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Biomedical subjects

R K Ockner

Publications and source records attributed to R K Ockner.

At least 55 records · Page 3Linked to original sources
PubMed1976

Fatty acid binding protein. Role in esterification of absorbed long chain fatty acid in rat intestine.

Fatty acid binding protein (FABP) is a protein of 12,000 mol wt found in cytosol of intestinal mucosa and other tissues, which exhibits high affinity for long chain fatty acids. It has been suggested that FABP (which may comprise a group of closely related proteins of 12,000 mol wt) participates in cellular fatty acid transport and metabolism. Although earlier findings were consistent with this concept, the present studies were designed to examine its physiological function more directly. Everted jejunal sacs were incubated in mixed fatty acid-monoglyceride-bile acid micelles, in the presence or absence of equimolar concentrations of either of two compounds which inhibit oleate binding to FABP:flavaspidic acid-N-methyl-glucaminate and alpha-bromopalmitate. Oleate uptake, mucosal morphology, and oxidation of [14C]acetate remained unaffected by these agents, but oleate incorporation into triglyceride was inhibited by 62-64% after 4 min. The inhibition by flavaspidic acid was reversible with higher oleate concentrations. The effect of these compounds on enzymes of triglyceride biosynthesis was examined in intestinal microsomes. Neither flavaspidic acid nor alpha-bromopalmitate inhibited acyl CoA:monoglyceride acyl-transferase. Fatty acid:coenzyme A ligase activity was significantly enhanced in the presence of partially purified FABP, probably reflecting a physical effect on the fatty acid substrate or on the formation of the enzyme-substrate complex. Activity of the enzyme in the presence of 0.1 mM oleate was only modestly inhibited by equimolar flavaspidic acid and alpha-bromopalmitate, and this effect was blunted or prevented by FABP. We conclude that in everted gut sacs, inhibition of triglyceride synthesis by flavaspidic acid and alpha-bromopalmitate could not be explained as an effect on fatty acid uptake or on esterifying enzymes in the endoplasmic reticulum but rather can be interpreted as reflecting inhibition of fatty acid binding to FABP. These findings lend further support to the concept that FABP participates in cellular fatty acid transport and metabolism. It is also possible that FABP, by effecting an intracellular compartmentalization of fatty acids and acyl CoA, may play a broader role in cellular lipid metabolism.

Acids↗
PubMed1976

Morphometric analysis of rat hepatocytes after total billary obstruction.

Using light and electron microscopic morphometric techniques, the effects of 48 hr of extrahepatic biliary obstruction on hepatocyte structure were examined in the rat. Liver cells near the portal area were compared to those in the centrilobular regions of the hepatic lobule. Observations on the normal animals confirm earlier evidence of quantitative differences in the surface density of organelles in hepatocytes located within different regions of the lobule. A striking difference in the quantity of the Golgi complex in the two areas of the lobule was noted for the first time, with the portal cells containing a significantly greater quantity of this organelle than centrolobular hepatocytes. After 48 hr of total obstruction, most of the previously reported qualitative changes in the canalicular and pericanalicular regions were confirmed. Morphometric analysis at the light-microscopic level showed an increase in the number of cells and a decrease in cell size in those cells near the portal area were compared to those in the centrolobular regions of the helar level demonstrated a significant decrease in both rough and smooth surfaced endoplasmic reticulum in cells of both zones, a finding in marked contrast to the hypertrophy of smooth endoplasmic reticulum suggested by other investigators on the basis of qualitative assessments. There was also a striking decrease in the amount of the Golgi complex, limited to cells in the portal regions. In addition, in all zones a decrease in the volume density of mitochondria and lysosomes was noted, whereas the volume of microbodies was increased. It is suggested that this loss in total membrane material within the cell may be secondary to the degranulation and decrease in total surface area of rough surfaced endoplasmic reticulum, an organelle thought to be responsible in part for the synthesis of new cellular membranes. These observations suggest that present concepts concerning the pathogenesis of cholestatic liver disease require reappraisal.

Animals↗
PubMed1975

Intestinal metabolism of plasma free fatty acids. Intracellular compartmentation and mechanisms of control.

