Very low density lipoprotein triglyceride transport in type IV hyperlipoproteinemia and the effects of carbohydrate-rich diets

Transport of plasma-free fatty acids (FFA) and of fatty acids in triglycerides of plasma very low density lipoproteins (VLDL-TGFA) was studied in two normal subjects, five patients with type IV hyperlipoproteinemia, and two patients with type I hyperlipoproteinemia. After intravenous pulse-labeling with albumin-bound 1-palmitate-(14)C, specific radioactivity of plasma FFA and VLDL-TGFA were determined at intervals up to 24 hr. The results were analyzed using several different multicompartmental models each compatible with the experimental data. Fractional transport of VLDL-TGFA was distinctly lower (no overlap) in the type IV patients than in the control subjects, both on a usual balanced diet (40% of calories from carbohydrate) and on a high-carbohydrate diet (80% of calories). However, net or total transport of VLDL-TGFA in the type IV patients was not clearly distinguishable from that in the control subjects, there being considerable overlap on either diet. The results suggest that in this group of type IV patients the underlying defect leading to the increased pool size of VLDL-TGFA is not overproduction but a relative defect in mechanisms for removal of VLDL-TGFA. Since some of these type IV patients had only a moderate degree of hypertriglyceridemia at the time they were studied, and since it is not established that patients with the type IV phenotype constitute a biochemically homogeneous population, the present results should not be generalized. Four studies were done (in two control subjects and two type IV patients) in which the kinetic parameters in the same individual were determined on the balanced diet and on the high-carbohydrate diet. All subjects showed an increase in VLDL-TGFA pool size. Using two of the models for analysis, all showed an increase in net transport of VLDL-TGFA; using the third model, three of the four studies showed an increase in VLDL-TGFA transport. The results are compatible with the interpretation that the carbohydrate-induced increase in VLDL-TGFA, both in controls and type IV patients, is at least in part due to an increased rate of production of VLDL-TGFA. The magnitude of the increase was approximately the same in controls and patients. Thus, metabolic adjustment to a high-carbohydrate regimen in these type IV patients may not be basically different from that in normal controls; the higher levels of VLDL-TGFA reached may simply be another reflection of a defective removal mechanism. An alternative interpretation, compatible with the data, would involve both a carbohydrate-induced increase in fractional rate of release of VLDL-TGFA from liver to plasma and a decrease in fractional removal of VLDL-TGFA from plasma without increase in net production rate. The simpler hypothesis of a single primary effect on net VLDL-TGFA production from FFA seems more likely.

ATP-dependent and cyclic AMP-dependent activation of rat adipose tissue lipase by protein kinase from rabbit skeletal muscle

Brief incubation of partially purified preparations of hormone-sensitive lipase from rat epididymal fat pads with ATP, Mg(++), cyclic adenosine 3':5'-monophosphate and rabbit muscle protein kinase (phosphorylase b kinase kinase) resulted in enhancement of lipolytic activity (44-93%). Little or no activation was observed when either the cofactor mixture or the protein kinase was omitted. When the fat pads were incubated with epinephrine prior to homogenization, addition of kinase and cofactors to the soluble supernatant fraction caused no activation whereas good activation was obtained in preparations from paired fat pads not exposed to epinephrine. The results indicate that the cyclic AMP-mediated activation of hormone-sensitive lipase in adipose tissue involves a protein phosphorylation step. Whether the lipase itself is phosphorylated and thus activated or whether the protein kinase is activating a mediating enzyme, in analogy with its action in the glycogen phosphorylase system, remains to be determined.

