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Phospholipid excretion and metabolism and thiol status in cyctic fibrosis Chen, Alice Ho-Wing

Abstract

Choline is an essential nutrient and is present in the diet predominantly (>90%) in phosphatidylcholine (PC). Large amounts of P C are also secreted into the intestine in bile. P C digestion and absorption requires pancreatic phospholipase A₂ (PLA₂) , which hydrolyzes PC to lysoPC and fatty acid. PLA₂ is inhibited at low intraluminal pH, which is common in patients with cystic fibrosis (CF). Choline deficiency results in hepatic steatosis, explained by the need for PC synthesis via the cytidine diphosphocholine pathway for secretion of triglyceride in very-low-density lipoprotein. In choline deficiency, the liver increases synthesis of PC via the phosphatidylethanolamine N-methylfransferase pathway in which methyl groups from methionine are transferred to phosphatidylethanolamine, forming P C and S-adenosylhomocysteine, which is then metabolized to homocysteine. Hepatic steatosis is common in patients with CF, but whether patients with CF malabsorb PC, which is a phospholipid (PL), is unknown. Current methods for fecal fat extraction extract triglyceride and fatty acid but do not quantitatively recover PL. This study aimed to establish a method for quantification of PL , PC and lysoPC excretion and to determine PL, PC and lysoPC excretion in children with CF and their associations with plasma thiols. This is a cross-sectional study of children with CF (n=18) and children without CF (n=8, control). All participants provided a venous blood sample, a 72h fecal sample and 5d food record. Fecal total fat and PL were recovered by sequential extraction with ethanol, ether, hexane, chloroform and methanol and were separated and quantified using high performance liquid chromatography (HPLC) with evaporative light scattering detector. Plasma thiols and PL were analyzed using HPLC . Compared with the controls, children with CF had significantly lower fat absorption (mean±SEM) (86± 1.6%, 94+1.2%), and higher excretion of fat (12.9±1.7g/d, 3.9±0.7g/d), PL (139±20mg/d, 66±18mg/d), P C (39±11mg/d, 2.6±1.0mg/d) and lysoPC (57+8.1mg/d, 22±6.2mg/d), respectively. Fecal PL excretion was significantly related to plasma homocysteine (r=0.64) and methionine (r=-0.49). PL, PC and lysoPC excretion is higher in children with C F and is associated with lower plasma methionine and higher homocysteine. Altered choline metabolism may occur in CF and may be important to the hepatic complications.

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