Adipose Tissue Extracts Research Articles

Detection of hormone-sensitive lipase in various tissues. I. Expression of an HSL/bacterial fusion protein and generation of anti-HSL antibodies.

Hormone-sensitive lipase (HSL) is an intracellular neutral lipase found in a variety of tissues, but primarily in adipose and steroidogenic tissues, that hydrolyzes triglycerides and cholesteryl esters. In the present studies, a portion of rat HSL cDNA was subcloned into a pET expression system and the resulting recombinant fusion protein was over-expressed in E. coli. The approximately 26 kD HSL/fusion protein was used to generate polyclonal antibodies in rabbits that recognize intact HSL (84 kD) in rat adipose tissue, ovary, adrenal, testis, heart, and lung, as well as in human adipose tissue. In addition, there was an approximately 89 kD protein observed in all rat tissues expressing the 84 kD protein. Unique to testes, there was an immunoreactive protein of approximately 102 kD in sexually immature rats, and additional immunoreactive proteins of approximately 113 kD and approximately 127 kD in sexually mature rats. The anti-HSL/fusion protein antibodies could remove approximately 60-80% of total neutral cholesterol esterase activity from extracts of rat adipose tissue and immunoprecipitated a single 84 kD protein after labeling of adipocytes with [32P]orthophosphate. The incorporation of 32P into the 84 kD HSL protein was stimulated 4-fold by incubation of adipocytes with isoproterenol. The half life of [35S]methionine-labeled HSL was approximately 4 h in normal rat adipocytes. The production of an HSL/fusion protein and generation of antibodies that recognize native HSL should be valuable tools in exploring the mechanisms regulating the expression of HSL activity and the function and localization of its immunoreactive proteins.

An anorectic agent from adipose tissue of overfed rats: Effects on feeding behavior

Parabiosis and blood-transfer studies with rodents suggest the existence of humoral factors capable of affecting energy balance. The nature and origin of these factors is undetermined. Aqueous extracts of adipose tissue from overfed rats significantly reduce food intake when administered intraperitoneally (IP) or intracerebroventricularly (ICV). We term the agent(s) responsible for this effect adipose satiety factor (ASF). A single IP dose of ASF, equivalent to 44 mg crude protein, suppresses cumulative food intake for over 12 h. ASF, prepared using a combination of adipose tissue from obese Zucker rats and overfed rats, is more potent per unit of protein than ASF prepared exclusively using adipose tissue from overfed rats. A single ICV dose of this hybrid preparation, equivalent to 14.6 μg of crude protein, suppresses cumulative food intake by 40% for up to 48 h. By ultrafiltration, the molecular weight associated with maximal ASF activity is between 30 and 100 kilodaltons (kDa). The behavioral specificity of ASF-induced anorexia is demonstrated using meal pattern, taste aversion, and differential starvation paradigms.

Expression and translation of the growth hormone-receptor gene in the guinea-pig.

The refractoriness of guinea-pigs to the growth-promoting actions of exogenous GH has been suggested to be due to a deficiency or defect in tissue GH receptors or in GH-receptor gene expression. GH-receptor mRNA was, however, demonstrated by Northern blot analysis and by the polymerase chain reaction in extracts of guinea-pig liver, adipose tissue, brain, hypothalamus and pituitary gland. High-affinity, low-capacity binding sites for radio-labelled ovine GH were also demonstrated on the plasma membranes of guinea-pig liver and were similar to those in rat liver. These results demonstrate that the unresponsiveness of guinea-pigs to exogenous GH is not due to the absence of GH receptors.

Adipose Tissue Glucose Transporters in NIDDM: Decreased Levels of Muscle/Fat Isoform

We investigated the mechanism of peripheral insulin resistance in the adipose tissue of obese and non-insulin-dependent diabetes mellitus (NIDDM) patients at the level of the glucose-transport effector system. Freshly isolated adipocytes from obese nondiabetic and obese NIDDM subjects had decreased insulin sensitivity and responsiveness for glucose-transport stimulation compared with control subjects, with more pronounced changes associated with obese NIDDM patients. The relative abundance of muscle/fat glucose-transporter isoform in the three groups of subjects was determined by Western-blot analysis of detergent-soluble adipose tissue extracts with monoclonal antibody 1F8. Obesity per se had no effect on adipose tissue muscle/fat glucose-transporter isoform (3150 +/- 660 vs. 4495 +/- 410 counts/min [cpm]/mg protein). Furthermore, decreased levels of muscle/fat isoform in adipose tissue of NIDDM patients were also reflected in isolated adipocytes. Our results demonstrate that insulin resistance in isolated adipocytes of NIDDM patients could at least partly be due to a significant depletion of adipose tissue muscle/fat glucose-transporter isoform.