Stimulation of insulin secretion by infusion of free fatty acids

The acute elevation of plasma free fatty acid (FFA) levels by direct infusion of sodium oleate into the plasma of conscious dogs was accompanied by the rapid onset of a 2- to 12-fold increase in plasma immunoreactive insulin, and, subsequently, a marked fall in plasma glucose, even in dogs receiving intravenous glucose throughout the infusion. The magnitude of both the insulin and glucose responses correlated with the mean FFA level during infusion. A large increase in plasma insulin and fall in glucose also occurred when glycerol was infused with oleate in order to simulate endogenous lipolysis more closely. Insulin levels in pancreaticoduodenal vein blood rose markedly during oleate infusion, while plasma ketone levels rose only slightly. In contrast to the effects of oleate infusion, elevation of plasma FFA to correspondingly high levels by triolein ingestion and intravenous heparin produced only small increases in plasma insulin, which did not correlate well with the FFA level reached, and small increases in plasma glucose.The results indicate that under certain conditions elevated FFA levels may be a potent stimulus of insulin secretion. This response is modified under other conditions such as during chylomicron removal under the influence of heparin. This effect may play a role in the regulation of lipolysis and ketone formation, but determination of the exact mechanism of FFA stimulation of the pancreas and its physiological significance will require further investigation.

Infusion of long-chain fatty acid anions by continuous-flow centrifugation

We have developed a method for the rapid infusion into plasma of large amounts of long-chain free fatty acids (FFA). Unanesthetized dogs were connected by a peripheral artery to a closed, continuousflow centrifuge from which cells and plasma emerged in separate lines. Sodium oleate was infused directly into the plasma line before cells and plasma were recombined and returned to the animal through a peripheral vein.The centrifugation procedure itself produced only small changes in circulating levels of glucose, FFA, and electrolytes. Plasma flow rates as high as 100 ml/min could be maintained, and centrifugations of 12 hr were accomplished without complications. During centrifugation, sodium oleate was infused at rates up to 80 muEq/kg per min for 2.5 hr; the maximum molar ratio of FFA to albumin without hemolysis was 10:1. Plasma FFA levels rose rapidly after infusions were started and reached constant elevated levels within 15-20 min. Oleate infusion at 10-50 muEq/kg per min produced a rise in plasma FFA proportional to the infusion rate. The maximum increment in plasma FFA above control values was 1.66 muEq/ml. When infusions ended, plasma FFA declined rapidly to control levels. Oleate infusion at rates below 30 muEq/kg per min did not reduce levels of other plasma FFA. Infusion at high rates was accompanied by a marked fall in blood glucose. This method permits adminsitration of long-chain fatty acids in sufficient quantities to study their individual metabolic effects, and provides a new way to supply lipid calories parenterally.

Kinetic studies of plasma free fatty acid and triglyceride metabolism in man

Plasma transport of free fatty acids (FFA) and triglyceride fatty acids (TGFA) was studied in seven subjects with normal lipid metabolism, one case of total lipodystrophy, and one case of familial hyperlipemia (Type V). Studies were carried out after intravenous injection of radioactive FFA, of lipoproteins previously labeled in vitro in the triglyceride moiety, or both. Computer techniques were used to evaluate a series of multicompartmental models, and a general model is proposed that yields optimum fitting of experimental data for both FFA and TGFA. The results show that as much as 20-30% of FFA leaving the plasma compartment in normal subjects is transported to an exchanging extravascular pool and quickly reenters the plasma pool as FFA. The rate of irreversible delivery of FFA from plasma to tissues averaged 358 muEq/min in normals. The lipodystrophy patient, despite the virtual absence of adipose tissue (confirmed at autopsy), had a plasma FFA concentration and a total FFA transport, both more than twice normal. Total TGFA transport ranged from 25 to 81 muEq/min in four normal controls. The rate constant for TGFA turnover in the patient with Type V hyperlipemia was so small that total transport could not be quantified from the data available; the TGFA half-life was over 500 min. In two normal subjects given injections of autologous lipoproteins labeled in vitro with triolein-(14)C and simultaneously given oleic acid-(3)H, it was shown that the time course for the disappearance of the TGFA in the in vitro labeled samples conformed almost exactly to that of the physiologically labeled lipoprotein TGFA synthesized from injected FFA (as evidenced by the simultaneous fitting of both sets of data using the same multicompartmental model and the same rate constants). Radioactivity appeared in the plasma FFA fraction at a significant rate after injection of plasma labeled in vitro with TGFA. It was estimated that as much as 50% of the total TGFA transported underwent rapid and rather direct conversion to FFA in the two normal subjects studied this way. The kinetic data suggest that such conversion of TGFA to FFA was not preceded by any extensive dilution, such as would result from complete mixing with tissue triglyceride stores.

