The interconversion and disposal of ketone bodies in untreated and injured post-absorptive rats

The lipogenic enzyme acetoacetyl-CoA synthetase and ketone body utilization for denovo lipid synthesis, a review

Acetoacetyl-CoA synthetase (AACS) is the key enzyme in the anabolic utilization of ketone bodies (KBs) for denovo lipid synthesis, a process that bypasses citrate and ATP citrate lyase. This review shows that AACS is a highly regulated, cytosolic, and lipogenic enzyme and that many tissues can readily use KBs for denovo lipid synthesis. AACS has a low micromolar Km for acetoacetate, and supply of acetoacetate should not limit its activity in the fed state. In many tissues, AACS appears to be regulated in conjunction with the need for cholesterol, but in adipose tissue, it seems tied to fatty acid synthesis. KBs are readily utilized as substrates for lipid synthesis in lipogenic tissues, including liver, adipose tissue, lactating mammary gland, skin, intestinal mucosa, adrenals, and developing brain. In numerous studied cases, KBs served several-fold better than glucose as substrates for lipid synthesis, and when present, KBs suppressed the utilization of glucose for lipid synthesis. Here, it is hypothesized that a physiological role for the utilization of KBs for lipid synthesis is a metabolic process of lipid interconversion. Fatty acids are converted to KBs in liver, and then, the KBs are utilized to synthesize cholesterol and other long-chain fatty acids in liver and nonhepatic tissues. The conversion of fatty acids to cholesterol via the KBs may be a particularly important example of lipid interconversion. Utilizing KBs for lipid synthesis is glucose sparing and probably is important with low carbohydrate diets. Metabolic situations and tissues where this pathway may be important are discussed.

[(11)C]-Acetoacetate PET imaging: a potential early marker for cardiac heart failure

The ketone body acetoacetate could be used as an alternate nutrient for the heart, and it also has the potential to improve cardiac function in an ischemic-reperfusion model or reduce the mitochondrial production of oxidative stress involved in cardiotoxicity. In this study, [(11)C]-acetoacetate was investigated as an early marker of intracellular damage in heart failure.

METHODS

A rat cardiotoxicity heart failure model was induced by doxorubicin, Dox(+). [(14)C]-Acetoacetate, a non-positron (β-) emitting radiotracer, was used to characterize the arterial blood input function and myocardial mitochondrial uptake. Afterward, [(11)C]-acetoacetate (β+) myocardial PET images were obtained for kinetic analysis and heart function assessment in control Dox(-) (n=15) and treated Dox(+) (n=6) rats. The uptake rate (K1) and myocardial clearance rate (k2or kmono) were extracted.

RESULTS

[(14)C]-Acetoacetate in the blood was increased in Dox(+), from 2 min post-injection until the last withdrawal point when the heart was harvested, as well as the uptake in the heart and myocardial mitochondria (unpaired t-test, p <0.05). PET kinetic analysis of [(11)C]-acetoacetate showed that rate constants K1, k2 and kmono were decreased in Dox(+) (p <0.05) combined with a reduction of 24% of the left ventricular ejection fraction (p <0.001).

CONCLUSION

Radioactive acetoacetate ex vivo analysis [(14)C], and in vivo kinetic [(11)C] studies provided evidence that [(11)C]-acetoacetate can assess heart failure Dox(+). Contrary to myocardial flow reserve (rest-stress protocol), [(11)C]-acetoacetate can be used to assess reduced kinetic rate constants without requirement of hyperemic stress response. The proposed [(11)C]-acetoacetate cardiac radiotracer in the investigation of heart disease is novel and paves the way to a potential role for [(11)C]-acetoacetate in cardiac pathophysiology.

