Lipogenesis from ketone bodies in rat brain. Evidence for conversion of acetoacetate into acetyl-coenzyme A in the cytosol

Lipogenesis in the brain of suckling rats. Studies on the mechansim of mitochondrial-cytosolic carbon transfer

1. Studies on the incorporation of [3-(14)C]pyruvate and d-3-hydroxy[3-(14)C]butyrate into the brain lipid fraction by brain homogenates of the suckling (7-day-old) rat have been carried out. 2. Whereas approximately twice as much CO(2) was evolved from pyruvate compared with 3-hydroxybutyrate metabolism, similar amounts of the radioactivity of these two precursors were incorporated into the lipid fraction. Furthermore, in both cases the incorporation into lipid was almost tripled when glucose (10mm) or NADPH (2.5mm) was added to the incubation media. 3. If 5mm-(-)-hydroxycitrate, an ATP-citrate lyase inhibitor, was added to the incubation the incorporation of carbon from pyruvate was inhibited to 39% of the control and from 3-hydroxybutyrate to 73% of the control, whereas CO(2) production from both precursors was not affected. 4. The incorporation from pyruvate or 3-hydroxybutyrate into lipids was not affected by the presence of 10mm-glutamate in the medium (to encourage N-acetylaspartate production). However, incorporation from pyruvate was inhibited by 21% in the presence of 5mm-amino-oxyacetate (a transaminase inhibitor) and by 83% in the presence of both hydroxycitrate (5mm) and amino-oxyacetate. 5. Incorporation from 3-hydroxybutyrate into brain lipids was inhibited by 20% by amino-oxyacetate alone, but by 55% in the presence of both hydroxycitrate and amino-oxyacetate. 6. It is concluded that the mechanism of carbon transfer from pyruvate into lipids across the mitochondrial membrane in the suckling rat brain is mainly via citrate and N-acetylaspartate. 3-Hydroxybutyrate, in addition to using these routes, may also be incorporated via acetoacetate formation and transport to the cytosol.

Acetoacetyl CoA thiolase deficiency: a cause of severe ketoacidosis in infancy simulating salicylism

A female child presented at one year of age with a febrile illness and loose stools, then developed severe ketoacidosis with vomiting; an apparent salicylate level of 11 mg/dl was measured. A sibling had died in similar circumstances nine years earlier. Investigation revealed that the child did not have salicylate intoxication, and that high levels of acetoacetate in blood and urine were giving readings indicative of the presence of salicylate on routine testing. Gas-liquid chromatographic analysis combined with mass spectrometry on urine samples revealed the presence of 2-methyl-acetoacetate, 2-methyl-3-hydroxybutyrate, and tiglyl glycine in appreciable amounts, indicating a defect in isoleucine catabolism located at the beta-ketothiolase step. The oxidation of 14C-isoleucine to CO2 in cultured fibroblasts confirmed that this pathway was defective. We present evidence that beta-ketothiolase deficiency is not simply a defect of isoleucine degradation; the deficient enzyme is the K+ dependent short-chain mitochondrial thiolase, which also plays a major catalytic role in ketone body and fatty acid oxidation.

Long and medium chain triglycerides increase plasma concentrations of ketone bodies in suckling rats

The potential of medium chain triglyceride (MCT) and long chain triglyceride (LCT) as sources of plasma ketones was investigated in suckling rats. Initially high concentrations of plasma ketones in 6-, 10, and 17-day-old rats increased 2- to 3-fold after acute feeding of MCT. This feeding had the same effect in fed or fasted adult rats. Corn oil (as a source of LCT) induced a large increase in the plasma ketone concentration of suckling rats and a relatively small but significant increase in fasted adult rats. The LCT treatment did not affect plasma ketone levels in fed adult rats. The results show clearly that feeding either LCT or MCT will enhance hyperketonemia in suckling rats. In the livers of all animals, regardless of age, the capacity for incorporation of [1(-14C)]octanoate into CO2 and acetoacetate far exceeded that for [1(-14C)]palmitate. The hyperketonemic action of LCT in suckling rats was accompanied by an increased activity of carnitine palmityltransferase and increased level of carnitine.

Control of glucose metabolism in isolated acini of the lactating mammary gland of the rat. The ability of glycerol to mimic some of the effects of insulin

Inhibition of glucose uptake by acetoacetate and relief of this inhibition by insulin found previously in slices of rat mammary gland [Williamson, McKeown & Ilic (1975) Biochem. J. 150. 145-152] was confirmed in acini, which represent a more homogeneous population of cells. Glycerol (1mM) behaved like insulin (50 minuits/ml) in its ability to relieve the inhibition of glucose (5 mM) utilization caused by acetoacetate (2 mM) in acini. Both glycerol and insulin reversed the increase in [citrate] and the decrease in [glycerol 3-phosphate] and the [lactate]/[pyruvate] ratio in the presence of acetoacetate. Lipogenesis from 3H2O, [3-14C] acetoacetate, [1-14C]- and [6-14C]-glucose was stimulated, whereas 14CO2 formation from [3-14C]acetoacetate was decreased. Neither insulin nor glycerol relieved the acetoacetate inhibition of glucose uptake when lipogenesis was inhibited by 5-(tetradecyloxy)-2-furoic acid. From measurements of [3-14C]acetoacetate incorporation into lipid in the various situations it is suggested that a cytosolic pathway for acetoacetate utilization may exist in rat mammary gland. In the absence of acetoacetate, glycerol inhibited glucose utilization by 60% and increased both [glycerol 3-phosphate] and the [lactate/[pyruvate] ratio. Possible ways in which glycerol may mimic the effects of insulin are discussed.