Activities of the enzymes of ketone body metabolism in the developing chick

The concentration of ketone bodies in the blood of the developing chick prior to and just after hatching were higher than those found in the adult. The activities of 3-oxo acid-CoA transferase and acetoacetyl-CoA thiolase in the heart, leg and pectoral muscle before and after hatching were higher than those of the adult. The activity of 3-hydroxybutyrate dehydrogenase increased constantly during incubation and after hatching in all three muscle tissues. In the liver the activities of the enzymes of ketone body synthesis increased during incubation and after hatching. It is suggested that the liver could provide fuel to the extrahepatic tissues of the developing chick and ketone bodies could contribute as fuel for oxidation in the skeletal muscle of the newly hatched bird.     

Activities of enzymes of ketone-body utilization in brain and other tissues of suckling rats

1. The activities of 3-hydroxybutyrate dehydrogenase and 3-oxo acid CoA-transferase in rat brain at birth were found to be about two-thirds of those of adult rat brain, expressed per g wet wt. The activities rose throughout the suckling period and at the time of weaning reached values about three times higher than those for adult brain. Later they gradually declined. 2. At birth the activity of acetoacetyl-CoA thiolase in rat brain was about 60% higher than in the adult. During the suckling period there was no significant change in activity. 3. In rat kidney the activities of the three enzymes at birth were less than one-third of those at maturity. They gradually rose and after 5 weeks approached the adult value. Similar results were obtained with rat heart. 4. The activity of glutamate dehydrogenase (a mitochondrial enzyme like 3-hydroxybutyrate dehydrogenase and 3-oxo acid CoA-transferase) also rose in brain and kidney during the suckling period, but at no stage did it exceed the adult value. 5. Throughout the suckling period the total ketone-body concentration in the blood was about six times higher than in adult fed rats, and the concentration of free fatty acids in the blood was three to four times higher. 6. It is concluded that the rate of ketone-body utilization in brains of suckling rats is determined by both the greater amounts of the key enzymes in the tissue and the high concentrations of ketone bodies in the blood. In addition, the low activities of the relevant enzymes in kidney and heart of suckling rats may make available more ketone bodies for the brain.     

The metabolic effects of prolonged starvation and refeeding in sturgeon and rainbow trout

The present study examines the particular metabolic strategies of the sturgeon Acipenser naccarii in facing a period of prolonged starvation (72 days) and subsequent refeeding (60 days) compared to the trout Oncorhynchus mykiss response under similar conditions. Plasma metabolites, endogenous reserves, and the activity of intermediate enzymes in liver and white muscle were evaluated. This study shows the mobilization of tissue reserves during a starvation period in both species with an associated enzymatic response. The sturgeon displayed an early increase in hepatic glycolysis during starvation. The trout preferentially used lactate for gluconeogenesis in liver and white muscle. The sturgeon had higher lipid-degradation capacity and greater synthesis of hepatic ketone bodies than the trout, although this latter species also showed strong synthesis of ketone bodies during starvation. During refeeding, the metabolic activity present before starvation was recovered in both fish, with a reestablishment of tissue reserves, plasmatic parameters (glucemia and cholesterol), and enzymatic activities in the liver and muscle. A compensatory effect in enzymes regarding lipids, ketone bodies, and oxidative metabolism was displayed in the liver of both species. There are metabolic differences between sturgeon and trout that support the contention that the sturgeon has common characteristics with elasmobranchs and teleosts.     

Ketone body metabolism during development

This paper briefly reviews the role of ketone bodies during development in the rat. Regulation of ketogenesis is in part dependent on the supply to the liver of medium- and long-chain fatty acids derived from mother's milk. The partitioning of long-chain fatty acids between the hepatic esterification and oxidation pathways is controlled by the concentration of malonyl-CoA, a key intermediate in the conversion of carbohydrate to lipid. As hepatic lipogenesis is depressed during the suckling period, [malonyl-CoA] is low and entry of long-chain acyl-CoA into the mitochondria for partial oxidation to ketone bodies is not restrained. Removal of ketone bodies by developing tissues is regulated by their availability in the circulation and by the activities of the enzymes of ketone body utilization. The patterns of activities of these enzymes differ among tissues during development so that the neonatal brain is an important site of ketone body utilization. The major role of ketone bodies in development is as an oxidative fuel to spare glucose, but they can also act as lipid precursors.     

Differential action of thyroid hormones on the activity of certain enzymes in rat kidney and brain.

