[en] During long-term dietary n-3 fatty acid supplementation, eicosapentaenoic acid (EPA) is not incorporated into phosphatidylinositol or -serine of human platelets in vivo and is not detectable in phosphatidic acid upon stimulation with thrombin. However, EPA is released from platelet phospholipids and metabolized to thromboxane B3 (TXB3). In contrast, in vitro, platelets incorporate [¹⁴C]EPA into phosphatidylinositol, whether they contain endogenous EPA in their cellular lipids or not. Following platelet stimulation, [¹⁴C]EPA appears in phosphatidic acid, as free fatty acid, and is transformed to TXB3. The authors conclude that the fatty acid compositions of platelet phospholipid subclasses are regulated with a high degree of specificity in vivo. Qualitative differences exist between in vivo and in vitro uptake of EPA into platelet phospholipid subclasses. After in vivo incorporation, EPA is released by action of a phospholipase A2

[en] We have developed an isotope dilution method for determination of deoxycholic acid pool size and input rate which employs oral administration of 50 mg of [24-¹³C]deoxycholic acid and serum sampling. The method has been validated by classical isotope dilution technique using [24-¹⁴C]deoxycholic acid and bile sampling in five patients with colonic adenomas. Excellent agreement between pool sizes and input rates determined with ¹³C/12C isotope ratio measurements in serum and ¹⁴C measurements in bile was obtained when isotope ratios were measured in the conjugated fraction of deoxycholic acid in serum. We conclude that pool size and input rate of deoxycholic acid can accurately be determined by blood sampling after oral administration of [24-¹³C]deoxycholic acid, therewith eliminating the use of radioactive tracers and the need for bile sampling

[en] As assessed by molecular sieve chromatography and quantitation by a specific radioimmunoassay, apoA-IV is associated in plasma with the triglyceride-rich lipoproteins, to a high density lipoprotein (HDL) subfraction of smaller size than HDL3, and to the plasma lipoprotein-free fraction (LFF). In this study, the turnover of apoA-IV associated to the triglyceride-rich lipoproteins, HDL and LFF was investigated in vivo in normal volunteers. Human apoA-IV isolated from the thoracic duct lymph chylomicrons was radioiodinated and incubated with plasma withdrawn from normal volunteers after a fatty meal. Radioiodinated apoA-IV-labeled triglyceride-rich lipoproteins, HDL, and LFF were then isolated by chromatography on an AcA 34 column. Shortly after the injection of the radioiodinated apoA-IV-labeled triglyceride-rich lipoproteins, most of the radioactivity could be recovered in the HDL and LFF column fractions. On the other hand, when radioiodinated apoA-IV-labeled HDL or LFF were injected, the radioactivity remained with the originally injected fractions at all times. The residence time in plasma of ¹²⁵I-labeled apoA-IV, when injected in association with HDL or LFF, was 1.61 and 0.55 days, respectively. When ¹²⁵I-labeled apoA-IV was injected as a free protein, the radioactivity distributed rapidly among the three plasma pools in proportion to their mass. The overall fractional catabolic rate of apoA-IV in plasma was measured in the three normal subjects and averaged 1.56 pools per day. The mean degradation rate of apoA-IV was 8.69 mg/kg X day

[en] Cholesterol turnover was studied in four baboons by injecting [¹⁴C]cholesterol 186 days and [³H]cholesterol 4 days before necropsy, and fitting a two- or three-pool model to the resulting specific activity-time data. At necropsy, cholesterol mass and specific activity were determined for the total body and for many tissues. The principal aim of this study was to estimate the extent of cholesterol synthesis in the side pools of the model, by computing the amount of side pool synthesis needed to equal the measured total body cholesterol. Central pool synthesis varied from 61 to 89% of the total cholesterol production rate. Moreover, the finding that the measured total body cholesterol fell within the range obtained from the kinetic analysis by using reasonable assumptions, provides evidence for the physiological validity of the model. A second aim of this study was to explore cholesterol turnover in various tissues. A pool model predicts that rapidly turning over tissues will have higher specific activities at early times and lower specific activities at later times after injection of tracer relative to slowly turning over tissues, except where significant synthesis occurs. Results in all four baboons were similar. Turnover rates for the different tissues loosely fell into three groups which were turning over at fast, intermediate, and slow rates. Finally, the magnitude of variation of cholesterol specific activity was moderate for several distributed tissues (fat, muscle, arteries, and the alimentary tract), but was small for liver. Cholesterol turnover in serial biopsies of skin, muscle, and fat could, however, be fitted with a single pool to estimate tissue turnover rates