Refsum's disease: characterization of the enzyme defect in cell culture

Refsum's disease (heredopathia atactica polyneuritiformis, HAP) is an inherited neurological disorder associated with storage of the branched-chain fatty acid, phytanic acid (3,7,11,15-tetramethylhexadecanoic acid). Cultured fibroblasts derived from skin biopsies of HAP patients did not contain elevated levels of phytanate, yet showed rates of phytanate-C-(14)C oxidation less than 3% of those seen in cells from control subjects. Cells of control subjects converted phytanate to alpha-hydroxyphytanate, to pristanate (the [n-1] homologue of phytanate) and to 4,8,12-trimethyltridecanoate, compounds previously identified as intermediates on the major pathway for phytanate metabolism in animals, providing the first direct evidence that this same oxidative pathway is operative in human cells. None of these breakdown products could be found after incubation of phytanate with HAP cells. Labeled alpha-hydroxyphytanate and labeled pristanate were oxidized at normal rates by HAP cells. Oxidation of the latter proceeded at normal rates both when added to the medium at very low tracer levels and at levels 100 times greater. Phytanate was incorporated into and released from lipid esters at normal rates by HAP cells. Elevated levels of free phytanate in the medium were no more toxic to HAP cells than to control cells over the 48- to 72-hr exposures involved in these studies, as evidenced by morphologic criteria and by ability to oxidize labeled palmitate. These findings are consistent with the hypothesis that the cells from HAP patients are deficient in a single enzyme involved in the alpha-hydroxylation of phytanate, while the enzymes involved in later steps are present at normal or near-normal levels.

Localization of the oxidative defect in phytanic acid degradation in patients with Refsum's disease

The rate of oxidation of phytanic acid-U-(14)C to (14)CO(2) in three patients with Refsum's disease was less than 5% of that found in normal volunteers. In contrast, the rate of oxidation of alpha-hydroxyphytanic acid-U-(14)C and of pristanic acid-U-(14)C to (14)CO(2), studied in two patients, while somewhat less than that in normal controls, was not grossly impaired. These studies support the conclusion that the defect in phytanic acid oxidation in Refsum's disease is located in the first step of phytanic acid degradation, that is, in the alpha oxidation step leading to formation of alpha-hydroxyphytanic acid. The initial rate of disappearance of plasma free fatty acid radioactivity after intravenous injection of phytanic acid-U-(14)C (t(1/2) = 5.9 min) was slower than that seen with pristanic acid-U-(14)C (t(1/2) = 2.7 min) or palmitic acid-1-(14)C (t(1/2) = 2.5 min). There were no differences between patients and normal controls in these initial rates of free fatty acid disappearance for any of the three substrates tested. There was no detectable lipid radioactivity found in the plasma 7 days after the injection of palmitic acid-1-(14)C or pristanic acid-U-(14)C in either patients or controls. After injection of phytanic acid-U-(14)C, however, the two patients showed only a very slow decline in plasma lipid radioactivity (estimated t(1/2) = 35 days), in contrast to the normals who had no detectable radioactivity after 2 days. Incorporation of radioactivity from phytanic acid-U-(14)C into the major lipid ester classes of plasma was studied in one of the patients; triglycerides accounted for by far the largest fraction of the total present between 1 and 4 hr.

Absorption of phytol from dietary chlorophyll in the rat

The fate of ingested chlorophyll-particularly of the phytol portion of the molecule-was studied. Uniformly (14)C-labeled pheophytin a (the Mg-free derivative of chlorophyll a) was prepared from an extract of tobacco leaves grown in (14)CO(2), and was administered by stomach tube to rats in which the thoracic duct had been cannulated. Only about 2% of the administered radioactivity was absorbed in 24 hr, largely into the thoracic duct lymph. Moreover, only a fraction of this lymph radioactivity was derived from phytol (i.e., was found in phytol, phytenic acid, or phytanic acid). The results indicated that not more than 1-2% of chlorophyll phytol is available for absorption by the rat. Similarly, after the administration of whole spinach or spinach extract (not labeled) to rats, only about 1% of the total phytol content was absorbed into the intestinal lymph. Nearly all of the administered phytol was found in the feces and the contents of the colon, and was still largely in the form of pheophytin. The study also indicated that little of the nonphytol portion of the chlorophyll molecule is absorbed.