Fatty acid metabolism in intestinal mucosa has been examined primarily in regard to lipid absorption. Since earlier studies suggested intestinal utilization of plasma free fatty acids (FFA), we investigated mucosal metabolism of plasma FFA in rats. Mucosal radioactivity (1 per cent of administered) was maximal 2 min after i.v. [14C]palmitate. Of mucosal 14C, 42 percent was in water-soluble metabolites, including CO2 and ketoacids, 28 percent in phospholipids, and only 16 per cent in triglycerides. The specific activity of mucosal triglyceride fatty acids (TGFA) was 11 times that of serum TGFA, confirming in situ synthesis. Double isotope experiments showed marked differences in the metabolism of fatty acids entering mucosa simultaneously from lumen and plasma. Whereas luminal fatty acids were chiefly esterified to triglyceride, plasma FFA were preferentially oxidized and incorporated into phospholipids. Crypts did not differ from villi, indicating that intestinal metabolism of plasma FFA is related to their site of entry into epithelial cells. Mucosal metabolism of i.v. [14C]palmitate was minimally affected by glucose administration. However, intraduodenal isocaloric ethanol inhibited mucosal oxidation of FFA by 60 per cent, and increased incorporation into triglycerides nearly twofold. During lipid absorption, mucosal uptake of plasma FFA doubled and incorporation into intestinal lymph triglycerides was increased sixfold. These studies demonstrate an intracellular compartmentation of fatty acids in the intestinal epithelium. In contrast to absorbed luminal fatty acids, plasma FFA in the fasting state are both an energy source and a substrate for the synthesis of tissue phospholipid. The fasting contribution of plasma FFA to mucosal and lymph triglyceride is minimal, but it increases during ethanol administration and fat absorption.

Animals↗
PubMed1974

Fatty acid-binding protein in small intestine. Identification, isolation, and evidence for its role in cellular fatty acid transport.

A soluble fatty acid-binding protein (FABP), mol wt approximately 12,000 is present in intestinal mucosa and other tissues that utilize fatty acids, including liver, myocardium, adipose, and kidney. This protein binds long chain fatty acids both in vivo and in vitro.FABP was isolated from rat intestine by gel filtration and isoelectric focusing. It showed a reaction of complete immunochemical identity with proteins in the 12,000 mol wt fatty acid-binding fractions of liver, myocardium, and adipose tissue supernates. (The presence of immunochemically nonidentical 12,000 mol wt FABP in these tissues is not excluded.) By quantitative radial immunodiffusion, supernatant FABP concentration in mucosa from proximal and middle thirds of jejuno-ileum significantly exceeded that in distal third, duodenum, and liver, expressed as micrograms per milligram soluble protein, micrograms per gram DNA, and micrograms per gram tissue. FABP concentration in villi was approximately three times greater than in crypts. Small quantities of FABP were present in washed nuclei-cell membrane, mitochondrial and microsomal fractions. However, the amount of FABP solubilized per milligram membrane protein was similar for all particulate fractions, and total membrane-associated FABP was only about 16% of supernatant FABP. Intestinal FABP concentration was significantly greater in animals maintained on high fat diets than on low fat; saturated and unsaturated fat diets did not differ greatly in this regard.The preponderance of FABP in villi from proximal and middle intestine, its ability to bind fatty acids in vivo as well as in vitro, and its response to changes in dietary fat intake support the concept that this protein participates in cellular fatty acid transport during fat absorption. Identical or closely related 12,000 mol wt proteins may serve similar functions in other tissues.

Adipose Tissue↗
PubMed1972

A binding protein for fatty acids in cytosol of intestinal mucosa, liver, myocardium, and other tissues.

A protein of molecular weight approximately 12,000 which binds long-chain fatty acids and certain other lipids has been identified in cytosol of intestinal mucosa, liver, myocardium, adipose tissue, and kidney. Binding is noncovalent and is greater for unsaturated than for saturated and medium-chain fatty acids. This protein appears to be identical with the smaller of two previously described cytoplasmic anion-binding proteins. Binding of long-chain fatty acids by this protein is greater than that of other anions tested, including sulfobromophthalein, and does not depend on negative charge alone. The presence of this binding protein may explain previously observed differences in intestinal absorption among fatty acids, and the protein may participate in the utilization of long-chain fatty acids by many mammalian tissues.

