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Biomedical subjects

K R Norum

Publications and source records attributed to K R Norum.

At least 91 records · Page 5Linked to original sources
PubMed1985

Hepatic retinol metabolism. Distribution of retinoids, enzymes, and binding proteins in isolated rat liver cells.

The main retinoids and some binding proteins and enzymes involved in retinol metabolism have been quantified in different types of rat liver cells. Hepatic perisinusoidal stellate cells contained 28-34 nmol of retinoids/10(6) cells, and parenchymal liver cells contained 0.5-0.8 nmol of retinoids/10(6) cells, suggesting that as much as 80% of more of total liver retinoids might be stored in stellate cells with the rest stored in parenchymal cells. Isolated endothelial cells and Kupffer cells contained very low levels of retinoids. More than 98% of the retinoids recovered in stellate cells were retinyl esters. Isolated parenchymal and stellate cell preparations both contained considerable retinyl palmitate hydrolase and acyl-CoA:retinol acyltransferase activities. Parenchymal cells accounted for about 75-80% of the total hepatic content of these two enzyme activities, with the rest located in stellate cells. On a cell protein basis, the concentrations of both of these activities were much greater in stellate cells than in parenchymal cells. In contrast, cholesteryl oleate and triolein hydrolase activities were fairly evenly distributed in all types of liver cells. Large amounts of cellular retinol binding proteins were also found in parenchymal and stellate cells. Although parenchymal cells accounted for more than 90% of hepatic cellular retinol binding protein, the concentration of the protein in stellate cells (per unit protein) was 22 X greater than that in parenchymal cells. Stellate cells were also enriched in cellular retinoic acid binding protein. Thus, both parenchymal and stellate cells contain substantial amounts of retinoids and of the enzymes and intracellular binding proteins involved in retinol metabolism. Stellate cells are particularly enriched in these several components.

Acyltransferases↗
PubMed1985

Hepatic uptake of [3H]retinol bound to the serum retinol binding protein involves both parenchymal and perisinusoidal stellate cells.

We have studied the hepatic uptake of retinol bound to the circulating retinol binding protein-transthyretin complex. Labeled complex was obtained from the plasma of donor rats that were fed radioactive retinol. When labeled retinol-retinol binding protein-transthyretin complex was injected intravenously into control rats, about 45% of the administered dose was recovered in liver after 56 h. Parenchymal liver cells were responsible for an initial rapid uptake. Perisinusoidal stellate cells initially accumulated radioactivity more slowly than did the parenchymal cells, but after 16 h, these cells contained more radioactivity than the parenchymal cells. After 56 h, about 70% of the radioactivity recovered in liver was present in stellate cells. For the first 2 h after injection, most of the radioactivity in parenchymal cells was recovered as unesterified retinol. The radioactivity in the retinyl ester fraction increased after a lag period of about 2 h, and after 5 h more than 60% of the radioactivity was recovered as retinyl esters. In stellate cells, radioactivity was mostly present as retinyl esters at all time points examined. Uptake of retinol in both parenchymal cells and stellate cells was reduced considerably in vitamin A-deficient rats. Less than 5% of the injected dose of radioactivity was found in liver after 5-6 h (as compared to 25% in control rats), and the radioactivity recovered in liver from these animals was mostly in the unesterified retinol fraction. Studies with separated cells in vitro suggested that both parenchymal and stellate cells isolated from control rats were able to take up retinol from the retinol-retinol binding protein-transthyretin complex. This uptake was temperature dependent.

Animals↗
PubMed1985

Retinol esterification in cultured rat liver cells.