Insulin resistance in normal rats infused with glucose for 72 h

Insulin resistance is accentuated during periods of poor metabolic control in human non-insulin-dependent diabetes mellitus. The role of hyperglycemia in this suppression of insulin action is not clear. If glucose impairs insulin action, then the effect should be reproducible in vivo in tissues of normal intact rats. To test this possibility, normal rats were continuously administered 50% glucose in water (60-66 mg.kg-1.min-1) via an indwelling jugular catheter. After 72 h, these animals were hyperglycemic, hyperinsulinemic, and glucosuric compared with control rats infused for 72 h with normal saline (P less than 0.01). Basal glucose uptake in vivo was greater in muscle of glucose-infused rats. Insulin-stimulated glucose uptake in vivo and in vitro (by perfused hindquarters and isolated adipocytes) were suppressed in the glucose-infused group (P less than 0.01). Glycogen synthase activity was reduced 40% in extracts of muscle and adipose tissue of hyperglycemic rats. Basal and isoproterenol-stimulated lipolysis were increased, whereas insulin suppression of lipolysis was blunted in adipocytes from glucose-infused animals (P less than 0.01). Glucose infusion did not alter insulin binding by isolated adipocytes or solubilized skeletal muscle insulin receptors. These results suggest that a 72-h in vivo glucose infusion impaired insulin action in muscle and adipose tissue of normal rats by inducing postbinding defects similar to those observed in human diabetes mellitus during intervals of deteriorated metabolic control.

Metabolism of [ 14C]4-chlorobiphenyl by hepatic microsomes isolated from razorbills, pigeons and rats

1. Analysis of individual PCB-isomers and congeners in extracts of adipose tissue from N = 12 razorbills suggested that 4-chlorobiphenyl was subjected to metabolism. 2. In vitro metabolism studies using [ 14C]-4-chlorobiphenyl as substrate showed that razorbills metabolise this substrate to [ 14C]4-chloro-4'-hydroxybiphenyl at an average rate of 20 pmol mg microsomal protein/min. For comparison, the metabolism of [ 14C]-4-chlorobiphenyl by pigeons and rats was also studied, and average rates in the formation of [ 14C]-4-chloro-4'-hydroxybiphenyl of 12 pmol mg microsomal protein min and 342 pmol mg microsomal protein min were estimated. 3. A comparison of the hepatic drug metabolising enzyme system of razorbills and pigeons showed similar concentrations of cytochrome P-450, cytochrome b 5 and comparable catalytic activies of cytochrome P-450-dependent monooxygenase, when assessed for HHDN epoxidase, PROD, EROD and the Phase II enzymes glutathiones- S-transferase, but were significantly lower, when compared with rats. The results obtained suggest fundamental differences in the catalytic activities of cytochrome P-450dependent monooxygenase between avian and mammalian species. 4. The present study, however, provides evidence that fish-eating seabirds have the ability to metabolically dispose of certain PCB isomers and congeners, which are amongst the most ubiquitously distributed pollutants in the ecosystem.

Protein-serine kinase from rat epididymal adipose tissue which phosphorylates and activates acetyl-CoA carboxylase. Possible role in insulin action

1. Most of the cyclic-nucleotide-independent acetyl-CoA carboxylase kinase activity in an extract of rat epididymal adipose tissue was evaluated from a Mono Q column by 0.175 M-NaCl at pH 7.4. The activity of the kinase in this fraction (fraction 1) was increased after exposure of intact tissue to insulin. 2. Incubation of purified adipose-tissue acetyl-CoA carboxylase with [gamma-32P]ATP and samples of fraction 1 led to the incorporation of up to 0.4 mol of 32P/mol of enzyme subunit. Most of the phosphorylation was on serine residues within a single tryptic peptide. This peptide, on the basis of two-dimensional t.l.c. analysis, h.p.l.c. and Superose 12 chromatography, appeared to be the same as the acetyl-CoA carboxylase peptide ('I'-peptide) which exhibits increased phosphorylation in insulin-treated tissue. 3. Phosphorylation of purified acetyl-CoA carboxylase by the kinase in fraction 1 was found to be associated with a parallel 4-fold increase in activity. However, increases in both phosphorylation and activity were much diminished if fraction 1 was treated by Centricon centrifugation to remove low-Mr components. Among these components was a potent inhibitor of acetyl-CoA carboxylase activity which appeared to be necessary for the kinase in fraction 1 to be fully active. 4. The inhibitor remains to be identified, but inhibition requires MgATP, although the inhibitor itself does not cause any phosphorylation of the carboxylase. No effects of insulin were observed on the activity of the inhibitor. 5. It is concluded that the kinase probably plays an important role in the mechanism whereby insulin brings about the well-established increases in phosphorylation and activation of acetyl-CoA carboxylase in adipose tissue.