In rat kidney several mitochondrial and soluble enzyme activities are stimulated by thyroid hormones and the mitochondrial membrane fluidity is also increased. However, the ketone metabolism enzyme activities of D-3-hydroxybutyrate dehydrogenase and of 3-oxoacid CoA-transferase are not significantly affected by the hyperthyroid state and the ketone body concentration is not greatly changed. Therefore, in hyperthyroid rats the response of the kidney, as far as the ketone bodies and their metabolizing enzymes are concerned, is at variance with that of the liver and the heart. In the brain of young rats, age 8-9 weeks, the activities of the enzymes of ketone body metabolism and those responsible for other metabolic pathways are not influenced by the hyperthyroid state. In these animals, however, the activities of two enzymes, NAD-isocitrate dehydrogenase and pyruvate kinase, are still stimulated by 28 and 41%, respectively. This can be probably related to the higher energy requirement for definitive brain maturation in young hyperthyroid rats.     

Induction of ketone body enzymes in glial cells

Ketone bodies serve a dual function in developing brain. They are important sources of energy for metabolism and serve as precursors for lipid synthesis. Astrocytes have two to three times higher activity than oligodendroglia for one of the enzymes involved in ketone body metabolism, 3-ketoacid-CoA transferase. Both glial cell types have similar levels of activity for beta-hydroxybutyrate dehydrogenase. Glucocorticoids and dibutytyl cAMP produce a significant stimulation of activity of both enzymes in astrocytes and oligodendroglia. However, the most striking induction in activity of the two enzymes is in the presence of hydrocortisone and sodium butyrate. There is a three- to eightfold stimulation with these effectors in both astrocytes and oligodendroglia. Thus, in brain the expression of ketone body enzyme activities is finely regulated by hormones and by agents that increase cAMP levels.     

Developmental Changes in the Folate-Dependent Enzymes of De Novo Purine Biosynthesis in Rat Brain

Abstract: The activities of the two folate-dependent enzymes in the de nova purine biosynthetic pathway (e.g., glycinamide ribonucleotide transformylase and aminoimidazolecarboxamide ribonucleotide transformylase), have been evaluated as a function of age in crude extracts from rat brain, liver, kidney, and spleen. The activities of the enzymes in brain are similar to those found in liver and kidney. In all tissues the activity of both enzymes was higher during early development, more than nine times above adult levels. In the CNS the enzymatic activities are apparently related to the periods of increased nucleic acid synthesis, with different activities being found in different regions during development. Our findings lend strong support to the suggestion that folic acid-dependent metabolism plays an important role during early development of the brain.     

Metabolic reorganization and signal transduction during estivation in the spadefoot toad

The maximal activities of 28 enzymes, representing multiple pathways of intermediary metabolism, were quantified in the brain, liver and skeletal muscle of spadefoot toads Scaphiopus couchii, comparing control toads with animals that had estivated for 2 months. Estivation-induced changes in brain enzyme activities were consistent with suppressed glycolysis and increased ketone body and amino acid catabolism. In liver, estivation resulted in reduced activities of eight enzymes representing carbohydrate, amino acid, ketone body and phosphagen metabolism, but the maximal activity of malic enzyme increased by 2.4-fold. Estivation led to a large-scale reorganization of skeletal muscle affecting most of the enzymes analyzed. Activities of enzymes of carbohydrate catabolism were generally elevated except for glycogen phosphorylase and hexokinase, whereas those of enzymes of fatty acid synthesis and ketone body metabolism were reduced. Increased glutamate dehydrogenase activities in both brain and muscle, as well as activities of other amino-acid-catabolizing enzymes in muscle, correlated with specific changes in the free amino acids pools in those tissues (reduced glutamine activity, increased glutamate, alanine and valine activities) that appear to be related to protein catabolism, for the purposes of elevating urea levels. The effects of estivation on signal transduction systems were also assessed. Total activities of protein kinases A and C (PKA and PKC) were largely unaltered in toad tissues during estivation (except for a 57% reduction in liver total PKC), but in seven organs there were strong reductions in the percentage of PKA present as the active catalytic subunit in estivating animals, and three contained a much lower percentage of membrane-bound active PKC during estivation. Activities of protein phosphatase types 1, 2A, 2B, and 2C were also frequently reduced during estivation. Overall, these results suggest that anuran estivation involves metabolic reorganization, including changing the maximal activities of key enzymes of intermediary metabolism as well as depressing the metabolic rate by suppressing signal transducing enzymes.     