Retinol esterification was examined in cultured hepatocytes and stellate cells from the rat. Esterification of [3H]retinol was linear for 2 h in both cell types. By increasing the concentration of retinol in the medium, there was a marked increase in retinol esterification in both cell types. The capacity for esterification of retinol was in the same order of magnitude in the two cell types at 3.5 microM-retinol in the medium. This represents a rate of retinol esterification which far exceeds that required to esterify the amount of retinol absorbed in the intestine. It was demonstrated in particulate homogenates from cultured hepatocytes that the esterification of retinol was dependent on acyl-CoA. Addition of 25-hydroxycholesterol or mevalonolactone promoted an increase in cholesterol esterification, whereas retinol esterification was unaffected, suggesting that cholesterol and retinol are esterified by two different enzymes. Some 80% of vitamin A in cultured hepatocytes is retinyl esters, mostly retinyl palmitate. By adding 87 microM-retinol in the medium the cells accumulated 100-fold free retinol and 2.5-3.0-fold retinyl esters within 1 h. When retinol-loaded cells were incubated without retinol, there was a marked decrease especially in free but also in esterified retinol. In the presence of 1 mM-oleic acid in the medium the amount of retinyl oleate was twice that in control cells.

Acyltransferases↗
PubMed1985

Chain-shortening of erucic acid and microperoxisomal beta-oxidation in rat small intestine.

The ability of rat small intestine to chain-shorten C22:1 fatty acids was investigated. Radioactive chain-shortened products, mainly C18:1, were demonstrated in intestinal-lymph lipids after intraluminal injection of [14-14C]erucic acid. Chain-elongation to C24:1 was also observed. Adaptation to a diet containing C22:1 fatty acids (partially hydrogenated-marine-oil diet) slightly increased the percentage of chain-shortened products. Microperoxisomal beta-oxidation activity, measured as CN(-)-insensitive palmitoyl-CoA-dependent NAD+ reduction, was detected in a microperoxisome-enriched fraction from mucosal scrapings. This activity was increased 1.9-fold by a soya-bean-oil diet, and 2.7-fold by a diet containing partially hydrogenated marine oil.

Animals↗
PubMed1985

Cholesterol esterification in human secretory endometrium and in endometrial cancer tissue. Demonstration of microsomal acyl-CoA-cholesterol acyl-transferase (ACAT) activity.

Human secretory endometrium and endometrial cancer tissue both contain acyl-CoA:cholesterol-acyltransferase (ACAT). The optimal assay conditions for measuring ACAT activity in microsomes prepared from these tissues were determined. The activity in normal secretory endometrium was 15.3 +/- 1.27 pmol cholesteryl ester formed per mg protein-1 per min-1, whereas 3 out of 4 endometrial cancers had 3-7-fold higher ACAT activity.

Adult↗
PubMed1985

Liver retinoids and retinol esterification in fetal and pregnant rats at term.

Retinol esterification in fetal rats and their mothers at term was studied in liver microsomes. The esterification rate was 0.28 +/- 0.05 nmol ester formed per milligram protein per minute, a value somewhat lower than that found in their mothers (0.44 +/- 0.11). The fetuses had significant amounts of liver retinoids. Analysis by high-performance liquid chromatography showed that the retinoid store consisted mainly of retinyl ester both in fetal and adult rat livers, but the fetal livers had higher percentages of free retinol and retinyl oleate than the adult livers. The presence of retinol esterification and a retinyl ester store in fetal rat liver at term is in accordance with the view that retinol brought to liver on retinol-binding protein can be taken up and retained there.

Acetyltransferases↗
PubMed1985

Retinol and retinyl esters in parenchymal and nonparenchymal rat liver cell fractions after long-term administration of ethanol.

Chronic ethanol consumption reduces the liver retinoid store in man and rat. We have studied the effect of ethanol on some aspects of retinoid metabolism in parenchymal and nonparenchymal liver cells. Rats fed 36% of total energy intake as ethanol for 5-6 weeks had the liver retinoid concentration reduced to about one-third, as compared to pair-fed controls. The reduction in liver retinoid affected both the parenchymal and the nonparenchymal cell fractions. Plasma retinol level was normal. Liver uptake of injected chylomicron [3H]retinyl ester was similar in the experimental and control group. The transport of retinoid from the parenchymal to the nonparenchymal cells was not found to be significantly retarded in the ethanol-fed rats. Despite the reduction in total retinoid level in liver, the concentrations of unesterified retinol and retinyl oleate were increased in the ethanol fed rats. Hepatic retinol esterification was not significantly affected in the ethanol-fed rats. Since our study has demonstrated that liver uptake of chylomicron retinyl ester is not impaired in the ethanol-fed rat, we suggest that liver retinoid metabolism may be increased.