Human adipose tissue hormone-sensitive lipase: identification and comparison with other species

The mRNA for human hormone-sensitive lipase (HSL) was identified using Northern blot analysis and a cDNA-probe for rat HSL. As in the rat, human adipose tissue expresses a single mRNA species of 3.3 kb. Using Western blotting with a polyclonal rabbit antibody towards rat adipose tissue HSL, the corresponding enzyme in human adipose tissue was identified with an apparent 88 kDa polypeptide, thus slightly larger than the rat and bovine 84 kDa, and the mouse and guinea-pig 82 kDa species. Additional evidence for the identification was provided by the inhibition of HSL diacylglycerol lipase activity by the anti-rat HSL antibody, and by NaF, DFP and Hg2+, known inhibitors of HSL. The concentration of the enzyme, as reflected by its activity per g tissue and the specific activity was about two thirds of that in the rat adipose tissue (200 g rats). The identification of the human enzyme protein made it possible to directly demonstrate its phosphorylation by cAMP-dependent protein kinase, thus extending the previous report regarding activation of the lipase with this kinase and ATP-Mg2+ in human adipose tissue extracts (Khoo, J.C., Aquino, A.A. and Steinberg, D. (1974) J. Clin. Invest. 53, 1124-1131).

Method for the determination of indole and skatole in pig fat

AbstractA quantitative method was developed to determine by gas‐liquid chromatography the concentrations of indole and skatole in pig fat. Methanolic extracts of subcutaneous adipose tissue were de‐fatted by cooling, concentrated using solid‐phase extraction columns, and analysed in a gas chromatograph fitted with a polar capillary column and a flame ionisation detector. The method detects indole and skatole to below 0‐002 mg kg−1 fat.

Distribution of 2,3,7,8-tetrachlorodibenzo-p-dioxin in human whole blood and its association with, and extractability from, lipoproteins.

Little is known about the specific mechanism of human toxicity or the long term health effects of TCDD exposure. Acute effects such as chloracne may appear and disappear before more long-term effects manifest themselves, such as possible immune system suppression. The distribution of TCDD in the human body has not been documented. Due to its lipophilic nature, however, it has been quantitated from extracts of adipose tissue, breast milk, and, more recently, in serum. The transport and eventual equilibration of most xenobiotic substances proceeds from the blood into tissues. Investigators have studied the partitioning of TCDD among lipoprotein species, but the potential associations with other serum proteins and red blood cells have not been thoroughly documented. In developing and documenting a method for quantifying TCDD in serum samples, the authors needed to determine its distribution and availability for extraction in the blood. They sought to determine the partitioning of TCDD (when added in vitro to whole blood), among the cellular, serum protein, and lipoprotein components.

Effect of insulin, S-adenosylhomocysteine, phospholipase C, n-butanol and Triton X-114 on alkaline phosphatase from isolated rat adipocyte plasma membranes

