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J A Ontko

Publications and source records attributed to J A Ontko.

At least 37 records · Page 2Linked to original sources
PubMed1984

Studies on the mechanism of hypertriglyceridemia in the genetically obese Zucker rat.

The possibility that impaired removal of lipoprotein triglyceride from the circulation may be a participating factor in the hypertriglyceridemia of the obese Zucker rat was examined. We found no significant differences in the heparin-released lipoprotein lipase (LPL) activities of the adipose tissue, skeletal muscle, and heart (expressed per gram of tissue) from the lean and obese Zucker rats. Furthermore, the kinetic properties of adipose tissue and heart LPL from the lean and obese rats were similar, indicating that the catalytic efficiency of the enzyme was unaltered in the obese animals. The postheparin plasma LPL activities of lean and obese rats were also similar. However, the postheparin plasma hepatic triglyceride lipase (H-TGL) activity in the obese rats was elevated. The higher activity of H-TGL could not alleviate the hypertriglyceridemia in these animals. Since hypertriglyceridemia in the obese rats could also be due to the hepatic production of triglyceride-rich lipoproteins which are resistant to lipolysis, we therefore isolated very low density lipoproteins (VLDL) from lean and obese rat liver perfusates and examined their degradation by highly purified human milk LPL. Although certain differences were observed in hepatic VLDL triglyceride fatty acid composition, the kinetic patterns of LPL-catalyzed triglyceride disappearance from lean and obese rat liver perfusate VLDL were similar. The isolated liver perfusate VLDL contained sufficient apolipoprotein C-II for maximum lipolysis. These results indicate that impaired lipolysis is not a contributing factor in the genesis of hypertriglyceridemia in the genetically obese Zucker rat. The hyperlipemic state may be attributed to hypersecretion of hepatic VLDL and consequent saturation of the lipolytic removal of triglyceride-rich lipoproteins from the circulation.

Adipose Tissue↗
PubMed1983

Dual effects of calcium on the oxidation of fatty acids to ketone bodies in liver mitochondria.

The addition of calcium ions (Ca2+) to rat liver mitochondria, under conditions of rapid accumulation of 10-40 nmol Ca2+/mg protein, inhibited the oxidation of long and medium chain fatty acids to ketone bodies, whereas higher quantities of Ca2+ activated the process. The mitochondrial NADH:NAD ratio exhibited corresponding depression and elevation. Both inhibitory and stimulatory actions of Ca2+ were operative in liver mitochondria from fed and fasted rats and appear to be localized in the mitochondrial inner membrane-matrix region. These observations may signify involvement of Ca2+ in the regulation of fatty acid oxidation and ketogenesis.

Animals↗
PubMed1983

Alterations in rat serum lipids and apolipoproteins following clofibrate treatment.

The in vivo effects of the hypolipidemic drug clofibrate (0.5 mmol/kg body wt daily p.o. for 7 days) on serum lipids and apolipoproteins have been studied in male rats. Clofibrate caused an increase in liver weight without affecting body weight. Triglyceride, total and free cholesterol and HDL cholesterol were decreased in sera of clofibrate-treated rats. The relative abundance, and accordingly the absolute quantities, of the polyunsaturated fatty acids linoleic (18:2), linolenic (18:3) and docosahexenoic (22:6) in serum triglyceride decreased in response to clofibrate treatment. The concentrations of serum apolipoproteins A-I, B and C-III were reduced in clofibrate-treated rats. The apolipoprotein E level was not altered. The distribution of apolipoproteins A-I, B, C-III and E between heparin-Mn supernatant and precipitate were unaffected. The unchanged C-III distribution indicates unaltered intravascular VLDL catabolism. Concurrent reductions in serum HDL cholesterol and ApoA-I in clofibrate-treated rats suggest a diminished production of lipoprotein particles containing ApoA-I. Reductions in serum ApoB and in the mass ratio of serum triglyceride to ApoB indicate a decrease in the number and size, respectively, of circulating triglyceride-rich lipoprotein particles. These observations suggest that the hypolipidemic effect of clofibrate in the normolipemic rat is caused mainly by diminished hepatic secretion, rather than by enhanced catabolism, of triglyceride-rich lipoproteins.