[en] The extent of oxidoreduction of the 3 alpha-, 7 alpha- and 12 alpha-hydroxyl groups in bile acids during the enterohepatic circulation in man was studied with the use of [3 beta-³H]-labeled deoxycholic acid and cholic acid, [7 beta-³H]-labeled cholic acid, and [12 beta-³H]-labeled deoxycholic acid and cholic acid. Each [³H]-labeled bile acid was given per os to healthy volunteers, together with the corresponding [24-¹⁴C]-labeled bile acid. The rate of oxidoreduction was calculated from the decrease in the ratio between ³H and ¹⁴C in the respective bile acid isolated from duodenal contents collected at different time intervals after administration of the labeled bile acids. The mean fractional conversion rate was found to be 0.29 day-1 for the 3 alpha-hydroxyl group in deoxycholic acid (n = 2), 0.18 day-1 for the 12 alpha-hydroxyl group in deoxycholic acid (n = 6), 0.09 day-1 for the 3 alpha-hydroxyl group in cholic acid (n = 3), 0.05 day-1 for the 7 alpha-hydroxyl group in cholic acid (n = 2), and 0.03 day-1 for the 12 alpha-hydroxyl group in cholic acid (n = 2). The extent of oxidoreduction of the 12 alpha-hydroxyl group in [12 beta-³H]-labeled deoxycholic acid given to two patients operated with subtotal colectomy and ileostomy was markedly reduced (less than 20% of normal)

[en] In studies where D-(-)-3-hydroxy[4,4,4-₂H³]butyrate is employed as isotopic tracer in vivo, we have described a selected ion monitoring, gas-liquid chromatography-mass spectrometry micromethod which measures [₂H³] tracer enrichment in 3-hydroxybutyrate and acetoacetate from 300-microliters blood samples. For plasma samples in the physiologic range, intra- and interassay precisions for each ketone averaged better than +/- 1% and +/- 2%, respectively. The use of the method was validated by comparing kinetic data obtained with the above tracer with simultaneous flux data obtained with conventional D-(-)-3-hydroxy[3-¹⁴C]butyrate tracer in five fasted rats

[en] A series of 3-monohydroxylated bile acids, in unlabeled and radioactive form, of varying side chain length and configuration at C-3 has been synthesized and rigorously characterized. They include: 3 alpha- and 3 beta-hydroxy-5 beta-androstane-17 beta-carboxylic acids (C20); 3 alpha- and 3 beta-hydroxy-5 beta-pregnan-21-oic acids (C21); 3 alpha- and 3 beta-hydroxy-23,24-bisnor-5 beta-cholan-22-oic acids (C22); 3 alpha- and 3 beta-hydroxy-24-nor-5 beta-cholan-23-oic acids (C23, norlithocholic and isonorlithocholic acids); and 3 beta-hydroxy-5 beta-cholan-24-oic acid (C24, isolithocholic acid). A novel approach to the degradation of lithocholic acid acetate to 24-norlithocholic acid is described. This degradation involves the photochemical modification of a Hunsdiecker reaction and Kornblum oxidation of the intermediate 23-bromide. The availability of these compounds makes it possible to study the metabolism and biological effects of short chain bile acids