Acyltransferases↗
PubMed1984

Clearance of acetyl low density lipoprotein by rat liver endothelial cells. Implications for hepatic cholesterol metabolism.

We have studied the hepatic uptake of human [14C] cholesteryl oleate labeled acetyl low density lipoprotein (LDL). Acetyl-LDL injected intravenously into rats was cleared from the blood with a half-life of about 10 min. About 80% of the injected acetyl-LDL was recovered in the liver after 1 h. Initially, most of the [14C]cholesterol was recovered in liver endothelial cells (about 60%). Some radioactivity (about 15%) was also recovered in the hepatocytes, while the Kupffer cells and stellate cells contained only small amounts of the label (less than 5%). About 1 h after injection, radioactivity started to disappear from endothelial cells and appeared instead in hepatocytes. Radioactivity subsequently declined in hepatocytes as well. After a lag phase of 4 h, significant amounts of radioactivity were recovered in bile. The in vitro uptake and hydrolysis of [14C]cholesteryl oleate-labeled acetyl-LDL were saturable in isolated rat liver endothelial cells. Native LDL does neither affect the uptake nor the hydrolysis of acetyl-LDL. Ammonia and monensin reduced the hydrolysis of acetyl-LDL in isolated liver endothelial cells. Furthermore, monensin at concentrations above 10 microM completely blocked the binding of acetyl-LDL to the liver endothelial cells, suggesting that the receptor for acetyl-LDL is trapped inside the cells. The liver endothelial cells may be involved in the protection against atherogenic lipoproteins, e.g. liver endothelial cells may mediate uptake of cholesterol from plasma and transfer of cholesterol to the hepatocytes for further secretion into the bile.

Animals↗
PubMed1984

Lymphatic absorption and transport of retinol and vitamin D-3 from rat intestine. Evidence for different pathways.

The lymphatic absorption and transport of retinol and vitamin D-3 from rat intestine has been studied. When rats were cannulated in the intestinal lymph duct and given an intraduodenal bolus of [3H]retinol and 14C-labelled vitamin D-3, 14C-labeled vitamin D-3 appeared later in the intestinal lymph than [3H]retinol and the rate of absorption of vitamin D-3 was still maximal at a time when that of retinol had declined. Both vitamins were absorbed via the lymphatic route in association with chylomicrons. Almost all the retinol was esterified, while vitamin D-3 appeared in the chylomicrons as free vitamin D-3. In vitro incubations and in vivo studies using hepatectomized and normal rats showed that the retinyl ester was a relatively nonexchangeable component of the chylomicrons and their remnants. Hence, all the vitamin A followed the remnants in their clearance from plasma. In contrast, significant amounts of vitamin D-3 were transferred from the chylomicrons to other plasma fractions. Therefore, only a fraction of this vitamin may be removed in association with the chylomicron remnants.

Animals↗
PubMed1984

The activity of acyl CoA: retinol acyltransferase in the rat: variation with vitamin A status.

Retinol esterification in the small intestine, liver and kidney of rats given a normal diet or a vitamin-A-free diet and of rats given large doses of vitamin A was studied. The active enzyme is a microsomal acyl CoA:retinol acyl transferase (ARAT). In the small intestine ARAT activity was 0.37 nmol ester/mg microsomal protein per min. Large doses of vitamin A increased the activity significantly, while the enzyme activity in the vitamin-A-deficient rats was in the range of that of the controls. Retinoic acid in physiological doses (0.064 mg three times per week) had no influence on ARAT activity. In the liver, ARAT activity of the controls was 0.58 nmol ester/mg microsomal protein per min. The activity was increased after large doses of vitamin A. It was not significantly reduced in vitamin-A-deficient animals. The kidney had a low, but significant ARAT activity, both in normal and vitamin-A-deficient animals and after large doses of vitamin A (range 0.08-0.14 nmol ester/mg microsomal protein per min). The vitamin-A-esterifying enzyme in the small intestine and liver of the rat seems to be influenced by the amount of retinol in the diet.