Changes in Lipogenic Capacity and Activities of Ketolytic and Lipogenic Enzymes in Brain Regions of Developing Rats

Oxidation of ketone bodies (KBs) generates acetyl coenzyme A (AcCoA), which can be further incorporated into fatty acid. We have determined the rates of lipogenesis from ketone bodies in developing rats and their relation to the activities of enzymes involved in the production of cytoplasmic AcCoA via different pathways in brain regions. In the cerebrum (Cbr), rates of fatty acid synthesis from [3-14C]acetoacetate ([3-14C]AcAc) were high during the early postnatal period but decreased rapidly thereafter until weaning. Although similar developmental patterns of synthesis characterized the cerebellum (Cbl), midbrain (Mb), brain stem (Bs), and thalamus (Th), maximal rates were highest in the Cbr and lowest in the Th. In all regions, synthetic rates were higher throughout the entire suckling period than in adulthood. There were not appreciable differences in synthetic rates among brain regions of adult rats. The developmental changes in rates AcAc incorporation into fatty acids were closely related to AcAcCoA synthetase activity, but not to activities of ATP-citrate lyase or AcCoA synthetase. During the early postnatal stage enhanced rates of lipogenesis were accompanied by increased activities of AcAcCoA synthetase in all regions, with the highest activity occurring in the Cbr. The sequence of reactions coupling AcAcCoA synthetase and AcAcCoA thiolase in cytoplasm may be an important pathway for generation of AcCoA from KBs for fatty acid synthesis in all regions of the developing brain. This interpretation is strengthened by evidence of concomitant increases in the activities of fatty acid synthetase and AcCoA carboxylase.     

Ketone body utilization for lipogenesis in the perfused liver of the obese Zucker rat

The purpose of these studies was to determine if the utilization of ketone bodies as a carbon source for lipogenesis could account for the decreased ketone body production by livers of obese Zucker rats, as well as contribute to the enhanced rates of fatty acid synthesis observed in these animals. Ketone body production was decreased from 822 mumol/liver in the lean to 538 mumol/liver in the obese genotype (P less than 0.05). The incorporation of ketone bodies into fatty acids was significantly greater in the obese rat liver (lean, 1.95 mumol of ketone bodies/liver, versus obese, 35.22 mumol/liver; P less than 0.025). The relative contribution of this pathway to the overall rate of fatty acid synthesis was not affected by genotype and accounted for only 3 to 4% of the fatty acids synthesized. The incorporation of ketone bodies into digitonin precipitable sterols was similar in the two genotypes (lean, 4.5 mmol/liver, versus obese 4.7 mumol/liver; NS). This accounted for 9.2 and 6.3% of the total sterol synthesis in lean and obese rat livers, respectively. The total incorporation of ketone bodies into lipid was 7.5 mumols in the lean rat livers and 42.0 mumoles in the obese (P less than 0.025). The net increase was 35 mumoles incorporated, whereas the net decrease in ketogenesis was 284 mumoles. Thus, although ketone body carbon utilization for lipid synthesis was increased in the liver of the obese rats, this pathway could only account for a fraction of the genotypic difference in ketone body production and was of relatively minor importance as a source of carbon for hepatic fatty acid synthesis in both lean and obese rats.     

Novel aspects of the activities and subcellular distribution of enzymes of ketone body metabolism in the liver and kidney of the goldfish, Carassius auratus

The metabolic organization of ketone body metabolism of liver and kidney of the goldfish Carassius auratus was assessed by measuring maximal activities, subcellular distribution, and stereoisomer preference of ketone body enzymes. These determinations indicate that the organization of ketone body metabolism in liver and kidney of goldfish differs from that of mammals in some respects. All the enzymes of ketone body metabolism were present in liver and kidney of goldfish, with the exception of hydroxymethylglutaryl-CoA (HMG-CoA) synthetase, which was not detected in liver. Two forms of beta-hydroxybutyrate dehydrogenase (betaHBDH) with different stereospecificity for beta-hydroxybutyrate (D- and L-beta-hydroxybutyrate) were detectable in liver and kidney. All of the ketone body enzymes measured in liver were mainly in the mitochondrial fraction, with the exception of D- and L-betaHBDH, which were cytosolic. In kidney, HMG-CoA synthase, together with HMG-CoA lyase and acetoacetyl CoA thiolase (AcoAT), were found mainly in the mitochondrial fraction. L-betaHBDH was mainly cytosolic in kidney, but by contrast with liver, D-betaHBDH was mainly found in the mitochondria, and SKT was distributed in both the mitochondrial and cytosolic compartments. J. Exp. Zool. 286:434-439, 2000.     