Refsum's disease: nature of the enzyme defect

Two siblings with Refsum's disease, an inherited disorder of lipid metabolism, oxidized intravenously injected uniformly labeled phytanic acid-C(14) at rates less than 5 percent of those found in normal subjects. The defect in oxidation of phytanic acid persisted in cultures of fibroblasts from the patients' skin. The rate of oxidation of the phytanic acid-C(14) was less than 1 percent of that found in cultures of fibroblasts from normal skin. However, pristanic acid, previously shown to be the first product of phytanic acid degradation, was oxidized at a normal rate in the patients' cultures. These results indicate that the enzymatic defect in Refsum's disease is in the first step of the pathway for degradation of phytanic acid, that is, in the unusual alpha-oxidative process that leads to a shortening of phytanic acid by one carbon atom.

Studies on the metabolic error in Refsum's disease

Studies utilizing mevalonic acid-2-(14)C and D(2)O as precursors failed to provide evidence for an appreciable rate of endogenous biosynthesis of phytanic acid in a patient with Refsum's disease. Orally administered tracer doses of phytol-U-(14)C were well absorbed both by seven normal control subjects (61 to 94%) and by two patients with Refsum's disease (74 and 80%). The fraction of the absorbed dose converted to (14)CO(2) in 12 hours was 3.5 and 5.8% in Refsum's disease patients and averaged 20.9% in seven control subjects. Labeled phytanic acid was demonstrated in the plasma of both control subjects and patients given phytol-U-(14)C, establishing phytol in the diet as a potential precursor of phytanic acid. This labeled phytanic acid had disappeared almost completely from the plasma of the seven control subjects by 24 to 48 hours, whereas it persisted at high concentrations in the plasma of the two patients for many days. We conclude that the phytanic acid accumulating in Refsum's disease is primarily of exogenous origin and that patients with Refsum's disease have a relative block in the degradation of phytanic acid and possibly other similar branched-chain compounds. This may relate to a deficiency in mechanisms for release of phytanic acid from stored ester forms or, more probably, to reactions essential to oxidative degradation of the carbon skeleton.

Release of free fatty acids from Ehrlich ascites tumor cells

Ehrlich ascites tumor cells release free fatty acids (FFA) during in vitro incubation in media that contain albumin. The released FFA are derived by lipolysis from endogenous lipid esters. Addition of glucose to the incubation medium greatly decreases the quantity of fatty acid released by the cells. Cyanide, which inhibits endogenous lipid oxidation but not lipolysis, increases the quantity of fatty acid released to media containing albumin and causes free fatty acid to accumulate in the cells in the absence of exogenous albumin. The release of fatty acid, either preformed or derived by lipolysis during prolonged incubations, occurs under conditions of net fatty acid uptake from the incubation medium. Net release of fatty acid from the cell occurs only when fatty acid-extracted albumin is present in the extracellular medium; extrapolation of the data suggests that net release will not occur under physiological conditions. It is postulated that free fatty acid uptake and release are independent processes, the direction of net fatty acid movement being determined by the relationship between cellular free fatty acid concentration (regulating efflux) and the molar ratio of free fatty acid to albumin in the extracellular medium (regulating uptake).