Alkaline phosphatase activity has been described in aqueous extracts of rat adipose tissue [l], in rat adipocyte plasma membrane fractions [2] and more recently in human adipose tissue [3]. Studies with hepatocytes have shown this to be a membrane-bound enzyme covalently linked to the polar head group of phosphatidylinositol (PI) [4]; this interaction is supported by the finding that PI-specific phospholipase C releases alkaline phosphatase from hepatocyte plasma membranes [5,6]. It has also been proposed that the butanol-extraction of alkaline phosphatase from plasma membranes is caused by an autolytic process derived from activation of a plasma membrane phospholipase, phospholipase D [7] or PI-specific phospholipase C [8]. Insulin stimulates phospholipid methylation in rat adipocyte plasma membranes [9,10] and releases a low molecular weight, acid-stable mediator which activates mitochondrial pyruvate dehydrogenase [ll]. Inhibition of insulin-stimulated phospholipid methylation by S-adenosylhomocysteine results in a significant increase in activity of the pyruvate dehydrogenase activator [12]. Saltiel et al [13] have suggested that insulin modulates the activity of both pyruvate dehydrogenase and low k, CAMP phosphodiesterase by activation of a PI-specific phospholipase C in plasma membranes, hydrolysis of a novel PI-glycan and release of a modulator of insulin action. Using polyacrylamide gel electrophoresis (PAGE), we describe the effect of phospholipase C, n-butanol, Triton X-114, insulin and S-adenosylhomocysteine on

Involvement of Cyclic AMP-Dependent Protein Kinase on the Phosphorylase Kinase Inhibition by Glucose-6-Phosphate in Adipose Tissue Extracts

In order to achieve further clarification of the regulation of glycogenolysis in adipose tissue, we studied the effect of glucose-6-phosphate on phosphorylase activation in Sephadex G-25 filtrate of adipose tissue. The activity of phosphorylase kinase was decreased by 50% and by 75% in the presence of 0.5 mM and 2 mM of glucose-6-phosphate, respectively. This inhibition could be partially prevented by 0.5 mM AMP. Furthermore, we investigated the influence of glucose-6-phosphate on the effect of cyclic-AMP-dependent protein kinase on the activation of phosphorylase. The addition of cyclic-AMP and cyclic-AMP-dependent protein kinase caused a decrease in the inhibition of the phosphorylase activation by glucose-6-phosphate. Also, the glucose-6-phosphate at physiological concentration, decreased adipose tissue cyclic-AMP-dependent protein kinase activity.

Monoclonal antibodies to avian lipoprotein lipase. Purification of the enzyme by immunoaffinity chromatography

Three monoclonal antibodies to avian lipoprotein lipase have been isolated by fusing spleen cells from immunized BALB/c mice with myeloma P3X-63 Ag 8. The antibodies were detected by their ability to bind immobilized lipoprotein lipase in enzyme-linked immunosorbent assay (ELISA) and by immunoprecipitation of purified enzyme in the presence of second (rabbit anti-mouse) antibodies. Two of these antibodies, CAL1-7 and CAL1-11, inhibited catalytic activity, whereas with CAL1-2 interaction with lipoprotein lipase could be demonstrated only in ELISA and in Western blot assays following denaturation of the enzyme with sodium dodecyl sulfate. An immunoadsorbent column was prepared by coupling immunopurified CAL1-11 to Sepharose-4B. When acetone powder extracts of adipose tissue were applied on the column, 70% of the catalytic activity bound to the matrix. Effective elution was achieved with 1.8 M NaCl, 40% glycerol, 5% acetone, 20 mM Chaps (3[(3-holamidopropyl)dimethylammonio]propanesulfonate), 0.5 mM EDTA, 1 mM phosphate (pH 6.5). After concentration of the active fractions on a heparin-Sepharose 4B column, the purified enzyme was obtained with an overall recovery of 25%. Sodium dodecyl sulfate polyacrylamide gel electrophoresis demonstrates that the preparation is homogeneous with a major band at M r 60900. Thus, avian adipose lipoprotein lipase has been purified by a one-step immunoaffinity followed by a concentrating step on heparin-Sepharose 4B.

The concentration of P-enolpyruvate carboxykinase protein in murine tissues in diabetes of chemical and genetic origin.