Acyltransferases↗
PubMed1984

Changes in the content of n-6 fatty acids in liver phospholipids in rats as a consequence of partially hydrogenated dietary oils.

The effects of unhydrogenated and partially-hydrogenated marine and rapeseed oils on the relative content of n-6 fatty acids in rat liver phospholipids were studied by gas-liquid chromatography. The type of dietary oil strongly affected the pattern of n-6 fatty acids in the liver phospholipids even after a relatively short feeding period (3 weeks). With diets deficient in linoleic acid, accumulation of the 20:3 n-9 fatty acid was observed in the animals receiving partially hydrogenated oils, but not in the animals receiving the unhydrogenated marine oil which contained relatively high amounts of polyunsaturated fatty acids of the n-3 family. The results presented are in line with reports in the literature that dietary trans fatty acids may inhibit the desaturation-elongation enzyme systems of rat liver. The observed effects did not appear to be directly correlated to the dietary content of very-long-chain monounsaturated fatty acids.

Animals↗
PubMed1984

Microsomal esterification of retinol in human liver.

Recent work has shown that esterification of retinol in microsomes from rat liver, mammary gland and small intestine and from human small intestine is catalyzed by an acyl CoA: retinol acyl transferase (ARAT). The current study demonstrates ARAT activity in human liver microsomes. At optimal incubation conditions the rate of retinyl ester formation due to ARAT (0.37 +/- 0.31 nmole ester formed X mg microsomal protein-1 X minute-1, mean +/- SD, n = 6) suggests that the enzyme is of physiological importance.

Acyltransferases↗
PubMed1984

A study of the small spherical high density lipoproteins of patients afflicted with familial lecithin: cholesterol acyltransferase deficiency.

We studied the effects of the lecithin:cholesterol acyltransferase reaction on the size and composition of the small spherical high density lipoproteins of patients afflicted with familial lecithin:cholesterol acyltransferase deficiency. We isolated these lipoproteins by preparative ultracentrifugation and rate zonal ultracentrifugation, determined their diameter by gradient gel electrophoresis, and then calculated their composition by relating measurements of their lipid and apolipoprotein content to particle volume. Our results revealed lipoprotein particles 6.0-6.2 nm in diameter that contained approximately 2 molecules of apolipoprotein A-I, 37-38 molecules of phospholipid, 3-9 molecules of unesterified cholesterol, 1-2 molecules of cholesteryl ester, and 1-2 molecules of triacylglycerol. Upon being incubated with lecithin:cholesterol acyltransferase and a source of additional unesterified cholesterol, these lipoproteins increased in content of total cholesterol and in particle size to form discrete lipoprotein products 6.6-8.6 nm in diameter. The increase in size occurred despite a net decrease in product unesterified cholesterol and phospholipid and though the net change in total lipid volume was small. Moreover, specific product lipoproteins, isolated by rate zonal ultracentrifugation, contained an increased amount of apolipoprotein A-I. These results seem best explained by a process involving lecithin:cholesterol acyltransferase-induced particle rearrangement reactions. The possibility that a similar process normally occurs in vivo deserves to be explored.

Electrophoresis, Polyacrylamide Gel↗
PubMed1983

Influence of diets on acyl-CoA:cholesterol acyltransferase and on acyl-CoA:retinol acyltransferase in villous and crypt cells from rat small intestinal mucosa and in the liver.

Cholesterol and retinol are both esterified with long-chain fatty acid within the mucosal cells of the small intestine. The reactions are catalyzed by microsomal acyl-CoA:cholesterol and acyl-CoA:retinol acyltransferases (EC 2.3.1.26, and EC 2.3.1.-, respectively). To gain more insight into the physiological importance of these acyltransferases, they were studied in villous and crypt cells from rats either fasting or on diets which varied in fat and cholesterol content. Both enzymes had a higher activity in villous than in crypt cells. The activities in villous cells varied with feeding and fasting and the composition of diet when the animals were killed postprandially. Acyl-CoA:cholesterol acyltransferase activity went up upon cholesterol feeding whereas retinol acyltransferase in the mucosa was reduced by high-fat diets. The liver cholesterol acyltransferase activity varied with diet, it increased with both cholesterol and fat feeding, whereas retinol acyltransferase activity remained relatively constant. The results obtained suggest that different diets are of importance for cholesterol and retinol acyltransferase activities both in the intestinal mucosa and in the liver. The variation in activities of the two acyltransferases suggests that they may be different enzymes.

