Metabolism of [3H]arachidonic acid- and [14C]linoleic acid-labelled chylomicrons in essential fatty acid-deficient rats

Digestion and Absorption of Milk Phospholipids in Newborns and Adults

Milk polar lipids provide choline, ethanolamine, and polyunsaturated fatty acids, which are needed for the growth and plasticity of the tissues in a suckling child. They may also inhibit cholesterol absorption by interacting with cholesterol during micelle formation. They may also have beneficial luminal, mucosal, and metabolic effects in both the neonate and the adult. The milk fat globule membrane contains large proportions of sphingomyelin (SM), phosphatidylcholine (PC), and phosphatidylethanolamine (PE), and some phosphatidylserine (PS), phosphatidylinositol (PI), and glycosphingolipids. Large-scale technical procedures are available for the enrichment of milk fat globule membrane (MFGM) in milk replacement formulations and food additives. Pancreatic phospholipase A2 (PLA2) and mucosal phospholipase B digest glycero-phospholipids in the adult. In the neonate, where these enzymes may be poorly expressed, pancreatic lipase-related protein 2 probably has a more important role. Mucosal alkaline SM-ase and ceramidase catalyze the digestion of SM in both the neonate and the adult. In the mucosa, the sphingosine is converted into sphingosine-1-phosphate, which is both an intermediate in the conversion to palmitic acid and a signaling molecule. This reaction sequence also generates ethanolamine. Here, we summarize the pathways by which digestion and absorption may be linked to the biological effects of milk polar lipids. In addition to the inhibition of cholesterol absorption and the generation of lipid signals in the gut, the utilization of absorbed choline and ethanolamine for mucosal and hepatic phospholipid synthesis and the acylation of absorbed lyso-PC with polyunsaturated fatty acids to chylomicron and mucosal phospholipids are important.

Is the ENaC Dysregulation in CF an Effect of Protein-Lipid Interaction in the Membranes?

While approximately 2000 mutations have been discovered in the gene coding for the cystic fibrosis transmembrane conductance regulator (CFTR), only a small amount (about 10%) is associated with clinical cystic fibrosis (CF) disease. The discovery of the association between CFTR and the hyperactive epithelial sodium channel (ENaC) has raised the question of the influence of ENaC on the clinical CF phenotype. ENaC disturbance contributes to the pathological secretion, and overexpression of one ENaC subunit, the β-unit, can give a CF-like phenotype in mice with normal acting CFTR. The development of ENaC channel modulators is now in progress. Both CFTR and ENaC are located in the cell membrane and are influenced by its lipid configuration. Recent studies have emphasized the importance of the interaction of lipids and these proteins in the membranes. Linoleic acid deficiency is the most prevailing lipid abnormality in CF, and linoleic acid is an important constituent of membranes. The influence on sodium excretion by linoleic acid supplementation indicates that lipid-protein interaction is of importance for the clinical pathophysiology in CF. Further studies of this association can imply a simple clinical adjuvant in CF therapy.

Metabolism of orally fed [3H]-eicosapentaenoic and [14C]-arachidonic acid in essential fatty acid-deficient rats

The metabolism of individual polyunsaturated fatty acids (PUFA) may be influenced differently by nutritional status and nutritional intake. In normal rats, radioactive arachidonic acid (20:4(n-6), is preferentially retained in tissue phospholipids compared to linoleic (18:2(n-6), or eicosapentaenoic acid (20:5(n-3). This study compares the fate of 20:4(n-6) and 20:5(n-3) acids in essential fatty acid-deficient (EFAD) rats. [3H]-20:5 and [14C]-20:4 were fed in a fish oil emulsion to EFAD rats. Tissue lipids were analysed for radioactivity at 1, 2 and 4 h. The conversion of [3H]-20:5 to docosapentaenoic acid (22:5) and docosahexaenoic acid (22:6) was examined using high performance liquid chromatography (HPLC). The recovery of 3H in small intestine was lower than that of 14C (26 vs. 36% after 4 h, p < 0.001), but was higher in the liver (26 of 3H vs. 22% of 14C, p < 0.01), kidneys (1.5 vs. 1.2%, p < 0.001) and colon (0.3 vs. 0.2%, p = 0.01). The percentages of 3H and 14C in phospholipids were investigated in intestine and liver, and were higher in EFAD rats than in normal rats, particularly for phosphatidylethanolamine (PE). The proportions of [3H]-20:5 transformed to 22:5 and 22:6 did not exceed 7% in the intestine and 10% in the liver. In conclusion, the metabolism of dietary fatty acids 20:4 and 20:5 differed less than in normal rats, mainly due to the preferential retention of both fatty acids in phospholipids.