Animals↗
PubMed1983

Cholesterol content as a possible determinant in the removal of chylomicron triglyceride by the liver.

The nature of chylomicron removal from the circulation and the role of the liver in this process have been investigated by measuring the removal of chylomicron triglyceride and cholesterol from the plasma of normal and functionally hepatectomized rats. The rate of removal of chylomicron triglyceride exceeded that of both free and esterified cholesterol in all animals. Whereas chylomicrons were completely removed from the circulation of normal rats soon after intravenous injection, chylomicron remnant particles accumulated in the plasma very low density lipoprotein and chylomicron fractions of hepatectomized rats. The rate of removal of chylomicron triglyceride from the blood of the hepatectomized rats decreased when the cholesterol content of the chylomicrons was elevated. In addition, more chylomicron triglyceride remained in the circulation of the hepatectomized rats when the chylomicrons were enriched with cholesterol and the remnant particles derived from these chylomicrons were of lower average density. These observations indicate that the cholesteryl ester content of the chylomicrons influences the degree of triglyceride removal in extrahepatic tissue capillaries and thereby the amount of residual triglyceride in the remnant particles subsequently removed by the liver.

Animals↗
PubMed1983

Concurrent inductions of avian hepatic lipogenesis, plasma lipids, and plasma apolipoprotein B by estrogen.

The inductions of hepatic fatty acid synthesis, estrogen-specific plasma proteins, plasma lipids, and apolipoproteins by a single subcutaneous injection of diethylstilbestrol (40 mg/kg body weight) have been examined in immature male turkeys. Estrogen induced the appearance of phosvitin, lipovitellin, and apoVLDL-II in the blood plasma. The highest concentrations of these estrogen-specific plasma proteins were observed 48 hr following hormone administration. Estrogen increased the concentration of triglyceride in the liver, predominantly those molecular species containing 16 carbon fatty acids (triglycerides with 53 and 55 carbon atoms). Liver cholesterol was present predominantly as free cholesterol. Although estrogen did not affect the concentrations of free or esterified cholesterol in the liver, the hormone increased the amount of cholesterol esterified with 20-carbon fatty acids and caused a corresponding decrease in cholesterol esterified with 18 carbon fatty acids. Estrogen treatment elevated the plasma triglycerides 55-fold, tripled the plasma phospholipid, and approximately doubled the plasma cholesterol. The de novo synthesis of fatty acids in the liver in vivo was stimulated by estrogen administration, as exhibited by increased 3H2O incorporation into the phospholipids and triglycerides of both liver and plasma. In contrast, hepatic cholesterol synthesis was unaffected. The amounts of newly synthesized triglyceride in the liver and plasma and the specific radioactivities of the plasma triglyceride following 1-hr in vivo labeling periods, 0, 24, 48, and 72 hr after estrogen injection indicate that increased hepatic fatty acid synthesis is a primary and major casuative factor in the development of estrogen-induced hyperglyceridemia in this avian species. The concentration of apolipoprotein B in the plasma increased in parallel with hepatic fatty acid synthesis and the appearance of newly synthesized triglyceride in the plasma, whereas the plasma apolipoprotein A-I level decreased. These observations indicate that in the avian liver estrogen causes a coordination of inductions in the conversion of carbohydrate to triglyceride and in the production of proteins (apolipoprotein B and apoVLDL-II) involved in the assembly of triglyceride-rich lipoprotein particles, leading to hypersecretion of these lipoproteins into the circulation.

Animals↗
PubMed1982

Structure-function relations between fatty acid oxidation and the mitochondrial inner-membrane--matrix region.