Measurement of beta-hydroxybutyrate in acute hyperketonemia in human brain

We report the measurement of D-beta-hydroxybutyrate (BHB) in the brains of six normal adult subjects during acute infusions of BHB. We used high field in vivo (1)H magnetic resonance (MR) spectroscopy in the occipital lobe in conjunction with an acute infusion protocol to elevate plasma BHB levels from overnight fasted levels (0.20 +/- 0.10 mM) to a steady state value of 2.12 +/- 0.30 mM. At this level of hyperketonemia, we determined a tissue BHB level of 0.24 +/- 0.04 mM. No increases in brain lactate levels were seen in these data. The concentrations of BHB and lactate were both considerably lower in comparison with previous data acquired in fasted adult subjects. This suggests that up-regulation of the monocarboxylic acid transporter occurs with fasting.

Mechanism of hypertriglyceridemia in human apolipoprotein (apo) CIII transgenic mice. Diminished very low density lipoprotein fractional catabolic rate associated with increased apo CIII and reduced apo E on the particles

Hypertriglyceridemia is common in the general population, but its mechanism is largely unknown. In previous work human apo CIII transgenic (HuCIIITg) mice were found to have elevated triglyceride levels. In this report, the mechanism for the hypertriglyceridemia was studied. Two different HuCIIITg mouse lines were used: a low expressor line with serum triglycerides of approximately 280 mg/dl, and a high expressor line with serum triglycerides of approximately 1,000 mg/dl. Elevated triglycerides were mainly in VLDL. VLDL particles were 1.5 times more triglyceride-rich in high expressor mice than in controls. The total amount of apo CIII (human and mouse) per VLDL particle was 2 and 2.5 times the normal amount in low and high expressors, respectively. Mouse apo E was decreased by 35 and 77% in low and high expressor mice, respectively. Under electron microscopy, VLDL particles from low and high expressor mice were found to have a larger mean diameter, 55.2 +/- 16.6 and 58.2 +/- 17.8 nm, respectively, compared with 51.0 +/- 13.4 nm from control mice. In in vivo studies, radiolabeled VLDL fractional catabolic rate (FCR) was reduced in low and high expressor mice to 2.58 and 0.77 pools/h, respectively, compared with 7.67 pools/h in controls, with no significant differences in the VLDL production rates. In an attempt to explain the reduced VLDL FCR in transgenic mice, tissue lipoprotein lipase (LPL) activity was determined in control and high expressor mice and no differences were observed. Also, VLDLs obtained from control and high expressor mice were found to be equally good substrates for purified LPL. Thus excess apo CIII in HuCIIITg mice does not cause reduced VLDL FCR by suppressing the amount of extractable LPL in tissues or making HuCIIITg VLDL a bad substrate for LPL. Tissue uptake of VLDL was studied in hepatoma cell cultures, and VLDL from transgenic mice was found to be taken up much more slowly than control VLDL (P < 0.0001), indicating that HuCIIITg VLDL is not well recognized by lipoprotein receptors. Additional in vivo studies with Triton-treated mice showed increased VLDL triglyceride, but not apo B, production in the HuCIIITg mice compared with controls. Tissue culture studies with primary hepatocytes showed a modest increase in triglyceride, but not apo B or total protein, secretion in high expressor mice compared with controls. In summary, hypertriglyceridemia in HuCIIITg mice appears to result primarily from decreased tissue uptake of triglyceride-rich particles from the circulation, which is most likely due to increased apo CIII and decreased apo E on VLDL particles. the HuCIIITg mouse appears to be a suitable animal model of primary familial hypertriglyceridemia, and these studies suggest a possible mechanism for this common lipoprotein disorder.

Models to interpret kinetic data in stable isotope tracer studies

In contrast to "weightless" radioactive tracers, stable isotope tracers have nonnegligible mass and are naturally present in the system, and the measured variable is a ratio of two isotopic species. These features do not allow stable isotopic tracer data analysis using straightforward analogy with radioactive tracer approaches, even though this practice is common. In this study, we present kinetic variables, models, and measurements for the analysis and interpretation of stable isotope tracer data. Assumptions and mathematical techniques for modeling the data when perturbation is both nonnegligible and negligible are discussed. Emphasis is placed on the rich information content of the dynamic portion of a stable isotope tracer curve and on the role of compartmental and noncompartmental modeling approaches for its interpretation. A presumed and commonly used analogy between the radioactive specific activity and stable isotopic enrichment is shown to be incorrect. We show that the proper analogue of specific activity is the tracer-to-tracee molar ratio. This variable is not a directly measurable one, but a formula is derived that allows its computation from the data. A method for reconstructing the time course in blood of the concentration component due to endogenous synthesis is presented. This allows measurement of the extent of the perturbation in the case where a nonweightless tracer is used. Special attention is given to data analysis originating from a multiple tracer experiment, a configuration necessary for studying more complex systems, e.g., the kinetics of interacting substrates.