Effects of dietary phytol and phytanic acid in animals

Feeding of phytol in large doses (2-5% by weight in the diet) led to accumulation of phytanic acid in the mouse, rat, rabbit, and chinchilla, the degree of accumulation depending upon the level of dietary intake. The relative concentration of phytanic acid, expressed as a percentage of the total fatty acids, was as high as 20-60% in liver and 30-40% in serum. Phytenic acid, which may be an intermediate in the conversion of phytol to phytanic acid, also accumulated. When phytol was withdrawn from the diet, tissue and serum concentrations of phytanic acid fell rapidly, which indicates the ability of the normal animal to metabolize phytanic acid readily. At high dosages in the diet, phytol inhibited growth and caused death within 1-4 weeks. In the mouse, dietary phytanic acid and dietary phytol fed in equivalent amounts were of comparable toxicity. Accumulation of tissue phytanic acid occurred more rapidly when phytanic acid was fed than when phytol was fed in equal amounts. In none of the animals fed either phytol or phytanic acid were there any signs of neurological defects. Histologic examination of rats fed phytol showed some fat accumulation, glycogen depletion, and karyokinesis in the liver. There were no pathologic changes in the retina or in the peripheral and central nervous system such as those described in Refsum's disease.

Relationship between fatty acid and glucose utilization in Ehrlich ascites tumor cells

Glucose greatly increased total free fatty acid (FFA) esterification by Ehrlich ascites tumor cells. However, the FFA concentration of the cells was not altered. Less exogenous FFA was oxidized to CO(2) at any given extracellular FFA:albumin molar ratio when glucose was available, but increasing amounts of radioactive CO(2) were produced as the FFA:albumin molar ratio was raised, even in the presence of glucose. It is suggested that glucose, by providing either energy or an excess of triose acceptor for fatty acid esterification, stimulated FFA uptake only indirectly, by increasing the utilization of FFA subsequent to initial uptake from the medium, i.e., by increasing the turnover rate of the cellular FFA pool. Availability of glucose decreased the oxidation of endogenous lipid radioactivity and the depletion of endogenous lipid ester radioactivity. Most of the radioactivity utilized was derived from phospholipids, and depletion of phospholipid radio-activity was spared when glucose was available. Depletion of cellular total lipid ester also was spared in the presence of glucose. Availability of FFA did not decrease total glucose uptake or its oxidation to CO(2). Glucose utilization by these cells appears not to be regulated by FFA availability in the manner that Randle and coworkers described for muscle.

Metabolism of phytol-U-14C and phytanic acid-U-14C in the rat

The metabolism of uniformly-labeled (14)C-phytol, (14)C-phytenic acid, and (14)C-phytanic acid was studied in the rat. Conversion of both phytol and phytenic acid to phytanic acid was demonstrated. Tracer doses of phytol-U-(14)C given orally were well absorbed (30-66%), and approximately 30% of the absorbed dose was converted to (14)CO(2) in 18 hr. After intravenous injection, 20% appeared in (14)CO(2) in 4 hr. Phytanic acid-U-(14)C given intravenously was oxidized at a comparable rate (22-37% in 4 hr) and was as rapidly oxidized as palmitic acid-1-(14)C (21% in 4 hr). Metabolism of these substrates was also studied in rats previously maintained on a diet containing 5% phytol by weight, which causes accumulation of phytanic acid, phytenic acid, and, to a lesser extent, phytol in blood and tissues. Despite the large body pools of preformed, unlabeled substrate in these animals, the fraction of an administered dose of phytol-U-(14)C or phytanic acid-U-(14)C converted to (14)CO(2) was not significantly diminished. These studies indicate that the rat has an appreciable capacity to degrade the highly branched carbon skeleton of phytol and its derivatives. Twenty-four hours after administration of phytol-U-(14)C, the lipid radioactivity remaining in the body was widely distributed among the tissues, highest concentrations being found in liver and adipose tissue. Four hours after intravenous administration of phytanic acid-U-(14)C, all of the major lipid classes in the liver contained radioactivity, most in triglycerides and phospholipids and least in cholesterol esters and lower glycerides. There was no demonstrable incorporation of mevalonate-2-(14)C or acetate-1-(14)C into liver phytanic acid when they were given intravenously to a rat previously fed phytol. Endogenous biosynthesis, if it occurs at all, must be extremely limited.

The effect of dietary fat on the turnover of cholic acid and on the composition of the biliary bile acids in man

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