Induction of brain D(--)-beta-hydroxybytrate dehydrogenase activity by fasting

In vivo assessment of β-hydroxybutyrate metabolism in mouse brain using deuterium (2 H) MRS

PURPOSE

To monitor the metabolic turnover of β-hydroxybutyrate (BHB) oxidation using 2 H-MRS in conjunction with intravenous administration of 2 H labeled BHB.

METHODS

Nine-month-old mice were infused with [3,4,4,4]-2 H4 -BHB (d4 -BHB; 3.11 g/kg) through the tail vein using a bolus variable infusion rate for a period of 90 min. The labeling of downstream cerebral metabolites from the oxidative metabolism of d4 -BHB was monitored using 2 H-MRS spectra acquired with a home-built 2 H surface coil on a 9.4T preclinical MR scanner with a temporal resolution of 6.25 min. An exponential model was fit to the BHB and glutamate/glutamine (Glx) turnover curves to determine rate constants of metabolite turnover and to aid in the visualization of metabolite time courses.

RESULTS

Deuterium label was incorporated into Glx from BHB metabolism through the tricarboxylic acid (TCA) cycle, with an increase in the level of [4,4]-2 H2 -Glx (d2 -Glx) over time and reaching a quasi-steady state concentration of ∼0.6 ± 0.1 mM following 30 min of infusion. Complete oxidative metabolic breakdown of d4 -BHB also resulted in the formation of semi-heavy water (HDO), with a four-fold (10.1 to ∼42.1 ± 7.3 mM) linear (R2  = 0.998) increase in its concentration by the end of infusion. The rate constant of Glx turnover from d4 -BHB metabolism was determined to be 0.034 ± 0.004 min-1 .

CONCLUSION

2 H-MRS can be used to monitor the cerebral metabolism of BHB with its deuterated form by measuring the downstream labeling of Glx. The integration of 2 H-MRS with deuterated BHB substrate provides an alternative and clinically promising MRS tool to detect neurometabolic fluxes in healthy and disease conditions.

Probing human sperm metabolism using 13C-magnetic resonance spectroscopy

STUDY QUESTION

Can 13C-Magnetic Resonance Spectroscopy (MRS) of selected metabolites provide useful information about human sperm metabolism and how glycolysis or oxidative phosphorylation are used by different sperm populations?

SUMMARY ANSWER

Sperm populations, prepared by density gradient centrifugation (DGC) and incubated with either 13Cu-glucose, 13Cu-fructose or 13C1-pyruvate, showed consistent evidence of metabolism generating principally lactate and more intermittently bicarbonate, and significantly more lactate was produced from 13Cu-glucose by vital or motile sperm recovered from the 40/80% interface compared to those from the pellet, which could not be accounted for by differences in the non-sperm cells present.

WHAT IS KNOWN ALREADY

Previous studies have focused on CO2 or other specific metabolite production by human sperm and there remains considerable debate about whether glycolysis and/or oxidative phosphorylation is the more important pathway for ATP production in sperm.

STUDY DESIGN, SIZE, DURATION

Sperm populations were prepared by DGC and subjected to 13C-MRS to answer the following questions. (i) Is it possible to detect human sperm metabolism of 13C substrates implicated in energy generation? (ii) What are the kinetics of such reactions? (iii) Do different sperm populations (e.g. '80%' pellet sperm and '40%' interface sperm) utilise substrates in the same way? Semen samples from 97 men were used in these experiments; 52 were used in parallel for aims (i) and (ii) and 45 were used for aim (iii).