P-enolpyruvate carboxykinase protein was measured by radioimmunoassay in liver, kidney, and adipose tissue extracts from alloxan- and streptozotocin-diabetic rats, in liver extracts from C57BL/KsJ-db+/db+ "diabetic" mice and in liver extracts from normal mice subjected to different dietary or hormonal states. The radioimmunoassay method measured tissue enzyme concentration (nanomoles/g) and total organ enzyme content (nanomoles/liver) independently of assayable activity (units/g). The "apparent" specific activity (units/nmol) was calculated from the maximum velocity and enzyme concentration data. Extracts of rat liver mitochondria and of skeletal muscle cytosol were also analyzed for P-enolpyruvate carboxykinase by the radioimmunoassay. In "chemical" diabetes, P-enolpyruvate carboxykinase by the radioimmunoassay. In "chemical" diabetes, P-enolpyruvate carboxykinase concentration increased approximately 3-fold over the fed, normal liver value of 0.89 microM, approximately 1.6-fold over the normal kidney value of 1.9 microM and approximately 2.9-fold over the normal adipose tissue value of 0.030 microM. Chemical diabetes caused the specific activity to decrease from 0.38 to approximately 0.27 units/nmol in liver and from 0.48 to approximately 0.32 units/nmol in kidney. Insulin replacement by in vivo injection not only promptly lowered the abnormally high enzyme concentration in both tissues but, paradoxically, decreased the apparent specific activities further to approximately 0.16 in liver and to 0.23 in kidney. In the db+/db+ diabetic mouse the liver enzyme increased from 0.22 microM at 5 weeks of age to 0.44 microM at 18 weeks when serum glucagon concentration is known to be highest and the pancreatic beta cells to be depleted of insulin. While the enzyme protein concentration increased 2-fold in the 18-week-old db+/db+ mouse, total liver enzyme content had actually increased 5-fold due to liver enlargement. In fasted normal mice, glucagon-treated normal mice, and alloxan/streptozotocin-treated mice, the concentration of liver enzyme increased significantly compared to the values in fed control mice. Pharmacological doses of dexamethasone did not induce the mouse enzyme. Rat liver mitochondria contained only trace quantities of immunoassayable enzyme which can be explained by contamination with cytosolic proteins. Rat skeletal muscle also contained only insignificant quantities of the enzyme or perhaps another cross-reacting, immunoassayable protein. The data obtained in diabetes before and after treatment show that complex mechanisms of control by insulin and glucagon do operate to regulate P-enolpyruvate carboxykinase in liver, kidney, and adipose tissue.

Coordination of glucose metabolism and NADPH formation in the adipose tissue and mammary gland during the lactation-weaning transition.

The capacity of fat cells from 48-hour weaned mid-lactating rats to synthesize fatty acids from (1-14C)-glucose and (6-14C)-glucose was markedly increased compared to adipocytes isolated from mid-lactating rats kept with their pups. Pentose shunt activity and glycolytic flux were also markedly increased in fat cells from weaned mid-lactating rats. Isolated fat cells from mid-lactating weaned rats, incubated with oleate and substrates that potentially may supply triosephosphate, showed a significantly stimulated triglyceride synthesis as measured by (1-14C)-oleate incorporation into the acyl moiety. The specific activities of malic enzyme, glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase were measured in adipose tissue and mammary gland extracts during pregnancy, lactation and weaning. These enzyme activities in the mammary gland increased in parallel from the beginning of pregnancy to the end of lactation. After weaning there was a sharp decrease in enzyme activities to levels similar to those found in mammary glands from virgin rats. In contrast, the activities of the lipogenic enzymes in adipose tissue were very low during lactation but increased rapidly after weaning.

Insulin-dependent and insulin-independent low Km cyclic AMP phosphodiesterase from rat adipose tissue.

Chromatographic analysis of a soluble extract of rat adipose tissue on DEAE-Sephacel resolves four distinct peaks of 3':5'-nucleotide phosphodiesterase (EC 3.1.4.17) activity. Kinetic investigation indicates that two of these fractions have a high affinity for cyclic AMP and show negative cooperative kinetic behavior at high substrate concentration. They differ in the degree of inhibition by cyclic GMP and in their response to insulin. If rat epididymal fat pads are incubated with insulin prior to homogenization, only one of the low Km cyclic AMP phosphodiesterase forms is stimulated.

Effects of Growth Hormone on the Oxidation of [1-14C]-Pyruvate in Adipose Tissue of Hypophysectomized Rats

The acute effects of growth hormone on the oxidation of [1-14C]-pyruvate to 14CO2 were studied in epididymal adipose tissue obtained from hypophysectomized rats. At concentrations ranging from 10 ng/ml to 1 microgram/ml, growth hormone increased the rate of pyruvate oxidation by 20-60%. A lag period of up to 30 min was required for the full effect of the hormone to develop. Addition of fructose to the incubation medium increased the rate of pyruvate oxidation in response to either growth hormone or insulin. The effects of 1 microgram/ml growth hormone were comparable in magnitude to those of 1 mU/ml insulin, and pyruvate oxidation in the presence of both agents was no greater than in the presence of either on its own. The enhancement of pyruvate oxidation by growth hormone, like that caused by insulin, probably results from activation of pyruvate dehydrogenase. Increased activity of pyruvate dehydrogenase was found in cell-free extracts of adipose tissue that had been exposed to either growth hormone or insulin. The response of tissue segments to growth hormone followed the same pattern as observed for other acute insulin-like effects of the hormones; it was transient and disappeared within 3 hours despite continued presence of the hormone. Previous exposure of the tissues to growth hormone made them refractory to the hormone upon reexposure.