Regional glucose and beta-hydroxybutyrate use by developing rat brain

Rates of glucose and D-beta-hydroxybutyrate use were determined in five brain regions of 20-day-old rats. The regions studied were cerebral cortex, thalamus, striatum, cerebellum, and brain stem. The tracers for determining rates of substrate use were [3H]fluorodeoxyglucose and [3-14C]-D-beta-hydroxybutyrate. Two or five minutes after isotope administration the animals were sacrificed in a 6-kW, 2450-MHz focused microwave device. Ten minutes prior to isotope administration the animals were injected intraperitoneally with normal saline or DL-beta-hydroxybutyrate (10 mmol/kg). Blood D-beta-hydroxybutyrate levels averaged 0.21 mumol/ml in saline-injected and 3.13 mumol/ml in hyperketonemic rats. Rates of glucose utilization were significantly heterogeneous between regions in both groups: thalamus greater than cerebral cortex greater than or equal to striatum greater than brain stem greater than cerebellum. These rates were 20-35% lower in hyperketonemic rats. Rates of D-beta-hydroxybutyrate use varied significantly between regions only in the saline group, with the brain stem rate being significantly lower than that in cortex or cerebellum. Regional rates of D-beta-hydroxybutyrate use did not correlate significantly with regional rates of glucose use in either the saline or the hyperketonemic groups. Regional rates of glucose use were strongly and positively correlated between conditions, as were regional rates of D-beta-hydroxybutyrate use. Thus, in 20-day-old rats, the regional heterogeneity of brain glucose use is similar to that in adult rats. D-beta-Hydroxybutyrate use is much less regionally heterogeneous.(ABSTRACT TRUNCATED AT 250 WORDS)

Ketone body transport in the human neonate and infant

Using a continuous intravenous infusion of D-(-)-3-hydroxy[4,4,4-2H3]butyrate tracer, we measured total ketone body transport in 12 infants: six newborns, four 1-6-mo-olds, one diabetic, and one hyperinsulinemic infant. Ketone body inflow-outflow transport (flux) averaged 17.3 +/- 1.4 mumol kg-1 min-1 in the neonates, a value not different from that of 20.6 +/- 0.9 mumol kg-1 min-1 measured in the older infants. This rate was accelerated to 32.2 mumol kg-1 min-1 in the diabetic and slowed to 5.0 mumol kg-1 min-1 in the hyperinsulinemic child. As in the adult, ketone turnover was directly proportional to free fatty acid and ketone body concentrations, while ketone clearance declined as the circulatory content of ketone bodies increased. Compared with the adult, however, ketone body turnover rates of 12.8-21.9 mumol kg-1 min-1 in newborns fasted for less than 8 h, and rates of 17.9-26.0 mumol kg-1 min-1 in older infants fasted for less than 10 h, were in a range found in adults only after several days of total fasting. If the bulk of transported ketone body fuels are oxidized in the infant as they are in the adult, ketone bodies could account for as much as 25% of the neonate's basal energy requirements in the first several days of life. These studies demonstrate active ketogenesis and quantitatively important ketone body fuel transport in the human infant. Furthermore, the qualitatively similar relationships between the newborn and the adult relative to free fatty acid concentration and ketone inflow, and with regard to ketone concentration and clearance rate, suggest that intrahepatic and extrahepatic regulatory systems controlling ketone body metabolism are well established by early postnatal life in humans.