Adipose Tissue↗
PubMed1971

An electron microscopic study of endogenous very low density lipoprotein production in the intestine of rat and man.

Previous studies have shown that in the absence of dietary lipid, intestinal lymph contains endogenous very low density lipoproteins (VLDL) which are identical to those in plasma in size, flotation rate, composition, and electrophoretic mobility. In order to document that these particles are produced in the mucosa of the small intestine itself, electron microscopic studies of rat and human intestinal mucosa were carried out. Small intestinal absorptive cells from rats fasted and restrained for 48 hr were rich in osmiophilic particles of the size of VLDL (300-1000 A). These particles were present in the endoplasmic reticulum and Golgi apparatus, and in intercellular spaces and lacteals; they were most abundant in mucosa from mid-jejunum. Similar particles were seen in jejunal mucosal biopsy specimens obtained from normal human volunteers after a 40-hr fast. After 6 hr of bile diversion or cholestyramine administration to fasted rats, the VLDL-sized particles virtually disappeared from the mucosa, suggesting that they were produced in the mucosa itself and depended upon the absorption of endogenous intralumenal lipid. These studies provide further evidence for the production of VLDL in absorptive cells of fasting rat and human intestine, and support the concept that the small intestine is a source of endogenous plasma VLDL.

Animals↗
PubMed1970

An electron microscopic and functional study of very low density lipoproteins in intestinal lymph.

Previous studies with fasting rats showed that the intestine produces endogenous very low density lipoproteins (VLDL) which resemble those in the plasma. Intestinal VLDL also were found to be important in lipid transport during absorption of saturated but not of unsaturated fat. These findings depended upon separations of a chylomicron-rich fraction (S(f) > 400) from VLDL (S(f) 20-400) by preparative ultracentrifugation methods based on particle flotation rates. The present studies correlate this method with electron microscopic measurement of lipoprotein particle size. Almost all intestinal lymph lipoprotein particles from fasting rats were less than 750 A in diameter, and could not be distinguished morphologically from plasma VLDL. Cholestyramine administration or bile diversion led to decreased lymph lipid output, correlating with marked reduction in VLDL. This supports the concept that lymph VLDL contain endogenous lipid which is reabsorbed from the intestinal lumen. During exogenous fatty acid absorption, lymph lipoprotein particle sizes were significantly smaller after administration of palmitate than after administration of linoleate, a finding consistent with ultracentrifugal evidence of the importance of VLDL in lipid transport during palmitate absorption. These studies fully confirm and extend earlier observations. Together, they show that the intestine is a source of endogenous VLDL in the fasting animal. In addition, significant quantities of exogenous lipid are transported in VLDL during palmitate absorption, whereas with linoleate absorption nearly all lipid is in chylomicrons. These findings indicate that the small intestine plays a role in lipoprotein metabolism which extends beyond the absorption of dietary fat.

Animals↗
PubMed1969

Effect of mixed micellar lipid on the absorption of cholesterol and vitamin D3 into lymph.

The absorption of endogenous cholesterol, labeled with tracer doses of cholesterol (14)C or cholesterol-(3)H and of near physiological doses of vitamin D(3)-(3)H was studied in rats with cannulated intestinal lymphatics. The effects of administering mixed micellar solutions of fatty acid, monoglyceride, and bile salt on the absorption of these labeled sterols was determined. It was observed that the specific activity of free cholesterol and the amounts of vitamin D(3) appearing in lymph were significantly increased during the intraduodenal administration of mixed micellar solutions of either linoleic or palmitic acid, in contrast to control rats receiving a micellar solution of taurocholate. These increases were related linearly to the lymph triglyceride level. In addition it was observed that when the linoleic acid solution was administered there was a more marked increase in the ratio of the specific activities of free and esterified cholesterol in lymph than with either the palmitic acid or taurocholate solutions. Additional studies in rats with intact lymphatics showed that the uptake of labeled cholesterol and vitamin D(3) from the intestinal lumen into the wall was similar whether the sterols were administered in taurocholate or in mixed micellar solution. These findings suggest that mixed micellar lipid increased the rate of appearance of labeled free cholesterol and vitamin D(3) in lymph by enhancing their transport out of the intestinal mucosa, rather than by an effect on uptake.

Animals↗