L-Leucine activates branched chain alpha-keto acid dehydrogenase in rat adipose tissue.

The activity of branched chain alpha-keto acid dehydrogenase in extracts of adipose tissue was elevated after homogenization of tissue segments which had been incubated in buffer containing 0.3 mM leucine. A maximum increase (4-fold) was observed in extracts of tissues incubated in buffer containing 2.5 mM leucine, alpha-Ketoisocaproate and leucine caused maximum increases which were of similar magnitude and which required the same length of incubation of the tissue segments (5 to 15 min). The effect of leucine on branched chain alpha-keto acid dehydrogenase activity was observed both in the presence and absence of insulin, which also increased the activity of the enzyme in tissue extracts. Intact adipose tissue segments oxidized [I-14C]leucine at a maximum rate approximately 4 times that of [1-(14)C]valine. The rate of valine oxidation by intact tissue segments was doubled by addition of 0.2 to 0.5 mM unlabeled leucine, but not isoleucine, to medium containing 2 mM [1-(14)C]valine. Leucine, but not valine, also stimulated the rate of oxidation of 2 mM [U-14C]isoleucine by intact tissue segments. These results suggest that branched chain alpha-keto acid dehydrogenase activity, which is thought to limit the rate of branched chain amino acid oxidation in adipose tissue, may be sensitive to changes in the concentration of leucine in rat blood.

Exercise and the cAMP system in rat adipose tissue II. Nucleotide catabolism.

In this study the effect of exercise training on the adenosine 3',5'-cyclic monophosphate (cAMP) system of rat adipose tissue has been investigated. The basal amount of cAMP for the exercising rats was 0.179 +/- 0.021 nmol/10(6) cells, the same value as for the controls. Phosphodiesterase activities (low and high Km) remained unaffected as a result of the program of treadmill running. Kinetic constants for the low- and high-Km phosphodiesterases revealed that the affinity of the enzymes for substrate (cAMP) was unaltered by physical training. Finally, ethyleneglycol-bis(beta-aminoethylether)-N,N'-tetraacetic acid, possibly through its effect on calmodulin, stimulated or inhibited (depending on concentration) phosphodiesterase activity in the same direction and to a similar extent in extracts of adipose tissue from runners and controls. Taken together, these data clearly demonstrate the exercise training has no effect on the cAMP system of adipose tissue in male rats.

Tissue distribution, excretion, and metabolism of 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) in the Golden Syrian hamster

The hamster has been reported to be the least sensitive mammalian species to the acute toxicity of 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD). The fate of a single dose of [ 3H]- or [ 14C]TCDD (650 μg/kg, ip or po) was assessed in male hamsters for up to 35 days following treatment. The greatest content (percentage dose/g tissue) of radioactivity was found in the liver, adipose tissue, and adrenals. The radioactivity in liver and adipose tissue was identified as unmetabolized TCDD. The rate of 3H or 14C elimination in urine and feces suggested a first-order process. Similar half-life of elimination ( t 1 2 ) values of 12.0 ± 2.0 and 10.8 ± 2.4 days (mean ± SD) were obtianed with ip administered [ 3H]- and [ 14C]TCDD, respectively. With both [ 3H]- and [ 14C]TCDD, approximately 35 and 50% of the radioactivity was eliminated in urine and feces, respectively. The t 1 2 for po administered [ 3H]TCDD was 15.0 ± 2.5 days. High-pressure liquid chromatography of the urine and bile of animals receiving [ 14C]TCDD revealed one major and several minor radioactive peaks, none of which corresponded to [ 14C]TCDD. The apparent absence of TCDD metabolites in extracts of liver or adipose tissue indicates that the biotransformed products of TCDD are readily excreted in urine and bile. The enhanced rate of metabolism and excretion of TCDD in hamsters relative to other species may in part contribute to, but not totally explain its unusual resistance to TCDD toxicity.