentration alone. This review highlights several studies over the past years that have contributed to knowledge of albumin metabolism. A short introduction is given for synthesis, formation and catabolism of albumin, after which an overview is given on how to measure albumin turnover including a general approach. Finally, albumin metabolism focused on patients with renal diseases will be discussed.

High incidence of symptomatic hyperammonemia in children with acute lymphoblastic leukemia receiving pegylated asparaginase.

Asparaginase is a mainstay of treatment of childhood acute lymphoblastic leukemia. Pegylation of asparaginase extends its biological half-life and has been introduced in the newest treatment protocols aiming to further increase treatment success. Hyperammonemia is a recognized side effect of asparaginase treatment, but little is known about its incidence and clinical relevance. Alerted by a patient with severe hyperammonemia after introduction of the new acute lymphoblastic leukemia protocol, we analyzed blood samples and clinical data of eight consecutive patients receiving pegylated asparaginase (PEG-asparaginase) during their treatment of acute lymphoblastic leukemia. All patients showed hyperammonemia (>50 µmol/L) and seven patients (88 %) showed ammonia concentrations > 100 µmol/L. Maximum ammonia concentrations ranged from 89 to 400 µmol/L. Symptoms varied from mild anorexia and nausea to headache, vomiting, dizziness, and lethargy and led to early interruption of PEG-asparaginase in three patients. No evidence of urea cycle malfunction was found, so overproduction of ammonia through hydrolysis of plasma asparagine and glutamine seems to be the main cause. Interestingly, ammonia concentrations correlated with triglyceride values (r = 0.68, p < 0.0001), suggesting increased overall toxicity.The prolonged half-life of PEG-asparaginase may be responsible for the high incidence of hyperammonemia and warrants future studies to define optimal dosing schedules based on ammonia concentrations and individual asparagine and asparaginase measurements.

Necrotizing enterocolitis and respiratory distress syndrome as first clinical presentation of mitochondrial trifunctional protein deficiency.

BACKGROUND: Newborn screening (NBS) for long-chain 3-hydroxy acyl-CoA dehydrogenase (LCHAD) deficiency does not discriminate between isolated LCHAD deficiency, isolated long-chain keto acyl-CoA (LCKAT) deficiency and general mitochondrial trifunctional protein (MTP) deficiency. Therefore, screening for LCHAD deficiency inevitably comprises screening for MTP deficiency, which is much less amenable to treatment. Furthermore, absence of a clear classification system for these disorders is still lacking. MATERIALS AND METHODS: Two newborns screened positive for LCHAD deficiency died at the age of 10 and 31 days, respectively. One due to severe necrotizing enterocolitis (NEC), cardiomyopathy and multiorgan failure and the other due to severe infant respiratory distress syndrome (IRDS) and hypertrophic cardiomyopathy. (Keto)-acylcarnitine concentration and enzymatic analysis of LCHAD and LCKAT suggested MTP deficiency in both patients. Mutation analysis revealed a homozygous HADHB c.357+5delG mutation in one patient and a homozygous splice-site HADHB mutation c.212+1G>C in the other patient.Data on enzymatic and mutation analysis of 40 patients with presumed LCHAD, LCKAT or MTP deficiency were used to design a classification to distinguish between these disorders. DISCUSSION: NEC as presenting symptom in MTP deficiency has not been reported previously. High expression of long-chain fatty acid oxidation enzymes reported in lungs and gut of human foetuses suggests that the severe NEC and IRDS observed in our patients are related to the enzymatic deficiency in these organs during crucial stages of development.Furthermore, as illustrated by the cases we propose a classification system to discriminate LCHAD, LCKAT and MTP deficiency based on enzymatic analysis.

The Proline/Citrulline Ratio as a Biomarker for OAT Deficiency in Early Infancy.

