Enzymatic changes associated with ketosis in long standing diabetes and prolonged starvation of rats

Physical training reverses defect in 3-ketoacid CoA-transferase activity in skeletal muscle of diabetic rats

To investigate one potential mechanism whereby physical training improves the plasma concentration of ketone bodies in experimental diabetes mellitus, we measured the activity of 3-ketoacid CoA-transferase, the key enzyme in the peripheral utilization of ketone bodies. Diabetes was induced with streptozotocin (50 mg/kg) and training carried out on a treadmill with a progressive 10-wk program. Diabetes resulted in an increase (P < 0.001) in plasma concentration of beta-hydroxybutyric acid in sedentary rats, which was partly reversed by training (P < 0.001). Diabetes was also associated with a decreased activity of 3-ketoacid CoA-transferase in gastrocnemius muscle. When expressed per total gastrocnemius, training increased the activity of 3-ketoacid CoA-transferase by 66% in nondiabetic rats (P < 0.001) and by 150% in diabetic rats (P < 0.001), the decrease present in diabetic rats being fully reversed by training. Simple linear regression between the log of 3-ketoacid CoA-transferase activity and the log of plasma beta-hydroxybutyric acid levels showed a statistically significant (r = 0.563, P < 0.001) negative correlation. The beneficial effects of training on plasma ketone bodies in diabetic rats are probably explained, at least in part, by an increase in ketone body utilization, mediated by an increase in skeletal muscle 3-ketoacid CoA-transferase activity.

Decreased rate of ketone-body oxidation and decreased activity of D-3-hydroxybutyrate dehydrogenase and succinyl-CoA:3-oxo-acid CoA-transferase in heart mitochondria of diabetic rats

Heart mitochondria from chronically diabetic rats ('diabetic mitochondria'), in metabolic State 3, oxidized 3-hydroxybutyrate and acetoacetate at a relatively slow rate, as compared with mitochondria from normal rats ('normal mitochondria'). No significant differences were observed, however, with pyruvate or L-glutamate plus L-malate as substrates. Diabetic mitochondria also showed decreased 3-hydroxybutyrate dehydrogenase and succinyl-CoA: 3-oxoacid CoA-transferase activities, but cytochrome content and NADH-dehydrogenase, succinate dehydrogenase, cytochrome oxidase and acetoacetyl-CoA thiolase activities proved normal. The decrease of 3-hydroxybutyrate dehydrogenase activity was observed in diabetic mitochondria subjected to different disruption procedures, namely freeze-thawing, sonication or hypoosmotic treatment, between pH 7.5 and 8.5, at temperatures in the range 6-36 degrees C, and in the presence of L-cysteine. Determination of the kinetic parameters of the enzyme reaction in diabetic mitochondria revealed diminution of maximal velocity (Vmax) as its outstanding feature. The decrease in 3-hydroxybutyrate dehydrogenase in diabetic mitochondria was a slow-developing effect, which reached full expression 2-3 months after the onset of diabetes; 1 week after onset, no significant difference between enzyme activity in diabetic and normal mitochondria could be established. Insulin administration to chronically diabetic rats for 2 weeks resulted in limited recovery of enzyme activity. G.l.c. analysis of fatty acid composition and measurement of diphenylhexatriene fluorescence anisotropy failed to reveal significant differences between diabetic and normal mitochondria. The Arrhenius-plot characteristics for 3-hydroxybutyrate dehydrogenase in membranes of diabetic and normal mitochondria were similar. It is assumed that the variation of the assayed enzymes in diabetic mitochondria results from a slow adaptation to the metabolic conditions resulting from diabetes, rather than to insulin deficiency itself.

Regulation of succinyl-CoA:3-oxoacid CoA-transferase in developing rat brain: responsiveness associated with prenatal but not postnatal hyperketonemia

Activities of ketone body-metabolizing enzymes in rat brain rise 3- to 5-fold during the suckling period, then fall more than 50% after weaning. Our purpose was to determine the mechanism of the developmental changes in activity of 3-oxoacid CoA-transferase in rat brain and to study its regulation by dietary modification. Purified rat brain 3-oxoacid CoA-transferase was used to generate specific antibody. Immunotitrations of the enzyme from brains of 4-, 24-, and 90-day-old rats indicated that changes in 3-oxoacid CoA-transferase activity during development are due to changes in content of the enzyme protein. Pulse-labeling studies showed that changes in enzyme specific activity reflected changes in its relative rate of synthesis, which increased 2.5-fold between the nineteenth day of gestation and the third postnatal day, remained at this high level until the twelfth postnatal day, and declined thereafter, returning by Day 38 to the level observed in utero. The enzyme is apparently degraded very slowly during early postnatal life. Fetal hyperketonemia induced by feeding pregnant rats a high-fat diet was associated with an increase in the relative rate of synthesis of 3-oxoacid CoA-transferase in brains of 19-day-old fetuses and newborn rats and with an increase in the specific activity of the enzyme at birth. To examine the role of postnatal hyperketonemia in the development of the enzyme in brains of suckling rats, neonates received intragastric cannulas and were fed, for up to 13 days, a modified milk formula low in fat. Postnatal hyperketonemia was abolished but cerebral 3-oxoacid CoA-transferase specific activity on Days 10 and 17 was not significantly affected. Thus, the physiological hyperketonemia caused by the high fat content of rat milk is not required for the normal development of 3-oxoacid CoA-transferase in rat brain.