PARTICIPANTS/MATERIALS, SETTING, METHODS

Sperm populations were prepared from ejaculates of healthy men using a Percoll/Phosphate Buffered Saline (PBS) DGC and then incubated with a range of 13C-labelled substrates (13Cu-glucose, 13Cu-fructose, 13C1-pyruvate, 13C1-butyrate, 13C3-lactate, 13C2,4-D-3-hydroxybutyrate, 13C5-l-glutamate, 13C1,2-glycine or 13Cu-galactose) along with penicillin/streptomycin antibiotic at 37°C for 4 h, 24 h or over 48 h for an estimated rate constant. Sperm concentration, vitality and motility were measured and, for a subset of experiments, non-sperm cell concentration was determined. A 9.4 T magnetic resonance spectrometer was used to acquire 1D 13C, inverse gated 1H decoupled, MRS spectra. Spectrum processing was carried out using spectrometer software and Matlab scripts to determine peak integrals for each spectrum.

MAIN RESULTS AND THE ROLE OF CHANCE

13Cu-glucose, 13Cu-fructose and 13C1-pyruvate were consistently converted into lactate and, to a lesser extent, bicarbonate. There was a significant correlation between sperm concentration and lactate peak size for 13Cu-glucose and 13Cu-fructose, which was not observed for 13C1-pyruvate. The lactate peak did not correlate with the non-sperm cell concentration up to 6.9 × 106/ml. The concentration of 13Cu-glucose, 13Cu-fructose or 13C1-pyruvate (1.8, 3.6, 7.2 or 14.4 mM) had no influence on the size of the observed lactate peak over a 4 h incubation. The rate of conversion of 13C1-pyruvate to lactate was approximately three times faster than for 13Cu-glucose or 13Cu-fructose which were not significantly different from each other. After incubating for 4 h, the utilisation of 13Cu-glucose, 13Cu-fructose or 13C1-pyruvate by sperm from the '40%' interface of the DGC was no different from those from the pellet when normalised to total sperm concentration. However, after normalising by either the vital or motile sperm concentration, there was a significant increase in conversion of 13Cu-glucose to lactate by '40%' interface sperm compared to pellet sperm (Vital = 3.3 ± 0.30 × 106 vs 2.0 ± 0.21 × 106; P = 0.0049; Motile = 7.0 ± 0.75 × 106 vs 4.8 ± 0.13 × 106; P = 0.0032. Mann-Whitney test P < 0.0055 taken as statistically significant). No significant differences were observed for 13Cu-fructose or 13C1-pyruvate.

LARGE SCALE DATA

Not applicable.

LIMITATIONS, REASONS FOR CAUTION

Only 13C labelled metabolites that accumulate to a sufficiently high concentration can be observed by 13C MRS. For this reason, intermediary molecules in the metabolic chain are difficult to observe without trapping the molecule at a particular step using inhibitors. Non-sperm cell concentration was typical of the general population and no link was found between these cells and the magnitude of the 13C-lactate peak. However, it is possible that higher concentrations than the maximum observed (6.9 × 106/ml) may contribute to exogenous substrate metabolism in other experiments.

WIDER IMPLICATIONS OF THE FINDINGS

13C-MRS can provide information on the underlying metabolism of multiple pathways in live sperm. Dysfunction in sperm metabolism, as a result of either impaired enzymes of lack of metabolisable substrate, could be detected in sperm by a non-destructive assay, potentially offering new treatment options to improve overall sperm quality and outcomes for reproduction.

STUDY FUNDING AND COMPETING INTERESTS

This work was supported by the Medical Research Council Grant MR/M010473/1. The authors declare no conflicts of interest.

Glucose, Lactate, β-Hydroxybutyrate, Acetate, GABA, and Succinate as Substrates for Synthesis of Glutamate and GABA in the Glutamine-Glutamate/GABA Cycle