Deficiency of ornithine-δ-aminotransferase (OAT) in humans results in gyrate atrophy. Early diagnosis may allow initiation of treatment before irreversible damage has occurred. However, diagnosis is commonly delayed well into adulthood because of the nonspecific character of initial symptoms. Here, we report findings in a neonate who was evaluated because of a positive family history of OAT deficiency. The reversed enzymatic flux in early infancy resulted in borderline low ornithine concentration - evoking urea cycle disturbances - and increased proline. In addition, plasma citrulline was low. Consequently, the proline/citrulline ratio in plasma was increased compared to controls. To find out whether amino acid profiling in neonatal dried blood spots is suitable to detect OAT deficiency, we evaluated the original newborn dried blood spots of two affected patients and compared it with a database of >450,000 newborns tested in Minnesota since 2004. Proline concentrations (777 and 1,381 µmol/L) were above the 99 percentile (776 µmol/L) of the general population, and citrulline concentrations (4.5 and 4.9 µmol/L) only just above the 1 percentile (4.37 µmol/L). The proline/citrulline ratio was 172.9 and 281.8, respectively. This ratio was calculated retrospectively in the normal population, and the 99 percentile was 97.6. Applying this ratio for NBS could lead to early and specific detection of neonatal OAT deficiency, with no additional expense to newborn screening laboratories quantifying amino acids. Given that early diagnosis of OAT disease can lead to earlier treatment and prevent visual impairment, further studies are indicated to evaluate whether newborn screening for OAT deficiency is warranted.

Liquid chromatography-tandem mass spectrometry assay for the quantification of free and total sialic acid in human cerebrospinal fluid.

BACKGROUND: Analysis of sialic acid (SA) metabolites in cerebrospinal fluid (CSF) is important for clinical diagnosis. In the present study, a high-performance liquid chromatography-tandem mass spectrometry (HPLC/MS/MS) method for free sialic acid (FSA) and total sialic acid (TSA) in human CSF was validated. METHODS: The method utilized a simple sample-preparation procedure of protein precipitation for FSA and acid hydrolysis for TSA. Negative electrospray ionisation was used to monitor the transitions m/z 308.2-->87.0 (SA) and m/z 311.2--> 90.0 ((13)C(3)-SA). Conjugated sialic acid (CSA) was calculated by subtracting FSA from TSA. We established reference intervals for FSA, TSA and CSA in CSF in 217 control subjects. The method has been applied to patients' samples with known differences in SA metabolites like meningitis (n=6), brain tumour (n=2), leukaemia (n=5), and Salla disease (n=1). RESULTS: Limit of detection (LOD) was 0.54 microM for FSA and 0.45 mM for TSA. Intra- and inter-assay variation for FSA (21.8 microM) were 4.8% (n=10) and 10.4% (n=40) respectively. Intra- and inter-assay variation for TSA (35.6 microM) were 9.7% (n=10) and 12.8% (n=40) respectively. Tested patients showed values of TSA above established reference value. CONCLUSION: The validated method allows sensitive and specific measurement of SA metabolites in CSF and can be applied for clinical diagnoses.

A 2D-DIGE approach to identify proteins involved in inside-out control of integrins.

Leukocyte integrins are functionally regulated by "inside-out" signaling, meaning that stimulus-induced signaling pathways act on the intracellular integrin tail and induce activation of the receptor at the outside. Both a change in conformation (affinity) and in clustering (avidity/valency) of the receptors has been described to occur. This inside-out signaling is essential for adequate migration of leukocytes to inflammatory sites; however, the exact underlying mechanism is not known. We used two variants of a mouse acute lymphocytic leukemia cell line (L1210), a suspension (L1210-S) and an adherent (L1210-A) variant that were characterized by nonactivated and activated integrins (beta(1), beta(2) and beta(3)), respectively. L1210-S and L1210-A cells were compared on protein expression profiles by two-dimensional fluorescence difference in-gel electrophoresis (2D-DIGE). We found 86 protein spots that were more than 1.25-fold different between L1210-A and L1210-S. Only 4 protein spots were more than 2.5-fold different. We identified 29 proteins by mass spectrometry among which were gelsolin, L-plastin, and Rho GTPase dissociation inhibitor 2. These proteins were upregulated in the L1210-A cells versus L1210-S, which was verified by Western blot analysis. Overexpression of gelsolin in U937 resulted in increased high affinity integrin expression and cell adhesion. Comparison of functionally different cell lines from similar origin by 2D-DIGE might be a successful approach to identify regulatory proteins involved in integrin inside-out control.

Enhanced apoB48 metabolism in lipoprotein lipase X447 homozygotes.

