Factors involved in the control of the activity of enzymes of hepatic ketogenesis during development of the rat

Enantioselective determination of 3-hydroxybutyrate in the tissues of normal and streptozotocin-induced diabetic rats of different ages

L-3-Hydroxybutyrate (3HB) and D-3HB are enantiomers that exist in various rat tissues, and the ratio of the 2 compounds is of importance since it may affect glucose utilization in cardiomyocytes. In this study, we determined the concentrations of L-3HB and D-3HB in the tissues of normal and streptozotocin (STZ)-induced diabetic rats of different ages by column-switching high-performance liquid chromatography using a fluorescence detection system. In normal rats, the levels of L-3HB peaked at 8 weeks of age in the cerebrum, liver, spleen, lung, kidney, adrenal gland, and heart and then decreased afterwards. The concentrations of L-3HB were the highest in the heart, with 26.24±13.74 μmol/mg protein. In addition, there was an increase in the levels of (D+L)-3HB, D-3HB, and L-3HB in the tissues of diabetic rats with time, whereas the ratios of L-3HB to (D+L)-3HB declined (46.44% vs. 21.03%, P<0.05, in heart tissue after 24 weeks of STZ treatment). Both the concentration and the ratio of L-3HB may be associated with disease conditions, and the determination of L-3HB may help clarify the role of L-3HB under physiological and pathological conditions.

Pathways and control of ketone body metabolism: on the fringe of lipid biochemistry

Ketone bodies become major body fuels during fasting and consumption of a high-fat, low-carbohydrate (ketogenic) diet. Hyperketonemia is associated with potential health benefits. Ketone body synthesis (ketogenesis) is the last recognizable step of lipid energy metabolism, a pathway that links dietary lipids and adipose triglycerides to the Krebs cycle and respiratory chain and has three highly regulated control points: (1) adipocyte lipolysis, (2) mitochondrial fatty acids entry, controlled by the inhibition of carnitine palmityl transferase I by malonyl coenzyme A (CoA) and (3) mitochondrial 3-hydroxy-3-methylglutaryl CoA synthase, which catalyzes the irreversible first step of ketone body synthesis. Each step is suppressed by an elevated circulating insulin level or insulin/glucagon ratio. The utilization of ketone bodies (ketolysis) also determines circulating ketone body levels. Consideration of ketone body metabolism reveals the mechanisms underlying the extreme fragility of dietary ketosis to carbohydrate intake and highlights areas for further study.

Neonatal ketosis is not rare: experience of neonatal screening using gas chromatography-mass spectrometry

The causes and effects of transient neonatal ketosis, discovered during a pilot study of screening for abnormalities in neonatal metabolism using gas chromatography-mass spectrometry, were investigated. Of the 21,342 neonates that were screened, 47 had significant ketosis. The organic acid profile accompanying ketosis in the urine of neonates followed the pattern of ketotic dicarboxylic aciduria in approximately half of the cases. Ketosis was more often found in neonates nourished by breast feeding (33 out of 47). Over half of the neonates showing ketosis (28 out of 47) were asymptomatic. When normal neonates and neonates testing positive for ketosis were compared, no statistically significant correlations were found with regard to birth mass, gestational period, or gender. However, neonates with ketosis tended to have low mass gain rates in the 5 days from birth and a statistically significant difference was found in this regard in comparison to normal neonates (P<0.0001). From the above results, development of ketosis in neonates was found to be possible even in normal subjects. Most ketosis in neonates was also found to depend largely on nourishment after birth. Existence of an asymptomatic ketosis category was also suggested.

Control of hepatic mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase during the foetal/neonatal transition, suckling and weaning in the rat

(1) We assayed active and total (i.e. active plus succinylated) 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase in mitochondria isolated from foetal, neonatal, suckling or weaned rats. (2) HMG-CoA synthase was substantially succinylated and inactivated in mitochondria isolated from term-foetal, (1-h-old, 6-h-old, 1-day-old) neonatal, suckling and high carbohydrate/low-fat (hc)-weaned rats. Succinylation of HMG-CoA synthase was very low in mitochondria isolated from the livers of foetal, 30-min-old neonatal and high-fat/carbohydrate-free (hf)-weaned rats. (3) There was a negative correlation between active HMG-CoA synthase and succinyl-CoA content in mitochondria isolated from term-foetal, suckling and hc-weaned rats. (4) Differences in active enzyme could not be entirely accounted for by differences in succinylation and inactivation of the synthase. Immunoassay confirmed that the absolute amounts of mitochondrial HMG-CoA synthase increased during the foetal/neonatal transition and decreased with hc weaning. The levels remained elevated with hf weaning. (5) From these data we propose that mitochondrial HMG-CoA synthase is controlled by two different mechanisms in young rats. Regulation by succinylation provides a mechanism for rapid modification of existing enzyme in response to changing metabolic states. Changes in the absolute amounts of HMG-CoA synthase provide a more long-term control in response to nutritional changes.

Intramitochondrial factors controlling hepatic fatty acid oxidation at weaning in the rat

Fatty acid oxidation was studied in isolated liver mitochondria of rats during the suckling-weaning transition. The oxidation rate of oleyl-CoA and palmitoylcarnitine was reduced 2.5-fold in rats weaned on a high-carbohydrate diet compared to suckling rats, when acetyl-CoA produced by beta-oxidation was directed towards ketone-body synthesis. Weaning on a high-fat diet minimized this change. Channeling of acetyl-CoA towards citrate synthesis doubled the oxidation rate of both substrates in HC-weaned rats. Thus, in addition to changes in carnitine palmitoyltransferase I activity, the beta-hydroxymethylglutaryl-CoA synthase pathway is also involved in the decreased fatty acid oxidation at weaning. This was confirmed by measurement of beta-hydroxymethylglutaryl-CoA synthase pathway activity.

Evidence that the development of hepatic fatty acid oxidation at birth in the rat is concomitant with an increased intramitochondrial CoA concentration

The development of hepatic fatty acid oxidation during the perinatal period in the rat was studied using isolated mitochondria. Ketone body synthesis from substrates entering at different levels of beta-oxidation was 2-3 times lower in mitochondria isolated from term-fetal liver than in 16-h-old newborn or adult liver mitochondria. The low rate of palmitoyl-L-carnitine oxidation in term-fetal mitochondria was linked neither to the low capacity of the respiratory chain nor to the removal of acetyl-CoA in the hydroxymethylglutaryl-CoA synthase pathway. The 2.5-times lower concentration of CoA found in term-fetal liver mitochondria when compared to 16-h-old or adult liver mitochondria might be the factor responsible for the low rate of fatty acid oxidation in term-fetal liver mitochondria.