The glutamine-glutamate/GABA cycle is an astrocytic-neuronal pathway transferring precursors for transmitter glutamate and GABA from astrocytes to neurons. In addition, the cycle carries released transmitter back to astrocytes, where a minor fraction (~25 %) is degraded (requiring a similar amount of resynthesis) and the remainder returned to the neurons for reuse. The flux in the cycle is intense, amounting to the same value as neuronal glucose utilization rate or 75-80 % of total cortical glucose consumption. This glucose:glutamate ratio is reduced when high amounts of β-hydroxybutyrate are present, but β-hydroxybutyrate can at most replace 60 % of glucose during awake brain function. The cycle is initiated by α-ketoglutarate production in astrocytes and its conversion via glutamate to glutamine which is released. A crucial reaction in the cycle is metabolism of glutamine after its accumulation in neurons. In glutamatergic neurons all generated glutamate enters the mitochondria and its exit to the cytosol occurs in a process resembling the malate-aspartate shuttle and therefore requiring concomitant pyruvate metabolism. In GABAergic neurons one half enters the mitochondria, whereas the other one half is released directly from the cytosol. A revised concept is proposed for the synthesis and metabolism of vesicular and nonvesicular GABA. It includes the well-established neuronal GABA reuptake, its metabolism, and use for resynthesis of vesicular GABA. In contrast, mitochondrial glutamate is by transamination to α-ketoglutarate and subsequent retransamination to releasable glutamate essential for the transaminations occurring during metabolism of accumulated GABA and subsequent resynthesis of vesicular GABA.

Clinical review: ketones and brain injury

Although much feared by clinicians, the ability to produce ketones has allowed humans to withstand prolonged periods of starvation. At such times, ketones can supply up to 50% of basal energy requirements. More interesting, however, is the fact that ketones can provide as much as 70% of the brain's energy needs, more efficiently than glucose. Studies suggest that during times of acute brain injury, cerebral uptake of ketones increases significantly. Researchers have thus attempted to attenuate the effects of cerebral injury by administering ketones exogenously. Hypertonic saline is commonly utilized for management of intracranial hypertension following cerebral injury. A solution containing both hypertonic saline and ketones may prove ideal for managing the dual problems of refractory intracranial hypertension and low cerebral energy levels. The purpose of the present review is to explore the physiology of ketone body utilization by the brain in health and in a variety of neurological conditions, and to discuss the potential for ketone supplementation as a therapeutic option in traumatic brain injury.

The metabolic basis of dual periodicity of feeding in rats

This article examines how the depletion and replenishment of various energy stores give rise to periodic eating and how constant body-energy levels are maintained over time.

Measures of the energy expended throughout the 24-hour feeding pattern in rats indicate that two different energy stores (one of small capacity and one of large) determine two superimposed feeding periodicities: one from meal to meal (prandial), the other from day to night (nycthemeral). The article reviews how experimental overrepletion or overdepletion of gastrointestinal content, blood glucose, or body fats affect food intake. These data suggest that gastrointestinal content determines both meal size and meal-to-meal periodicity. Other evidence indicates that glucose uptake rate in tissues, which is modulated by fat synthesis and fat mobilization, affects the periodic onset of feeding and the difference between nocturnal and diurnal postprandial satiety.

There follows an examination of the neuroendocrine bases for the interacting mechanisms governing energy input and output balance and of the role of the ventromedial hypothalamus in body-fat regulation and the lateral hypothalamus in feeding.

Cerebral metabolic adaptation and ketone metabolism after brain injury

The developing central nervous system has the capacity to metabolize ketone bodies. It was once accepted that on weaning, the 'post-weaned/adult' brain was limited solely to glucose metabolism. However, increasing evidence from conditions of inadequate glucose availability or increased energy demands has shown that the adult brain is not static in its fuel options. The objective of this review is to summarize the body of literature specifically regarding cerebral ketone metabolism at different ages, under conditions of starvation and after various pathologic conditions. The evidence presented supports the following findings: (1) there is an inverse relationship between age and the brain's capacity for ketone metabolism that continues well after weaning; (2) neuroprotective potentials of ketone administration have been shown for neurodegenerative conditions, epilepsy, hypoxia/ischemia, and traumatic brain injury; and (3) there is an age-related therapeutic potential for ketone as an alternative substrate. The concept of cerebral metabolic adaptation under various physiologic and pathologic conditions is not new, but it has taken the contribution of numerous studies over many years to break the previously accepted dogma of cerebral metabolism. Our emerging understanding of cerebral metabolism is far more complex than could have been imagined. It is clear that in addition to glucose, other substrates must be considered along with fuel interactions, metabolic challenges, and cerebral maturation.