Brain uptake and metabolism of ketone bodies in animal models

As a consequence of the high fat content of maternal milk, the brain metabolism of the suckling rat represents a model of naturally occurring ketosis. During the period of lactation, the rate of uptake and metabolism of the two ketone bodies, beta-hydroxybutyrate and acetoacetate is high. The ketone bodies enter the brain via monocarboxylate transporters whose expression and activity is much higher in the brain of the suckling than the mature rat. beta-Hydroxybutyrate and acetoacetate taken up by the brain are efficiently used as substrates for energy metabolism, and for amino acid and lipid biosynthesis, two pathways that are important for this period of active brain growth. Ketone bodies can represent about 30-70% of the total energy metabolism balance of the immature rat brain. The active metabolism of ketone bodies in the immature brain is related to the high activity of the enzymes of ketone body metabolism. Thus, the use of ketone bodies by the immature rodent brain serves to spare glucose for metabolic pathways that cannot be fulfilled by ketones such as the pentose phosphate pathway mainly. The latter pathway leads to the biosynthesis of ribose mandatory for DNA synthesis and NADPH which is not formed during ketone body metabolism and is a key cofactor in lipid biosynthesis. Finally, ketone bodies by serving mainly biosynthetic purposes spare glucose for the emergence of various functions such as audition, vision as well as more integrated and adapted behaviors whose appearance during brain maturation seems to critically relate upon active glucose supply and specific regional increased use.     

Lipid, ketone body and oxidative metabolism in the African lungfish, Protopterus dolloi following 60 days of fasting and aestivation

The potential importance of lipids and ketone bodies as fuels in the African lungfish, Protopterus dolloi, and the role of oxidative metabolism, were examined under control, fasted and aestivated conditions. In aestivating but not fasting lungfish, the activities of citrate synthase (CS) and cytochrome c oxidase (CCO) (enzymes of oxidative metabolism) showed tissue-specific changes. Significant reductions in CS activity occurred in the kidney, heart, gill and muscle, and in CCO in the liver and kidney tissues. Aestivation, but not fasting, also had a tissue-specific effect on mitochondrial state 3 respiration rates (using succinate as a substrate), with a >50% reduction in the liver, yet no change within muscle mitochondria. There is no indication that enzymes involved in lipid catabolism are up-regulated during periods of fasting or aestivation; however, both 3-hydroxyacyl CoA dehydrogenase (HOAD) and carnitine palmitoyl CoA transferase (CPT) activities were sustained in the liver despite the approximately 42% reduction in CCO activity, potentially indicating lipid metabolism is of importance during aestivation. Lungfish are able to utilize both the d- and l-stereoisomers of the ketone body beta-hydroxybutyrate (beta-HB); however, beta-HB does not appear to be an important fuel source during aestivation or fasting as no changes were observed in beta-HB tissue levels. This study demonstrates that an important aspect of metabolic depression during aestivation in lungfish is the tissue-specific down regulation of enzymes of aerobic metabolism while maintaining the activities of enzymes in pathways that supply substrates for aerobic metabolism.     

Streptozotocin diabetes increases mRNA expression of ketogenic enzymes in the rat heart

BackgroundDiabetic cardiomyopathy develops in insulin-dependent diabetic patients who have no hypertension, cardiac hypertrophy or vascular disease. Diabetes increases cardiac fatty acid oxidation, but cardiac hypertrophy limits fatty acid oxidation. Here we examined effects of diabetes on gene expression in rat hearts.MethodsWe used oligonucleotide microarrays to examine effects of insulindependent diabetes in the rat heart. RTQ PCR confirmed results of microarrays. Specific antibodies were used to examine changes in the mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase 2 (HMGCS2).ResultsA surprising result of diabetes was increased mRNA encoding all enzymes of the ketone body synthesis pathway. Increased mRNA expression for these enzymes was confirmed by RTQ PCR. The mRNA encoding HMGCS2, the rate-controlling enzyme, was 27 times greater in diabetic hearts. Total HMGCS2 protein increased 8-fold in diabetic hearts, but no difference was found in HMGCS2 protein in control vs. diabetic liver.ConclusionsInsulin-dependent diabetes induced the enzymes of ketone body synthesis in the heart, including HMGCS2, as well as increasing enzymes of fatty acid oxidation.General significanceThe mammalian heart does not export ketone bodies to other tissues, but rather is a major consumer of ketone bodies. Induction of HMGCS2, which is normally expressed only in the fetal and newborn heart, may indicate an adaptation by the heart to combat “metabolic inflexibility” by shifting the flux of excess intramitochondrial acetyl-CoA derived from elevated fatty acid oxidation into ketone bodies, liberating free CoA to balance the acetyl-CoA/CoA ratio in favor of increased glucose oxidation through the pyruvate dehydrogenase complex.     