Decreasing the osmolarity of the incubation media for rat liver mitochondria, by lowering the KCl or sucrose content, greatly increased the oxidation of palmitate, palmitoyl-CoA, palmitoyl-(-)-carnitine, octanoate and butyrate, but only slightly affected the oxidation of succinate, glutamate plus malate, and 3-hydroxybutyrate. Kinetic studies indicated that the increased rate of fatty acid oxidation commenced at the onset of incubation in the hypotonic media and remained relatively constant. This was accompanied by an increase in the 3-hydroxybutyrate: acetoacetate ratio, a higher rate of oxygen consumption, an increase in the extramitochondrial adenine-nucleotide phosphorylation-state ratio, and a greater state of reduction of the respiratory chain cytochromes b, c, and oxidase. Incubation of mitochondria in the hypotonic media caused a rapid decrease in the intermembrane and intracrystal spaces and concurrent expansion of the matrix, without changing the volume of the mitochondria. These observations indicate that expansion of the inner-membrane-matrix compartment of mitochondria in vitro specifically activates fatty acid oxidation at the site of beta-oxidation or at the site of its interaction with the respiratory chain, with a consequent increase in the states of reduction and phosphorylation of the mitochondria. Evidence is also presented which suggests that the activation of mitochondrial fatty acid oxidation by Ca2+ is initiated by a Ca2+-induced alteration of the structure or volume of the inner-membrane-matrix region.

Animals↗
PubMed1980

Evaluation of malonyl-CoA in the regulation of long-chain fatty acid oxidation in the liver. Evidence for an unidentified regulatory component of the system.

Palmitate oxidation by liver mitochondria from fed and starved rats exhibited markedly different sensitivities to inhibition by malonyl-CoA. In the mitochondrial system from fed rats, 50% inhibition required 19 muM-malonyl-CoA, whereas the mitochondria from starved rats were by comparison refractory to malonyl-CoA. Inhibition by malonyl-CoA was completely reversed by increasing the molar ratio of fatty acid to albumin. Results indicate that the potential effectiveness of malonyl-CoA as an inhibitor of fatty acid oxidation in the liver is dependent on an unidentified regulatory component of the system. The functional activity of this component is modified by the nutritional state, and its site of action is at the mitochondrial level.

Acyl Coenzyme A↗
PubMed1980

Production of apolipoproteins E and A-I by rat hepatocytes in primary culture.

Hepatocytes were isolated from normal adult rat livers and cultured in a modified HI-WO/BA medium. A nearly confluent monolayer was established at the plating concentration employed. The hepatocytes synthesized ansd secreted albumin at rates similar to those observed in vivo. The cells secreted triacylglycerol in the absence of fatty acid substrate. Under these conditions the most abundant triacylglycerol molecular species contained 53 carbons. Incubation with oleic acid markedly increased triacylglycerol secretion predominantly in the form containing a total carbon number of 57. Approx. 80% of the secreted cholesterol was in the free form and this was unaffected by oleic acid. Employing monospecific antibodies constant rates of synthesis and secretion of apolipoproteins E and A-I were demonstrated by quantitative electroimmunoassay of the cell culture media. The rates of albumin, apolipoprotein E, and apolipoprotein A-I production were 1480, 170 and 60 microgram/h per g cell protein, respectively.

Albumins↗
PubMed1976

Direct effects of fructose metabolism on fatty acid oxidation in a recombined rat liver mitochondria-hish speed supernatant system.

The effects of fructose on the oxidation of [1-(14)C]palmitate in a rat liver mitochondria-high speed supernatant system have been investigated. This model system permitted study of the direct effects of fructose and the metabolism of fructose on fatty acid oxidation in the near absence of fatty acid esterification. Fructose inhibited the utilization of albumin-bound [1-(14)C] palmitate in the mitochondria-supernatant system, but did not affect fatty acid utilization by isolated liver mitochondria. Although fructose decreased the ATP content in the mitochondrial-supernatant system, the level of ATP throughout the incubation period was sufficient for maximal fatty acid activation. Fructose decreased the conversion of [1-(14)C]palmitate to 14CO2 and depressed the formation of total labeled oxidation products (14CO2 + 14C-labeled ketone bodies) in this system. The results suggest that fructose metabolism inhibited fatty acid oxidation in the mitochondria-supernatant system by competitive substrate oxidation and thereby decreased utilization of the added [1-(14)C]palmitate. The ihibition of L-[L-(14)C]palmitoylcarnitine oxidation, fructose was in all respects similar to its inhibition of palmitate oxidation, indicating that the site of fructose interaction was within the beta-oxidation sequence. These observations support the concept (Ontko, J.A. [1972] J. Biol. Chem. 247, 1788-1800) that the reciprocal changes in esterification and oxidation of palmitate caused by fructose in liver cells are primarily mediated via inhibitory effects on long-chain fatty acid oxidation.