Turnover of succinyl-CoA:3-oxoacid CoA-transferase in glioma and neuroblastoma cells. Specific influence of acetoacetate in neuroblastoma cells

The specific activity of succinyl-CoA:3-oxo-acid CoA-transferase (3-oxoacid CoA-transferase, EC 2.8.3.5) increases significantly during growth in culture in both mouse neuroblastoma N2a and rat glioma C6 cells. To investigate the mechanism(s) responsible for this, antibody specific for rat brain 3-oxoacid CoA-transferase was raised in rabbits. Immunotitrations of 3-oxoacid CoA-transferase from neuroblastoma and glioma cells on days 3 and 7 of growth after subculture showed that the ratio of 3-oxoacid CoA-transferase activity to immunoprecipitable enzyme protein remained constant, indicating that differences in specific activity of the enzyme at these times in both cell types reflect differences in concentration of enzyme protein. In glioma cells, the relative rate of 3-oxoacid CoA-transferase synthesis was about 0.04-0.05% throughout 9 days in culture. In contrast, the relative rate of synthesis of 3-oxo-acid CoA-transferase in neuroblastoma cells was about 0.07-0.08% on days 3, 5 and 7 after subculture, but fell to 0.052% on day 9. The degradation rates of total cellular protein (t1/2 = 28 h) and 3-oxoacid CoA-transferase (t1/2 = 46-50 h) were similar in both cell lines. The rise in specific activity of the enzyme in both cell lines from days 3 to 7 without a significant increase in the relative rate of synthesis reflects a slow approach to steady-state conditions for the enzyme secondary to its slow degradation. Differences in 3-oxoacid CoA-transferase specific activity between the two cell lines are apparently due to a difference of about 60% in relative rates of enzyme synthesis. The presence of 0.5 mM-acetoacetate in the medium significantly increased the specific activity of 3-oxoacid CoA-transferase in neuroblastoma cells during the early exponential growth phase. This treatment increased the relative rate of synthesis of 3-oxoacid CoA-transferase by 23% (P less than 0.025) in these cells on day 3, suggesting that substrate-mediated induction of enzyme synthesis is a mechanism of regulation of 3-oxoacid CoA-transferase.

Activities of 3-oxo acid-CoA transferase and acetoacetyl-CoA synthetase in brain and liver of the developing chick

The 3-oxo acid-CoA transferase and acetoacetyl-CoA synthetase activities were estimated in subcellular fractions isolated from the cerebral hemispheres, the optic lobes, the cerebellum and the liver of the chick between the twentieth day of embryonic life and the thirtieth day of postnatal development. In the 3 areas of chick brain, the transferase activity increases during the whole postnatal development. Transferase is located both in mitochondria and microsomes unlike in the rat brain where this enzyme is specifically mitochondrial. There is no detectable activity of synthetase in the chick brain. These differences in enzyme localization imply that acetoacetate is converted into acetoacetyl-CoA only by 3-oxo acid-CoA transferase in the mitochondria and cytoplasm of the chick brain, whereas, in the rat brain, this reaction is performed by 3-oxo acid-CoA transferase in mitochondria and by acetoacetyl-CoA synthetase in the cytosol. There is no detectable activity either for transferase or for synthetase in the chick liver.

[Effects of parenteral infusion of glycerol on glycerol kinase and adenosintriphosphate in rat kidneys (author's transl)]

Effects of intravenous glycerol infusions on glycerol kinase and adenosintriphosphate in rat kidneys. Intravenous infusions of glycerol with rates near and above the average maximal turnover capacity (0,74 g.kg-1.h-1) cause alterations of the kidneys such as weight increase, decrease of protein content and activity of glycerol kinase with excretion of this enzyme in the urine. At infusion rates of 0,6 g.kg-1.h-1 and more a decrease of the ATP-content of the kidneys is observed. Effects on the liver are not consistent and only demonstrable at infusion rates above the maximal turnover rate of glycerol. The results are discussed with regard to osmotic nephrosis. The appearance of glycerol kinase in the urine is discussed as an early symptom of kidney damage.

Comparative studies on 3-oxo acid coenzyme A transferase from various rat tissues

1. Tissue activities, intracellular distribution as well as selected kinetic and molecular properties of succinyl-CoA-3-oxo acid CoA transferase (EC 2.8.3.5), which is an initiator of ketone body usage, were examined in rat kidney, heart, brain, skeletal muscle and liver. 2. The activities of the transferase in these tissues are similar to reported values and are somewhat affected by the homogenization medium. Higher recoveries of activity are obtained when a phosphate buffer is used during the homogenization; Tris solutions containing sucrose and mannitol lead to only slightly lower recoveries, but can be used in studies to determine the subcellular localization of the transferase activity. 3. A close correlation was observed between the relative activities of citrate synthase (a mitochondrial marker enzyme) and CoA transferase in the cytoplasmic, particulate and mitochondrial fractions from the five tissues. 4. The K(m) values for acetoacetate (measured in two different ways), the ratio of V(max.) values for the two enzyme-catalysed half-reactions, and succinate product inhibition are quite similar for the enzyme from each tissue. 5. The enzymes are also similar in molecular weight (with an approx. mol.wt. of 100000 as determined by gel filtration). All show an active band in isoelectric-focusing studies with pI 7.6, except for the enzyme from heart (pI 6.8). 6. The results demonstrate a mitochondrial origin for CoA transferase in these rat tissues and support the proposition that CoA transferase is a ketolytic enzyme, i.e. an enzyme uniquely involved in the complete oxidation of ketone bodies. The structural and functional similarities of these transferases suggest that factors other than differences in K(m) values account for differences in the utilization of ketone bodies by various tissues.