Factors affecting concentrations of acetoacetate and d-3-hydroxybutyrate in haemolymph and tissues of the adult desert locust

The ketone bodies acetoacetate and d-3-hydroxybutyrate are found in the haemolymph, the fat body, and the flight muscles of the adult desert locust. Acetoacetate is the major ketone body in the haemolymph and the flight muscles, but in the fat body d-3-hydroxybutyrate usually predominates. The concentration of acetoacetate in the haemolymph varies with age, and increases during starvation and flight and also after the injection of corpus cardiacum homogenate; it is little affected by stress and there are no differences between the sexes. Ketone bodies appear to be formed in the fat body and are oxidized by the fat body, the flight muscles, and the testes. All the tissues oxidize acetoacetate much more readily than d-3-hydroxybutyrate, and the flight muscles of fed locusts oxidize acetoacetate much more readily than the fat body or the testes. In starved locusts the ability of the fat body and the flight muscles to oxidize ketone bodies is greatly reduced, but utilization by the testes remains normal. Thus the flight muscles appear to be the major consumers of ketone bodies in fed locusts, and the testes the major consumers in starved locusts. It is suggested that ketone bodies are formed in the fat body during the mobilization of the triglyceride lipid reserves, and are either oxidized by the fat body or transported by the haemolymph to the flight muscles and other tissues to be used as a respiratory fuel.     

Allosteric activation of brain hexokinase by magnesium ions and by magnesium-ion–adenosine triphosphate complex

1. The highest blood concentrations of ketone bodies were found at 5 days of age, after which time the concentration fell to reach the adult value by 30 days of age. 2. Both mitochondrial and cytoplasmic hydroxymethylglutaryl-CoA synthase activities were detected, with highest activities being found in the mitochondria at all stages of development. Activity of the mitochondrial enzyme increases rapidly immediately after birth, showing a maximum at 15 days of age, thereafter falling to adult values. The cytoplasmic enzyme, on the other hand, increased steadily in activity after birth to reach a maximum at 40 days of age, after which time activity fell to adult values. 3. Both mitochondrial and cytoplasmic aceto-acetyl-CoA thiolase activities were detected, with the mitochondrial enzyme having considerably higher activities at all stages of development. The developmental patterns for both enzymes were very similar to those for the corresponding hydroxymethylglutaryl-CoA synthases. 4. The activity of heart acetoacetyl-CoA transferase remains constant from late foetal life until the end of the suckling period, after which time there is a gradual threefold increase in activity to reach the adult values. The activity of brain 3-oxo acid CoA-transferase increases steadily after birth, reaching a maximum at 30 days of age, thereafter decreasing to adult values, which are similar to foetal activities. Although at all stages of development the specific activity of the heart enzyme is higher than that of brain, the total enzymic capacity of the brain is higher than that of the heart during the suckling period.     

Formation of free acetate by isolated perfused livers from normal, starved and diabetic rats

Isolated rat livers perfused in an open system exhibited a continous net release of free acetate. Upon intraportal infusion of hexanoate the net release of total ketone bodies and of free acetate increased significantly in livers from fed and 48 hours starved rats. The ratio ketone body production/acetate production during infusion of hexanoate was similar with livers from fed and starved rats. Livers from diabetic rats, however, did not only exhibit a higher rate of ketone body and acetate production, but also a significant decrease of the ratio ketone body production/acetate production. Intraportal infusion of oleate led also to an enhanced release of free acetate. An examination of the activities of 5 enzymes involved in ketone body and acetate metabolism showed no correlation with the higher rate of acetate production by diabetic livers.     