Acyltransferases↗
PubMed1983

Effects of fasting on plasma lipids and cholesterol esterification in plasma, liver and intestinal mucosa in the char (Salmo alpinus L.).

1. CoA-dependent cholesterol esterification, measured as esterification of 3H-cholesterol, was demonstrated in homogenates of liver and intestinal mucosa of the char (Salmo alpinus L.). 2. Plasma concentration of triacylglycerols, unesterified and total cholesterol were significantly reduced to 43, 58 and 72% of the control values, respectively, after 6 weeks fasting. 3. The rate of cholesterol esterification in plasma and liver homogenate was significantly lower in the fasted fish compared to the controls, but the esterification activity in the homogenate of intestinal mucosa increased twofold in the fish fasted for 6 weeks.

Animals↗
PubMed1983

Acyl pattern of adipose tissue triglycerides, plasma free fatty acids, and diet of a group of men participating in a primary coronary prevention program (the Oslo Study).

The acyl pattern of adipose tissue triglycerides and of plasma free fatty acids were determined after 7 yr of diet intervention on elevated plasma cholesterol in 42 men taking part in the smoking-lipid trial of the Oslo Study. Twenty-two of the men were advised to change dietary habits (mainly reduce saturated fat) whereas the remaining 20 were controls. The adipose tissue from men in the intervention group contained relatively more linoleic and linolenic acids and relatively less saturated and monounsaturated fatty acids compared to men in the control group. There were strong correlations between the relative content of several fatty acids in adipose tissue triglycerides and plasma free fatty acid. Furthermore, there was a close correlation between the intake of polyunsaturated fatty acids found in a dietary survey done 2 to 3 yr before this study and the relative content of polyunsaturated fatty acids in adipose tissue.

Adipose Tissue↗
PubMed1983

Transport of cholesterol.

.ur current model for cholesterol transport is summarized in Figure 10. In this figure we have put together the various steps in cholesterol transport that were described previously in this review. Under normal conditions, cholesterol metabolism and transport are well regulated. If the transport system is overloaded for a long time, however, hypercholesterolemia caused mainly by increased plasma LDL may develop in several species, including humans. Under such circumstances reverse transport of cholesterol may also fail, giving rise to deposits of cholesterol. Tissue macrophages may be responsible for this lipid accumulation, because receptor-mediated (adsorptive) endocytosis of lipoprotein-associated cholesterol in these cells is not under negative-feedback control. The deposits are mainly found in tissues poorly supplied with blood and lymph: the skin, tendons, the cornea, and arteries. Overload of cholesterol transport may be the result of too much fat and cholesterol in the diet, giving rise to cholesterol-rich lipoproteins from the gut and to increased production of liver (formula; see text) VLDL, which in humans ends up as LDL. In many individuals, however, no hypercholesterolemia is seen, even after eating large amounts of a "western" diet for decades; others may develop increased LDL on a relatively "prudent" diet. Obviously many of the factors and mechanisms in cholesterol transport are influenced by genetic factors. Although studies of several inborn errors of lipid metabolism have given information about some mechanisms, the quantitatively more important differences in genetic patterns, which determine whether or not a western diet will result in hyperlipidemia, are not well known. Perhaps studies of different forms of apoB and apoE and of HDL subgroups and hyper-alpha-lipoproteinemia will explain why certain individuals develop hypercholesterolemia and premature atherosclerosis. All the recent information related to cholesterol metabolism and transport gives rise to new questions. There are many problems of interest for future research: What are the metabolic differences between the apoB produced in the liver and that produced in the gut? To what extent is the protein moiety of LDL modified in the plasma of blood and lymph and in interstitial tissue? Are such modifications important to whether LDL uptake goes through the classic LDL pathway or through the macrophage (i.e., scavenger?) pathway? Are some changes in apoB important for liver recognition of LDL?(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