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Small Intestine and Liver Microsomal Triacylglycerol Transfer Protein in the Bovine and Rat: Effects of Dietary Coconut Oil

Unité de Recherches sur les Herbivores, Equipe Nutriments et Métabolismes, Institut National de la Recherche Agronomique, Centre de Recherches de Clermont Ferrand-Theix, 63122 Saint-Genès Champanelle, France

Corresponding author: D. Gruffat; e-mail: gruffat{at}clermont.inra.fr.

The bovine liver is characterized by a chronic low capacity to secrete triacylglycerols (TAG). In situations favoring their hepatic synthesis, such as coconut oil feeding, TAG accumulate, leading to a lipid infiltration in the liver of preruminant calves. To assess the possible role of the microsomal TAG transfer protein (MTP) in this phenomenon and to put into evidence a tissue-specific regulation in the bovine species, we compared by Western blot the content in both MTP subunits in the liver and in different portions of the small intestine in preruminant calves and in growing rats receiving coconut oil or beef tallow as the sole source of fat in the diet. The pattern of MTP distribution was similar between calf and rat tissues, the jejunum being the major site for both MTP expression and intestinal absorption of dietary lipid endproducts. Concentrations of the MTP large and small subunits were 10- to 20-fold lower and 2- to 3-fold lower, respectively, in calf than in rat tissues, including the liver. Coconut oil in the diets of calves and rats did not significantly affect the expression of MTP large subunits even though TAG content was strongly increased 12-fold in the calf liver. These results clearly indicated that calf liver handled fat metabolically in a manner different from rat liver. However, present experimental conditions did not allow proof that MTP was directly related to the accumulation of fat in calf liver.

Key Words: coconut oil • liver • microsomal triacylglycerol transfer protein • small intestine

Abbreviation key: apo B = apolipoprotein B, BT = beef tallow, CO = coconut oil, DAU = densitometric arbitrary unit, FA = fatty acids, MTP = microsomal triacylglycerol transfer protein, PDI = protein disulfide isomerase, TAG = triacylglycerol, VLDL = very low density lipoprotein.