Different metabolites might reduce food intake by the mitochondrial generation of reducing equivalents

To test the possibility that hypophagic effects of injected metabolites are linked to a particular oxidative step in their metabolism, cumulative food intake in rats after subcutaneous injection of various metabolites or their immediate oxidation products was investigated. Glycerol was compared with dihydroxyacetone, L-malate with oxaloacetate, and L-lactate with pyruvate. Subcutaneous injection of 4.5 mmol/kg metabolic body weight (BW0.75) of glycerol or malate reduced food intake significantly, whereas the same doses of dihydroxyacetone or oxaloacetate did not. Lactate or pyruvate (4.5 mmol/kg BW0.75, each) both reduced food intake significantly. However, even higher doses (7.7 mmol/kg BW0.75) of lactate or pyruvate failed to affect food intake when rats were fed a high fat (HF) diet, which is known to decrease the activity of the enzyme pyruvate dehydrogenase. In an additional experiment, subcutaneous injection of lactate or pyruvate (7.7 mmol/kg BW0.75, each) increased plasma levels of lactate and pyruvate 2 h after the injection in HF-rats more than in rats fed the usual high-carbohydrate diet. The data are consistent with the hypothesis that the increased generation of reducing equivalents in the mitochondria, brought about by the oxidation of injected glycerol, malate, lactate, or pyruvate, reduces food intake in rats.

Ketone kinetics and D-(-)-3-hydroxybutyrate-induced inhibition of feeding in rats

For further investigation of the mechanism involved in D,L-3 hydroxybutyrate (DL3HB)-induced inhibition of feeding [5], we measured cumulative food intake following subcutaneous injections of DL3HB or D-(-)-3-hydroxybutyrate (D3HB), and plasma ketone levels following subcutaneous injections of D3HB or acetoacetate (ACAC) in rats. Subcutaneously injected DL3HB (10 mmoles/kg body weight) and D3HB (5 mmoles/kg body weight) reduced food intake a similar amount. After injection of D3HB or ACAC (5 mmoles/kg body weight) plasma concentrations of the injected ketone bodies reached a similar high level at 20 min and then decreased with time (40 and 60 min). Even though ACAC failed to reduce food intake, injected ACAC disappeared faster from the circulation than injected D3HB. The data are consistent with the idea that oxidation of injected D3HB to ACAC reduces food intake in rats.

A double-isotope method for the measurement of ketone-body turnover in the rat. Effect of L-alanine

The synthesis of 4-3H-labelled ketone bodies, and their use along with 14C-labelled ketone-body precursors, is employed using an 'in vivo' rat infusion model to measure ketone-body turnover. The use of two isotopes is necessary to measure ketone-body turnover when ketogenesis may occur from more than one precursor such as glucose and fatty or amino acids. Requirements of isotopic equivalence in terms of metabolic similarity, valid stoichiometry and the lack of differences in the kinetics of relevant enzymes is demonstrated for the 4-3H- and 14C-labelled ketone bodies. The hypoketonaemic effect of L-alanine is shown by two distinct phases after the administration of L-alanine. During the first 12 min after alanine administration ther was a 50% decrease in acetoacetate and a 30% decrease in 3-hydroxybutyrate production, with no significant change in the utilization of either compound. The hypoketonaemic action of alanine during the following 16 min was primarily associated with an uptake of 3-hydroxybutyrate that was somewhat greater than the increase in its production. There were essentially equivalent decreases in production and utilization of acetoacetate, resulting in no significant net change in the level of this ketone body in the blood.

Different effects of subcutaneous D,L-3-hydroxybutyrate and acetoacetate injections on food intake in rats

Cumulative food intake following subcutaneous injection of D,L-3-hydroxybutyrate (DL3HB) or acetoacetate (AcAc) was investigated in rats, because ketone bodies might contribute to food intake regulation according to Kennedy's lipostatic hypothesis. In addition, the metabolic effects of DL3HB-injections were studied by measuring the levels of plasma D3HB, plasma non-esterified fatty acids (NEFA), plasma glycerol, blood glucose, and liver glycogen. Subcutaneously injected DL3HB (10 mmoles/kg body weight) significantly reduced feeding while equimolar AcAc did not. DL3HB-injection increased plasma D3HB and decreased plasma NEFA and plasma glycerol 1-2 hours after the injection but did not affect blood glucose or liver glycogen content. The data suggest that oxidation of D3HB to AcAc contributes to the inhibition of feeding following subcutaneous DL3HB-injection in rats.