RATIONALE: Lipoprotein lipase (LPL) X447 homozygotes are characterized by enhanced conversion of TRL apoB100. Here, we set out to investigate whether this LPL variant is also associated with enhanced apoB48 clearance. Therefore, we evaluated apoB48 kinetics in X447 homozygotes in the fed state by infusion of isotope L-[1-(13)C]-valine and subsequent compartmental modeling. METHODS AND RESULTS: ApoB48 metabolism was assessed in five X447 homozygotes (X/X genotype) and five S447 homozygotes (S/S genotype). Subjects were continuously fed and received infusion of stable isotope L-[1-(13)C]-valine. Results were analyzed by SAAM II modeling. Fasting (2.4-fold, p=0.02) as well as non-fasting (1.6-fold, p=0.09) apoB48 concentration was increased in the X447 homozygotes compared to S447 homozygotes. In addition, the X447 homozygotes exhibited a 1.7-fold higher apoB48 poolsize (p=0.04). Interestingly, apoB48 fractional catabolic rate (FCR) was 1.9-fold higher (p=0.007) and apoB48 synthesis was more than two-fold higher (p=0.006) in the X447 homozygotes compared to S447 homozygotes. CONCLUSION: In the present study, we show that X447 homozygotes exhibit enhanced apoB48 clearance. Previously, these homozygotes were shown to present with enhanced apoB100 TRL conversion. Combined, this LPLS447X gain of function variant affects apoB48 as well as apoB100 TRL metabolism.

Endogenous cholesterol synthesis is associated with VLDL-2 apoB-100 production in healthy humans.

Subjects with high plasma cholesterol levels exhibit a high production of VLDL apolipoprotein B-100 (apoB-100), suggesting that cholesterol is a mediator for VLDL production. The objective of the study was to examine whether endogenous cholesterol synthesis, reflected by the lathosterol-cholesterol ratio (L-C ratio), affects the secretory rates of different VLDL subfractions. Ten healthy subjects were studied after overnight fasting. During a 10 h primed, constant infusion of 13C-valine (15 micromol/kg/h), enrichment was determined in apoB-100 from ultracentrifugally isolated VLDL-1 and VLDL-2 by gas chromatography mass spectrometry. The synthesis rates of VLDL-1 apoB-100 and VLDL-2 apoB-100, catabolism, and transfer were estimated by compartmental analysis. Mean VLDL-1 apoB-100 pool size was 90 +/- 15 mg, and mean VLDL-2 apoB-100 pool size was 111 +/- 14 mg. Absolute synthesis rate of VLDL-1 apoB-100 was 649 +/- 127 mg/day and 353 +/- 59 mg/day for VLDL-2 apoB-100. There was a strong association between the absolute synthesis rate of VLDL-2 apoB-100 and L-C ratio (r 2 = 0.61, P < 0.01). In contrast, no correlation was observed between L-C ratio and absolute synthesis rate of VLDL-1 apoB-100 (r 2 = 0.302, P = 0.09). In conclusion, these data provide additional support for an independent regulation of VLDL-1 apoB-100 and VLDL-2 apoB-100 production. Endogenous cholesterol synthesis is correlated only with the VLDL-2 apoB-100 production.

Increased albumin and fibrinogen synthesis rate in patients with chronic renal failure.

BACKGROUND: Hypoalbuminemia and hyperfibrinogenemia are frequently observed in patients with chronic renal failure (CRF) and are both associated with cardiovascular diseases. The mechanisms responsible for hypoalbuminemia and hyperfibrinogenemia in CRF are unknown. METHODS: In the present study, both albumin and fibrinogen kinetics were measured in vivo in predialysis patients (N = 6), patients on peritoneal dialysis (N = 7) and control subjects (N = 8) using l-[1-13C]-valine. RESULTS: Plasma albumin concentration was significantly lower in patients on peritoneal dialysis compared to control subjects (P < 0.05). Plasma fibrinogen was significantly increased in both predialysis patients (P < 0.01) as well as patients on peritoneal dialysis (P < 0.001) in comparison to control subjects. In contrast to albumin, fibrinogen is only lost in peritoneal dialysate and not in urine. The absolute synthesis rates (ASR) of albumin and fibrinogen were increased in patients on peritoneal dialysis (ASR albumin, 125 +/- 9 mg/kg/day versus 93 +/- 9 mg/kg/day, P < 0.05; ASR fibrinogen, 45 +/- 4 mg/kg/day versus 29 +/- 3 mg/kg/day, P < 0.01) compared to control subjects. Albumin synthesis is strongly correlated with fibrinogen synthesis (r2 = 0.665, P < 0.0001, N = 21). In this study, the observed hypoalbuminemia in patients on peritoneal dialysis is likely not explained by malnutrition, inadequate dialysis, inflammation, metabolic acidosis, or insulin resistance. We speculate that peritoneal albumin loss is of relevance. CONCLUSION: Synthesis rate of albumin and fibrinogen are coordinately up-regulated. Both albumin and fibrinogen are lost in peritoneal dialysis fluid. To compensate protein loss, albumin synthesis is up-regulated, but the response, in contrast to predialysis patients, does not fully correct plasma albumin concentrations in peritoneal dialysis patients. The increase in fibrinogen synthesis introduces an independent risk factor for atherosclerosis, since plasma fibrinogen pool is enlarged.