Absorption and incorporation into tissue lipids of 3H-arachidonic- and 14C-linoleic acid: effects of ethanol in jejunal tissue cultures and in vivo

The effects of ethanol in the rat on the absorption and incorporation of 3H-arachidonic (20: 4, n-6) and 14C-linoleic acid (18:2, n-6) into tissue lipids was examined in jejunal tissue cultures in vivo. The pattern of incorporation earlier seen in the small intestine in vivo, ie. a preferential incorporation of 3H-20: 4 in comparison to 14C-18: 2 into phospholipids (PL), particularly phosphatidylethanolamine (PE) and phosphatidylinositol (PI) was seen in the jejunal tissue cultures. 10 mM ethanol slightly decreased the incorporation of 3H-20: 4 into PE and PI, but did not change the partitioning of labelled fatty acids into phospholipids and triacylglycerols (TG). Ethanol (100 mM) decreased the incorporation of both 3H and 14C into both PL and TG and caused a moderate increase in the TG/PL radioactivity ratio. Rats were also infused intraduodenally with either 10% ethanol or saline and given 3H-20: 4 and 14C-18: 2 in Intralipid. Radioactivity of tissue lipids were analysed after 1, 2 and 3 h. Ethanol did not significantly influence the absorption or the retention in small intestine, liver or heart of 3H or 14C, or the time course for the 3H- and 14C lipid radioactivity in serum. The distribution of 3H and 14C between total nonpolar lipids and individual PLs was also similar in the two groups, 3H-20: 4 thus exhibiting the same preferential incorporation into PLs, compared to 14C-18: 2, in both groups. Significant amounts of 3H and 14C appeared in phosphatidylethanol (PEth) of the small intestine, heart and liver, when the tissues from the ethanol-infused animals were stored frozen before extraction. When the tissues were extracted immediately after the experiment, the proportions of 3H and 14C migrating as PEth did not exceed 0.5%. Although sufficient amounts of ethanol and the phospholipase D activity necessary for PEth formation were thus present in the tissues examined, little PEth formation thus occurred during the ethanol infusion. In the ethanol treated group about 1% of the 3H and 14C radioactivity of the small intestine was in ethyl ester. Intraduodenal infusion of 10% ethanol in vivo thus has little acute effects on the metabolism of absorbed 18: 2 and 20: 4 in the intestinal mucosal cells. Formation of some ethyl ester occurs, however.

The effect of essential fatty acid deficiency on hepatic bile salt sulphotransferase in rats

Hepatic bile salt sulphotransferase (BSS) activity and the contents of unconjugated oestradiol-17 beta (E2) and conjugated oestrone (cE1) in liver tissue was significantly lower in young essential fatty acid (EFA) deficient female rats than in female control rats. No corresponding differences were found between male EFA deficient and control rats. A significant sex difference, with higher values in females, was found for BSS activity and E2 and cE1 contents in control rats but not in EFA deficient rats. The decrease in hepatic BSS activity in female rats caused by EFA deficiency may be mediated via a decreased estrogenic action on the liver.

Adaptive regulation of lipoprotein lipase and salt-resistant lipase activities in essential fatty acid deficiency: an experimental study in the rat

Lipoprotein lipase (LPL) activities of postheparin plasma, heart, lungs, and adipose tissue, and salt-resistant lipase (hepatic lipase, SRL) activities of postheparin plasma, liver, and adrenals were examined in essential fatty acid deficient (EFAD) rats and in age-matched controls. The LPL activity of heart was higher in the deficient than in the control rats, but did not differ in the other tissues. The SRL activity of postheparin plasma was twofold higher, and that of liver and adrenals approximately 50% higher in the group with EFA deficiency. It is suggested that SRL exhibits an adaptive up-regulation in EFA deficiency. This up-regulation may be linked to a role for the enzyme in the transport of polyenoic fatty acids.