Changes in cardiac output and its distribution after non-haemorrhagic injury in the rat

In the unanaesthetized rat cardiac output, measured by a thermodilution technique, fell rapidly following a 4h period of bilateral hind-limb ischaemia. There was a significant positive correlation between body temperature and total cardiac output after injury. There were also striking changes in the distribution of cardiac output; for example, hepatic artery flow was markedly increased, whereas splanchnic flow fell passively with the reduction in cardiac output. There were suggestions of regional differences in the reduction of blood flow within the brain.

Influence of acetate on the metabolism of beta-hydroxybutyrate in the perfused hind-quarter of the rat

The effect of acetate, at concentrations normally found during ethanol combustion in intact animals and man in vivo, on the uptake and oxidation of beta-hydroxybutyrate by the perfused rat hind-quarter was studied. The addition of acetate did not significantly affect the total uptake of beta-hydroxybutyrate, but caused a 40% decrease in the oxidation of beta-hydroxybutyrate to CO2. The oxidation of 14C-beta-hydroxybutyrate to 14CO2 accounted for about 10% of the total oxygen consumption by the perfused muscle in the absence of acetate. In the presence of acetate this figure was reduced to about 5%. The addition of beta-hydroxybutyrate did not significantly affect the metabolic fate of 14C-acetate. It is concluded that acetate is preferred as oxidative substrate to beta-hydroxybutyrate. The inhibited oxidation of beta-hydroxybutyrate by acetate did not affect the concentration ratio between beta-hydroxybutyrate and acetoacetate in the medium at the end of the perfusion, indicating that the ability of the muscle tissue to restore an increased redoxlevel, "exported" from the liver during ethanol combustion to extrahepatic tissues, was not impaired by acetate.

Blood D-3-hydroxybutyrate and the regulation of plasma concentrations of free fatty acids in the fasted rat

Acetoacetate or DL-3-hydroxybutyrate were infused into fasted rats at rates comparable to the endogenous rate of ketone body release. Blood samples taken before and during infusion were analyzed for concentrations of ketone bodies, free fatty acids, and immunoreactive insulin. During infusion of acetoacetate or DL-3-hydroxybutyrate, blood concentrations of total ketone bodies doubled. While the HBA/AcAc ratios remained constant in rats infused with DL-3-hydroxybutyrate, the ratios decreased significantly during infusion of acetoacetate, reflecting increases in acetoacetate concentrations without proportionate increases in concentrations of D-3-hydroxybutyrate. Blood concentrations of free fatty acids decreased more rapidly in the rats infused with DL-3-hydroxybutyrate than in the rats infused with acetoacetate. The decrease in free fatty acid concentrations was related to increases in blood D-3-hydroxybutyrate in both the DL-3-hydroxybutyrate-infused rats and in the acetoacetate-infused rats. Plasma immunoreactive insulin concentrations were significantly higher in the DL-3-hydroxybutyrate-infused rats than in the acetoacetate-infused rats. Acetoacetate and DL-3-hydroxybutyrate were infused into depancreatized, functionally hepatectomized fasted rats at rates comparable to those used in the intact fasted rats. Only small decreases in free fatty acid concentrations were observed during infusion. From these experiments it was concluded that increases in blood concentrations of D-3-hydroxybutyrate were more effective than acetoacetate in causing decreases in free fatty acid concentrations in the fasting rat. The pancreas is required for this effect.

