Enzymes of glycerolipid synthesis in small-intestinal mucosa of foetal and neonatal guinea pigs

Lipin 2/3 phosphatidic acid phosphatases maintain phospholipid homeostasis to regulate chylomicron synthesis

The lipin phosphatidic acid phosphatase (PAP) enzymes are required for triacylglycerol (TAG) synthesis from glycerol 3-phosphate in most mammalian tissues. The 3 lipin proteins (lipin 1, lipin 2, and lipin 3) each have PAP activity, but have distinct tissue distributions, with lipin 1 being the predominant PAP enzyme in many metabolic tissues. One exception is the small intestine, which is unique in expressing exclusively lipin 2 and lipin 3. TAG synthesis in small intestinal enterocytes utilizes 2-monoacylglycerol and does not require the PAP reaction, making the role of lipin proteins in enterocytes unclear. Enterocyte TAGs are stored transiently as cytosolic lipid droplets or incorporated into lipoproteins (chylomicrons) for secretion. We determined that lipin enzymes are critical for chylomicron biogenesis, through regulation of membrane phospholipid composition and association of apolipoprotein B48 with nascent chylomicron particles. Lipin 2/3 deficiency caused phosphatidic acid accumulation and mammalian target of rapamycin complex 1 (mTORC1) activation, which were associated with enhanced protein levels of a key phospholipid biosynthetic enzyme (CTP:phosphocholine cytidylyltransferase α) and altered membrane phospholipid composition. Impaired chylomicron synthesis in lipin 2/3 deficiency could be rescued by normalizing phospholipid synthesis levels. These data implicate lipin 2/3 as a control point for enterocyte phospholipid homeostasis and chylomicron biogenesis.

Aging and cholesterol uptake in the rabbit jejunum: role of the bile salt micelle and the unstirred water layer

A previously validated in vitro technique was used to determine the effect of aging upon the rate of uptake of cholesterol into the jejunum of suckling, young, and older rabbits. Cholesterol uptake was greater in suckling than in older animals, over a wide range of durations of incubation and varying concentrations of cholesterol or bile acid. The rate of uptake of cholesterol in the young animals was intermediate between the values seen in the suckling and older rabbits. This greater uptake of cholesterol in the younger than in the older animals persisted when the effective resistance of the unstirred water layer (UWL) was varied by stirring the bulk phase. In contrast, the uptake of medium- and long-chain length fatty acids was greater in the young than in the older animals when UWL was low, but the converse was true when UWL was high at each rate of stirring of the bulk phase. The UWL was lower in the younger than in the older rabbits. Thus, the differences in the in vitro uptake of cholesterol into the jejunum of rabbits of varying age is due to the greater passive permeability properties and greater functional membrane surface area of the jejunum of young animals, and the lower effective resistance of the overlying unstirred water layer.

Co-ordinate changes in enzymes of fatty acid synthesis, activation and esterification in rabbit mammary gland druing pregnancy and lactation

1. The activities of fatty acid synthetase, acyl-CoA synthetase, glycerol phosphate acyltransferase and phosphatidate phosphatase were measured in the mammary glands of rabbits from day 16 of pregnancy to day 15 of post partum. 2. There were significant correlations between the increases in activities of these enzymes during this period. This was the case whether the activities were expressed per mg of homogenate protein, per g wet wt. of tissue or per total wet weight of the whole glands. The only exception was the lack of correlation between the activities of fatty acid synthetase and of phosphatidate phosphatase per g wet wt. of tissue. 3. These co-ordinate increases are discussed in relation to the changes which occur in fatty acid metabolism in the mammary gland during pregnancy and lactation.