Developmental regulation of D-beta-hydroxybutyrate dehydrogenase in rat liver and brain

The activities of ketone-metabolizing enzymes in rat brain increase 3- to 5-fold during the suckling period before decreasing to the adult level after weaning. We have observed that a similar developmental pattern also exists for D-beta-hydroxybutyrate dehydrogenase (BDH) in rat liver. Utilizing antibodies prepared against the purified protein we determined that the changes in BDH activities in both brain and liver are due to changes in the amount of BDH in the mitochondria. In vitro translations of isolated RNA followed by immunoprecipitation revealed that the increase in BDH activity and content was correlated with an increase in the level of functional BDH-mRNA in both liver and brain.

Elevated blood ketone and glucagon levels cannot account for 1,3-butanediol induced cerebral protection in the Levine rat

1,3-Butanediol is an ethanol dimer that induces systemic ketosis. It has previously been shown to increase hypoxic survival time and reduce neurologic deficit in several experimental preparations. The aim of this study was to determine if the mechanism of 1,3-butanediol-induced cerebral protection was elevation of blood ketone levels, blood glucagon levels, or both. Blood beta-hydroxybutyrate levels, glucagon levels, or both produced by a previously reported protective dose of 1,3-butanediol (47 mmol/kg) were simulated by direct i.v. infusion of the ketone beta-hydroxybutyrate and glucagon separately and in combination, and the effect on hypoxic survival time in instrumented Levine rats (unilateral carotid ligation and hypoxic exposure) was determined. To test if the mechanism was a direct or osmotic effect of the alcohol, an equimolar dose of ethanol (47 mmol/kg) was administered and the effect on hypoxic survival time was compared with that produced by 1,3-butanediol. As in previous studies, 1,3-butanediol significantly increased hypoxic survival time (241% of control, Scheffe p less than 0.05). Various doses of beta-hydroxybutyrate and glucagon were infused to approximate the blood levels of beta-hydroxybutyrate and glucagon produced by a protective dose of 1,3-butanediol. Although beta-hydroxybutyrate or glucagon infusions produced blood levels of these substances that were comparable with those produced by administering butanediol, they failed to prolong hypoxic survival time as long as 1,3-butanediol. No correlation was detected between hypoxic survival time and blood levels of beta-hydroxybutyrate, glucagon, insulin, or glucose. An equimolar dose of ethanol did not significantly increase hypoxic survival time.(ABSTRACT TRUNCATED AT 250 WORDS)

Glycemia, ketonemia, and brain enzymes of ketone body utilization in suckling and adult rats undernourished from intrauterine life

The effect of undernutrition from the 16th day of pregnancy up to 70th day of life on blood glucose and ketone bodies and on several brain mitochondrial enzymes related to energy metabolism or biosynthetic function was investigated. Undernutrition in perinatal period was established by means of a food restriction to pregnant rats and, later, to the lactating mother; undernourished postweaned rats received half the diet consumed by the controls. Body and brain weight from undernourished rats was less than controls throughout the entire period studied. Glycemia and ketonemia were also always lower than controls. Cytochrome c oxidase, citrate synthase, 3-hydroxybutyrate dehydrogenase, 3-oxoacid coenzyme A transferase, and acetoacetyl-coenzyme A thiolase activities during the suckling period were in most stages lower than controls; subsequently, activities in undernourished rats reached or surpassed the control values. These results could explain the "catch up" phenomenon in several ultrastructural parameters found by other authors in undernourished postweaned rats.