Transferrin synthesis is increased in nephrotic patients insufficiently to replace urinary losses.

The urinary loss of transferrin is sufficient to reduce plasma transferrin concentrations in the nephrotic syndrome. Hypotransferrinemia may lead to iron loss and microcytic anemia. The mechanism responsible for the hypotransferrinemia in the nephrotic syndrome is, however, unknown. In the present study, synthesis rate of transferrin was measured in vivo in nephrotic patients (n = 7) compared with control subjects (n = 6) using L-[1-(13)C]-valine. Plasma transferrin and iron concentration in the patients were significantly lower than in control subjects (transferrin, 1.39 +/- 0.08 versus 2.57 +/- 0.11 g/L, P < 0.0001; iron, 10.2 +/- 0.8 versus 21.1 +/- 4.5 micromol/L, P = 0.02). Furthermore, albuminuria correlated with transferrinuria (r(2) = 0.901, P = 0.001). The absolute synthesis rate of transferrin was increased in the patients (10.0 +/- 1.1 versus 7.4 +/- 0.7 mg/kg per d, P = 0.07), although this value failed to achieve significance. C-reactive protein, plasma iron, and proteinuria did not correlate with transferrin synthesis. In contrast, transferrin synthesis correlated with albumin synthesis (r(2) = 0.648, P = 0.03; n = 7). The present study indicates that increased transferrin synthesis occurs in nephrotic patients but is insufficient to compensate for urinary losses. Because, overall, no significant relationship was found between transferrin synthesis and either C-reactive protein or iron, it is unlikely that inflammation suppresses or that iron deficiency stimulates increased transferrin synthesis in these patients. The correlation between transferrin synthesis and albumin synthesis suggests that transferrin synthesis is a component of a general response in hepatic protein synthesis in the nephrotic syndrome. This suggests that a therapeutic approach to maximize plasma transferrin concentrations in nephrotic patients should be aimed primarily at reducing urinary protein excretion.

A broad-based metabolic approach to study VLDL apoB100 metabolism in patients with ESRD and patients treated with peritoneal dialysis.

BACKGROUND: Dyslipidemia is often observed in patients with end-stage renal disease (ESRD) and is associated with cardiovascular diseases. Peritoneal dialysis treatment may further deteriorate the lipoprotein abnormalities, suggesting that peritoneal dialysis alters lipid metabolism. METHODS: To study the mechanisms involved in these abnormalities in peritoneal dialysis, we measured insulin sensitivity, free fatty acids release, de novo lipogenesis (DNL), very low-density lipoprotein (VLDL) apoB100 kinetics and cholesterol synthesis in vivo in ESRD (N= 6), peritoneal dialysis patients (N= 5), and controls (N= 7) using stable isotopes. RESULTS: Insulin sensitivity, as assessed by an euglycemic hyperinsulinemic clamp, tended to be lower in ESRD and peritoneal dialysis compared to controls [P= 0.08 by analysis of variance (ANOVA)]. Free fatty acid release during the euglycemic hyperinsulinemic clamp tended to be higher in ESRD and peritoneal dialysis compared to controls (P= 0.08 by ANOVA), while DNL and fractional cholesterol synthesis were normal. VLDL-1 apoB100 (P < 0.05) and VLDL-2 apoB100 pool sizes (P < 0.05) were significantly higher in peritoneal dialysis patients compared to controls. The increased VLDL-1 apoB100 pool size was explained by increased VLDL-1 apoB100 synthesis (P < 0.05) in combination with reduced VLDL-1 apoB100 catabolism (P < 0.01), while the increased VLDL-2 apoB100 pool was explained by reduced catabolism (P < 0.01). CONCLUSION: Both VLDL-1 apoB100 and VLDL-2 apoB100 pool sizes are increased in peritoneal dialysis patients, due to disturbances both in synthesis and catabolism. VLDL-1 apoB100 production is, at least partially, explained by increased free fatty acid availability secondary to peripheral insulin resistance, thus identifying insulin resistance as potential therapeutic target in peritoneal dialysis patients.