Urea synthesis in the liver and kidney of developing sheep

In order to establish if the urea found in foetal fluids in sheep could be of foetal origin and whether there are changes in the ability of ovine liver to synthesise urea during foetal and postnatal development, the rates of urea production from ammonium and bicarbonate ions have been measured in liver and kidney slices from animals aged from 50 days conceptual age to 16 weeks after birth, and in pregnant and non-pregnant ewes. The activities of five enzymes directly involved in the biosynthesis of urea have also been determined.Urea was found to be synthesised by foetal liver from at least 50 days conceptual age at rates similar to those observed in adult ewes. Highest rates of urea synthesis per unit weight of liver were found immediately after birth. In the liver there were significant positive correlations between the rates of urea synthesis by slices and the activities of carbomoyl phosphate synthase (ammonia) (EC 2.7.2.5), argininosuccinate synthetase (EC 6.3.4.5) and argininosuccinate lyase EC 4.3.2.1). Ornithine carbomoyl transferase (EC 2.1.3.3) activity was highest in the livers of ruminating animals. Hepatic arginase activity (EC 3.5.3.1) was highest during the late foetal life and in the mature foetuses the activity was ten-fold greated than that in maternal liver.Urea was not synthesised from ammonia and bicarbonate in kidney slices and neither ornithine carbomoyl transferase activity nor argininosuccinate synthetase activity could be detected. The activity of renal arginase was at least 70 times less than that found in the liver and the highest activity was found in ruminating lambs.The changes observed in the activities of the urea cycle enzymes during development have been contrasted with those reported to occur in other species. It is concluded that there is no single factor regulating the activities of the five enzymes directly concerned with urea synthesis during development. The results support the hypothesis that in mammals the ability of the liver to synthesise urea in foetal life is related to renal development.     

Changes in drug-metabolizing activities in the livers of suckling rats as a result of treatment of the lactating mothers with phenobarbitone and chlorpromazine

1. The activities in rat tissues of 3-oxo acid CoA-transferase (the first enzyme involved in acetoacetate utilization) were found to be highest in kidney and heart. In submaxillary and adrenal glands the activities were about one-quarter of those in kidney and heart. In brain it was about one-tenth and was less in lung, spleen, skeletal muscle and epididymal fat. No activity was detectable in liver. 2. The activities of acetoacetyl-CoA thiolase were found roughly to parallel those of the transferase except for liver and adrenal glands. The high activity in the latter two tissues may be explained by additional roles of thiolase, namely, the production of acetyl-CoA from fatty acids. 3. The activities of the two enzymes in tissues of mouse, gerbil, golden hamster, guinea pig and sheep were similar to those of rat tissues. The notable exception was the low activity of the transferase and thiolase in sheep heart and brain. 4. The activities of the transferase in rat tissues did not change appreciably in starvation, alloxan-diabetes or on fat-feeding, where the rates of ketone-body utilization are increased. Thiolase activity increased in kidney and heart on fat-feeding. 5. The activity of 3-hydroxybutyrate dehydrogenase did not change in rat brain during starvation. 6. The factors controlling the rate of ketone-body utilization are discussed. It is concluded that the activities of the relevant enzymes in the adult rat do not control the variations in the rate of ketone-body utilization that occur in starvation or alloxan-diabetes. The controlling factor in these situations is the concentration of the ketone bodies in plasma and tissues.     

Enzyme activities support the use of liver lipid-derived ketone bodies as aerobic fuels in muscle tissues of active sharks

Few data exist to test the hypothesis that elasmobranchs utilize ketone bodies rather than fatty acids for aerobic metabolism in muscle, especially in continuously swimming, pelagic sharks, which are expected to be more reliant on lipid fuel stores during periods between feeding bouts and due to their high aerobic metabolic rates. Therefore, to provide support for this hypothesis, biochemical indices of lipid metabolism were measured in the slow-twitch, oxidative (red) myotomal muscle, heart, and liver of several active shark species, including the endothermic shortfin mako, Isurus oxyrinchus. Tissues were assayed spectrophotometrically for indicator enzymes of fatty acid oxidation (3-hydroxy-o-acyl-CoA dehydrogenase), ketone-body catabolism (3-oxoacid-CoA transferase), and ketogenesis (hydroxy-methylglutaryl-CoA synthase). Red muscle and heart had high capacities for ketone utilization, low capacities for fatty acid oxidation, and undetectable levels of ketogenic enzymes. Liver demonstrated undetectable activities of ketone catabolic enzymes but high capacities for fatty acid oxidation and ketogenesis. Serum concentrations of the ketone beta-hydroxybutyrate varied interspecifically (means of 0.128-0.978 micromol mL(-1)) but were higher than levels previously reported for teleosts. These results are consistent with the hypothesis that aerobic metabolism in muscle tissue of active sharks utilizes ketone bodies, and not fatty acids, derived from liver lipid stores.