Two-day starvation does not alter the kinetics of blood--brain barrier transport and phosphorylation of glucose in rat brain

The blood-brain barrier (BBB) transport and brain phosphorylation of glucose were assessed in conscious rats subjected to 2 days of starvation. Although plasma glucose decreased, no significant changes in brain blood flow, BBB glucose transport, or 2-deoxy-D-glucose phosphorylation were observed. The data suggest that adaptive changes of brain glucose metabolism previously observed in starvation are located beyond the initial steps of brain entry and phosphorylation.

The effect of a high fat diet on pyruvate decarboxylase deficiency without central nervous system involvement

A nine-year-old Japanese boy with low pyruvate decarboxylase activity in fibroblasts showed no central nervous symptoms except for muscle fatigue. The pyruvate decarboxylase activities in fibroblasts of the patient and two control subjects were 0.407 +/- 0.083, 1.029 +/- 0.137 and 1.607 +/- 0.096 mumoles/g protein/30 min, respectively. The Michaelis-Menten constant (Km) was the same in the patient and controls. There was no inhibitor of pyruvate decarboxylase in the patient's fibroblasts. A high fat diet has been given to the patient for five years. At present he does not complain of any kind of muscle fatigue, except after severe exercise. Mental and physiological development of the patient are within the normal ranges. However, trials of orally administered thiamine hydrochloride or thiamine hydrochloride combined with lipoamide did not improve his muscle fatigue.

Ketone-body metabolism in glioma and neuroblastoma cells

We have examined the metabolism of ketone bodies in neuroblastoma C1300 and glioma C6 cells, two established lines of neural origin. The three ketone body-metabolizing enzymes are present in cells of both lines in the relative proportions normally found in brain (D-3-hydroxybutyrate dehydrogenase less than acetoacetyl-CoA thiolase less than 3-ketoacid CoA-transferase), the activities of the first two are higher in glioma cells than in neuroblastoma, and that of the third is 2-fold higher in neuroblastoma cells than in glioma cells. The specific activity of 3-ketoacid CoA-transferase (EC 2.8.3.5) in both cell lines increased as the cultures achieved confluence, then decreased. Ketone bodies and especially acetoacetate are preferred substrates for synthesis of neural lipids in cells of both lines. The incorporation of glucose carbon into lipids is significantly reduced in cells of both lines in the presence of ketone bodies. Addition of acetoacetate but not DL-3-hydroxybutyrate to the culture medium resulted in a significant increase in the activity of 3-ketoacid CoA-transferase and also in the rate of acetoacetate oxidation in neuroblastoma cells but not glioma cells. These findings indicate that specific differences exist in the capacity of these two cell lines to metabolize ketone bodies and also that substrate-level regulation of the ketone body-metabolizing pathway exists. These two lines therefore provide a potentially useful system in which the mechanisms of regulation of these enzymes may be examined.

Starvation-induced changes in transport of ketone bodies across the blood-brain barrier

The permeability of the blood brain barrier (BBB) to beta-hydroxybutyrate (beta-HB) was computed in fed and starved (five days) rats by the simultaneous measurement of cerebral blood flow (diffusible indicator method-123I-iodoantipyrine) and brain uptake of 14C-beta-HB (relative to a 3H2O reference). The results from the present study demonstrate that the movement of beta-HB across the BBB in rat is by a carrier-mediated process. During starvation, total movement (carrier-mediated and diffusionary) of this ketone body into brain was observed to be enhanced because of an increase in the diffusionary loss across the cerebral capillary. The calculated transport kinetics also suggest that the beta-HB molecule has a greater affinity for the transport (mediator) protein during starvation, although the maximal rate of uptake by brain due to a carrier processes mediated Vmax is decreased either because there is a smaller quantity of the mediating molecule or because there is trans inhibition by a high cellular concentration of beta-HB or some analog.

Ketone bodies are selectively used by individual brain regions

Close study of 3-hydroxybutyrate uptake by brain suggests that its metabolism is limited by permeability. Furthermore, the permeability characteristics vary from region to region; areas known to have no blood-brain barrier show the highest rate of utilization. The results imply that rather than substitute fuels, ketone bodies should be considered supplements which partially supply specific areas but are incapable of supporting the entire energy requirement of all brain regions.