Acetoacetates↗
PubMed1973

Localization of the enzymes of ketogenesis in rat liver mitochondria.

The localization of the enzymes of ketogenesis in isolated rat liver mitochondria has been investigated. Mitochondrial subfractions were isolated after disruption of this subcellular organelle by (a) hypotonic lysis in water, which permitted the ultracentrifugal separation of the soluble and membranous compartments of the mitochondrion, or by (b) a procedure involving swelling, contraction, and ultrasonic treatment, which permitted the isolation from discontinuous sucrose gradients of subfractions rich in intermembrane space protein, outer membrane, and inner membrane-matrix particles. Two membrane subfractions were invariably present as distinct bands at the lower interface of the discontinuous gradient. The upper of these two bands was found to be a highly purified preparation of outer mitochondrial membrane. Subfractions rich in matrix and in inner membrane were isolated from inner membrane-matrix particles after hypotonic treatment. The content of the various mitochondrial compartments in all subfractions was assessed from their enzymic and electron microscopic characteristics. The ketogenic activity of each subfraction was determined by measuring its capacity to form ketone bodies from acetyl CoA. The activity of this process was markedly enhanced by dithiothreitol. These measurements of ketone body formation, together with assays of individual enzymes of the ketogenic pathway, show that thiolase, HMGCoA synthase, and HMGCoA cleavage enzyme are localized in the matrix of the inner membrane-matrix particles. The rates of ketone body formation indicate that the HMGCoA synthase is the rate-limiting enzyme of the pathway in subfractions of high matrix content. Studies with sodium chloride indicate that a large portion of the HMGCoA synthase, which remains present in membrane subfractions derived from water-treated mitochondria, is bound by ionic interaction to component(s) of the membrane.

Acetyltransferases↗
PubMed1973

Effects of ethanol on the metabolism of free fatty acids in isolated liver cells.

Ethanol inhibited the oxidation and enhanced the esterification of albumin-bound [1-(14)C]palmitate incubated with isolated rat liver cells. Ethanol decreased the conversion of [1-(14)C]palmitate to (14)CO(2) and (14)C-labeled ketone bodies and enhanced the incorporation of [1-(14)C]palmitate into glycerolipids, especially triglyceride; cholesteryl ester synthesis was unaffected. The half-maximal effective ethanol concentration for each of these processes was 6-10 micro g/ml and a maximum effect was produced by about 50 micro g/ml. Ethanol oxidation was required for each of these alterations, since the effects were completely abolished by pyrazole. The energy obtainable from ethanol oxidation was in excess of the energy deficit from decreased fatty acid oxidation. However, ethanol did not affect O(2) consumption, indicating that ethanol oxidation replaced the oxidation of both fatty acids and other substrates. Ethanol inhibited the citric acid cycle in the intact liver cells by 20-30%. The major site of inhibition was alpha-ketoglutarate oxidation. Results suggest that ethanol inhibited alpha-ketoglutarate dehydrogenase in the mitochondria of hepatocytes by elevating the mitochondrial NADH:NAD ratio. A minor site of inhibition of ethanol oxidation was detected between succinate and citrate. It is suggested that ethanol inhibits fatty acid oxidation in hepatocytes by competitive substrate oxidation, resulting in an increased availability of long-chain free fatty acids; this thereby enhances esterification, leading to accumulation of liver triglyceride.

Animals↗