Effect of ischaemic limb injury on the rates of metabolism of ketone bodies in starved rats

1. Rats starved for 30h were injected with trace amounts of [3-14C]acetoacetate and beta-hydroxy[3-14C]butyrate 1h after ischaemic limb injury in a 20 degrees C environment, and the concentrations and radioactivities of blood ketone bodies were determined at intervals. 2. Starvation alone raised the rates of production and utilization of beta-hydroxybutyrate plus acetoacetate about 3.7-fold, but lowered their metabolic clearance rates by about 50%. In the starved rat ketone-body oxidation could account for up to 30% of whole body O2 consumption. 3. Injury in starved rats lowered the rates of production and utilization of both beta-hydroxybutyrate and acetoacetate, the combined fall of about 37% slightly exceeding the concomitant fall in whole-body O2 consumption. The concentration of beta-hydroxybutyrate decreased after injury, but its metabolic clearance rate was unaltered; the concentration of acetoacetate rose slightly and its metabolic clearance rate fell.

The estimation of rates of utilization of glucose and ketone bodies in the brain of the suckling rat using compartmental analysis of isotopic data

The brains of 18-day-old rats utilize glucose and ketone bodies. The rates of acetyl-CoA formation from these substrates and of glycolysis were determined in vivo from the labelling of intermediary metabolites after intraperitoneal injection of d-[2-(14)C]glucose, l(+)-[3-(14)C]- and l(+)-[U-(14)C]-lactate and d(-)-3-hydroxy[(14)C]butyrate. Compartmental analysis was used in calculating rates to allow for the rapid exchange of blood and brain lactate, the presence in brain of at least two pools each of glucose and lactate, and the incomplete equilibration of oxaloacetate with aspartate and of 2-oxoglutarate with glutamate. Results were as follows. 1. Glucose and ketone bodies labelled identical pools of tricarboxylate-cycle metabolites, and were in every way alternative substrates. 2. The combined rate of oxidation of acetyl-CoA derived from pyruvate (and hence glucose) and ketone bodies was 1.05mumol/min per g. 3. Ketone bodies contributed 0.11-0.53mumol/min per g in proportion to their concentration in blood, with a mean rate of 0.30mumol/min per g at 1.24mm. 4. Pyruvate and ketone bodies were converted into lipid at 0.018 and 0.008mumol/min per g respectively. 5. Glycolysis, at 0.48mumol/min per g, was more rapid in most rats than pyruvate utilization by oxidation and lipid synthesis, resulting in a net output of lactate from brain to blood. 6. Rates of formation of brain glutamate, glutamine and aspartate were also measured. Further information on the derivation of the models has been deposited as Supplementary Publication SUP 50034 (18 pages) at the British Library, Lending Division (formerly the National Lending Library for Science and Technology), Boston Spa, Yorks. LS23 7QB, U.K., from whom copies can be obtained on the terms indicated in Biochem. J. (1973) 131, 5.

The design of experiments using isotopes for the determination of the rates of disposal of blood-borne substrates in vivo with special reference to glucose, ketone bodies, free fatty acids and proteins

1. The two well-known methods of estimating rates of irreversible disposal (R) of blood-borne substrates in vivo by isotope experiments involve estimating the specific radioactivity (S) of the substrate in blood either after single intravenous injection of labelled substrate or during its infusion at a constant rate. The value of R is calculated from the S-time curve, usually by assuming: (i) a metabolic steady state with respect to substrate, (ii) the passage of all substrate through the blood, and (iii) the absence of certain types of recycling via blood. 2. In a theoretical investigation we show how experiments can be performed and R calculated from analyses of blood when one or more of the above assumptions is unjustified, by using glucose, ketone bodies, plasma free fatty acids and proteins as examples. In general the methods require single injection procedures, with estimation of the total quantity of label in the substrate in blood and the substrate concentration instead of only S. Such values give estimates of R with standard errors even when only one blood specimen is taken from each of a group of animals, as is convenient when working with small animals or substrates in low concentration, and when the animals are in a non-steady state in which constant infusion procedures are invalid. 3. Similar methods give the fraction of label injected as one compound which passes through another (the isotopic yield). 4. The methods are not always applicable, and cannot be applied to plasma proteins in some pathological conditions. A questionnaire for assessing their applicability is given.