[The ketone bodies in the hemolymph of Biomphalaria glabrata under starvation and infection with Schistosoma mansoni (author's transl)]

The metabolism of the snail Biomphalaria glabrata stressed by five days' starvation as well as by infection with Schistosoma mansoni was examined with regard to the metabolism of ketone bodies. Previous studies in the metabolism of this host--parasite relationship always resulted in changes in the same direction with starvation as well as with infection. Contrary to that the concentration of acetoacetate and beta-hydroxybutyrate measured in the hemolymph decreased significantly with starvation but increased significantly with infection. The following problems concerning the ketone body metabolism are discussed: on the one hand the differences between infected and starved snails, and on the other hand the differences between the snails and the mammals as well as in the invertebrates so far investigated.

Effect of hyperphenylalaninaemia on lipid synthesis from ketone bodies by rat brain

The effect of hyperphenylalaninaemia on the metabolism of ketone bodies in vivo and in vitro by developing rat brain was investigated. The incorporation in vivo of [14C]acetoacetate into cerebral lipids was decreased by both chronic (for 3 days) and acute (for 6h) hyperphenylalaninaemia induced by injecting phenylalanine into 1-week-old rats. In studies in vitro it was observed that the incorporation of the radioactivity from [14C]acetoacetate and 3-hydroxy[14C]butyrate into cerebral lipids was inhibited by phenyl-pyruvate, but not by phenylalanine. Phenylpyruvate also inhibited the incorporation of 3H from 3H2O into lipids by brain slices metabolizing either 3-hydroxybutyrate or acetoacetate in the presence of glucose. These findings suggest that the decrease in the incorporation in vivo of [14C]acetoacetate into cerebral lipids in hyperphenylalaninaemic rats is most likely caused by phenylpyruvate and not by phenylalanine. Phenylpyruvate as well as phenylalanine had no inhibitory effects on ketone-body-catabolizing enzymes, namely 3-hydroxybutyrate dehydrogenase, 3-oxo acid CoA-transferase and acetoacetyl-CoA thiolase, in rat brain. Phenylpyruvate but not phenylalanine inhibited the activity of the 2-oxoglutarate dehydrogenase complex from rat and human brain. These findings suggest that the metabolism of ketone bodies is impaired in brains of untreated phenylketonuric patients, and in turn may contribute to the diminution of mental development and function associated with phenylketonuria.

3-hydroxybutyrate dehydrogenase in tissues from normal and ketonaemic sheep

1. In liver, rumen epithelium and kidney cortex of the sheep, a dehydrogenase active against dl-3-hydroxybutyrate occurred in both the cytosol and particulate fractions of the tissues. In brain, heart, skeletal and smooth muscles, the enzyme occurred only in the particulate fraction. 2. Enzyme activity in the cytoplasmic fraction of liver and rumen epithelium was similar with either d(-)-3-hydroxybutyrate or dl-3-hydroxbutyrate, but was less with acetoacetate as the substrate. The cytosol fraction of kidney cortex showed very little activity with d(-)-3-hydroxybutyrate, confirming that most of the activity with dl-3-hydroxybutyrate was with the l(+) isomer in this tissue. 3. 3-Hydroxybutyrate dehydrogenase activities in the cytosol and particulate fractions of liver, rumen epithelium and kidney cortex and in the particulate fraction of brain tissue were not stimulated by phosphatidylcholine, unlike the enzyme in sheep muscle and in tissues of other species. 4. The activity of 3-hydroxybutyrate dehydrogenase was not increased significantly in any of the tissues of ketonaemic sheep. 5. Comparison of rates of 3-hydroxybutyrate production in vivo with the enzyme activity in ketogenic tissue suggested that in sheep the maximum rate of production might be limited by this activity.