[en] The origin of the cholesterol needed by the cornea for growth and cell turnover was addressed by comparing absolute rates of sterol synthesis with rates of sterol accumulation during early development of the rabbit. Linearity of incorporation of ³H₂O and [¹⁴C]mevalonate into digitonin-precipitable sterols with time of incubation in vitro and a lack of accumulation of ¹⁴C in intermediates of sterol biosynthesis indicated that tritiated water can validly be used to measure rates of sterol synthesis by the cornea. The rate of sterol synthesis per unit weight of rabbit cornea was constant between 14 and 60 days of age at an average 1.03 nmol of ³H of ³H₂O incorporated/mg dry cornea per 8 h. Essentially all of the synthesized cholesterol and most of the cholesterol mass was present in corneal epithelium. The cumulative sterol synthesized over the 46-day period studied exceeded the observed rate of cholesterol accumulation by sixfold. Cholesterol synthesized in excess of the growth requirement was likely used to support turnover of the epithelium which was estimated at 9 days. Removal of cholesterol from the cornea by excretion into tear fluid and clearance by high density lipoproteins are also considered

[en] Lipoprotein secretion by Caco-2 cells, a human intestinal cell line, was studied in cells grown on inserts containing a Millipore filter (0.45 micron), separating secretory products from the apical and basolateral membranes into separate chambers. Under these conditions, as observed by electron microscopy, the cells formed a monolayer of columnar epithelial cells with microvilli on the apical surface and tight junctions between cells. The electrical resistances of the cell monolayers were 250-500 ohms/cm2. Both ¹⁴C-labeled lipids and ³⁵S-labeled proteins were used to assess lipoprotein secretion. After a 24-hr incubation with [¹⁴C]oleic acid, 60-80% of the secreted triglyceride (TG) was in the basolateral chamber; 40% of the TG was present in the d less than 1.006 g/ml (chylomicron + VLDL) fraction and 50% in the 1.006 less than d less than 1.063 g/ml (LDL) fraction. After a 4-hr incubation with [³⁵S]methionine, apolipoproteins were found to be major secretory products with 75-100% secreted to the basolateral chamber. Apolipoproteins B-100, B-48, E, A-I, A-IV, and C-III were identified by immunoprecipitation. The d less than 1.006 g/ml fraction was found to contain all of the major apolipoproteins, while the LDL fraction contained primarily apoB-100 and apoE; the HDL (1.063 less than d less than 1.21 g/ml) fraction principally contained apoA-I and apoA-IV. Mn-heparin precipitated all of the [³⁵S]methionine-labeled apoB-100 and B-48 and a majority of the other apolipoproteins, and 80% of the [¹⁴C]oleic acid-labeled triglyceride, but only 15% of the phospholipid, demonstrating that Caco-2 cells secrete triglyceride-rich lipoproteins containing apoB

[en] Apolipoprotein E (apoE) synthesis has been examined in rat and guinea pig tissues using in vitro translation and [³⁵S]methionine labeling of tissue slices. A number of tissues not involved in lipoprotein synthesis synthesize a protein very similar to apoE, including the spleen, adrenal, kidney, testis, ovary, heart, and lung. Although the intestine is involved in lipoprotein synthesis, apoE synthesis could not be detected in intestinal mucosa. The protein synthesized by the extrahepatic tissues was identified as apoE by its electrophoretic mobility, its immunologic reactivity with a monospecific antibody and by limited proteolysis mapping with Staphylococcus aureus V8 protease. ApoE represented between 0.02 and 0.7% of the total protein synthesized in the extrahepatic tissues, indicating that apoE mRNA is a fairly abundant mRNA in these tissues. ApoE mRNA was also detected by hybridization with a rat apoE cDNA clone, which hybridized to a single mRNA 1250 nucleotides in length in rat liver and in extrahepatic tissues. Hybridization of the apoE clone to rat genomic DNA demonstrated that the apoE gene was more heavily methylated in intestinal mucosa, which did not synthesize apoE, than in liver, testis, or kidney. ³⁵S labeling of peritoneal macrophages revealed that both rat and guinea pig macrophages synthesized and secreted apoE in vitro. Rhesus aortic smooth muscle cells also synthesized and secreted apoE. The possible functions of apoE synthesized in the peripheral tissues are considered