Pathways of acetyl CoA production for lipogenesis from acetoacetate, beta-hydroxybutyrate, pyruvate and glucose in neonatal rat lung

The lipogenic enzyme acetoacetyl-CoA synthetase and ketone body utilization for denovo lipid synthesis, a review

Acetoacetyl-CoA synthetase (AACS) is the key enzyme in the anabolic utilization of ketone bodies (KBs) for denovo lipid synthesis, a process that bypasses citrate and ATP citrate lyase. This review shows that AACS is a highly regulated, cytosolic, and lipogenic enzyme and that many tissues can readily use KBs for denovo lipid synthesis. AACS has a low micromolar Km for acetoacetate, and supply of acetoacetate should not limit its activity in the fed state. In many tissues, AACS appears to be regulated in conjunction with the need for cholesterol, but in adipose tissue, it seems tied to fatty acid synthesis. KBs are readily utilized as substrates for lipid synthesis in lipogenic tissues, including liver, adipose tissue, lactating mammary gland, skin, intestinal mucosa, adrenals, and developing brain. In numerous studied cases, KBs served several-fold better than glucose as substrates for lipid synthesis, and when present, KBs suppressed the utilization of glucose for lipid synthesis. Here, it is hypothesized that a physiological role for the utilization of KBs for lipid synthesis is a metabolic process of lipid interconversion. Fatty acids are converted to KBs in liver, and then, the KBs are utilized to synthesize cholesterol and other long-chain fatty acids in liver and nonhepatic tissues. The conversion of fatty acids to cholesterol via the KBs may be a particularly important example of lipid interconversion. Utilizing KBs for lipid synthesis is glucose sparing and probably is important with low carbohydrate diets. Metabolic situations and tissues where this pathway may be important are discussed.

Chemistry and biochemistry of (-)-hydroxycitric acid from Garcinia

(-)-Hydroxycitric acid [(-)-HCA] is the principal acid of fruit rinds of Garcinia cambogia, Garcinia indica, and Garcinia atroviridis. (-)-HCA was shown to be a potent inhibitor of ATP citrate lyase (EC 4.1.3.8), which catalyzes the extramitochondrial cleavage of citrate to oxaloacetate and acetyl-CoA: citrate + ATP + CoA --> acetyl-CoA + ADP + P(i) + oxaloacetate. The inhibition of this reaction limits the availability of acetyl-CoA units required for fatty acid synthesis and lipogenesis during a lipogenic diet, that is, a diet high in carbohydrates. Extensive animal studies indicated that (-)-HCA suppresses the fatty acid synthesis, lipogenesis, food intake, and induced weight loss. In vitro studies revealed the inhibitions of fatty acid synthesis and lipogenesis from various precursors. However, a few clinical studies have shown controversial findings. This review explores the literature on a number of topics: the source of (-)-HCA; the discovery of (-)-HCA; the isolation, stereochemistry, properties, methods of estimation, and derivatives of (-)-HCA; and its biochemistry, which includes inhibition of the citrate cleavage enzyme, effects on fatty acid synthesis and lipogenesis, effects on ketogenesis, other biological effects, possible modes of action on the reduction of food intake, promotion of glycogenesis, gluconeogenesis, and lipid oxidation, (-)-HCA as weight-controlling agent, and some possible concerns about (-)-HCA, which provides a coherent presentation of scattered literature on (-)-HCA and its plausible mechanism of action and is provocative of further research.

Pulmonary surfactant lipid synthesis from ketone bodies, lactate and glucose in newborn rats

The contribution of acetoacetate (AcAc), beta-hydroxybutyrate (beta OHB), lactate and glucose to pulmonary surfactant lipid synthesis in three- to five-day-old rats was measured. Minced lung tissue was incubated with 3H2O and [3-14C]AcAc, [3-14C]beta OHB, [U-14C]lactate or [U-14C]glucose, and the radioactivity incorporated into surfactant lipids was measured. When expressed as nmol of substrate incorporated/g lung tissue per four hr, lactate was incorporated more rapidly than other substrates into total surfactant lipids and phosphatidylcholine (PC). There was no difference in the rates of incorporation of lactate, AcAc or glucose into disaturated PC (DSPC). Substrates other than glucose were incorporated almost exclusively into fatty acids, whereas 60-80% of glucose incorporated into surfactant phospholipids was found in fatty acids, with the remaining in glyceride-glycerol. When expressed as nmol acetyl units incorporated/g lung tissue per four hr, the rates of AcAc, lactate and glucose incorporation into total surfactant fatty acids were comparable. Glucose incorporation into DSPC and PC was greater than that of AcAc and lactate. When glucose was the only exogenous substrate added to the incubation medium, it contributed 37% of total surfactant fatty acids synthesized de novo. In the presence of other substrates, the contribution of glucose to de novo fatty acid synthesis dropped to 14-20%. In the presence of unlabeled glucose, 14C-labeled AcAc, lactate and beta OHB contributed 52%, 40% and 19%, respectively, of the total fatty acids synthesized de novo. The rate of beta OHB incorporation into surfactant lipids was only about 50% that of other substrates and was accompanied by low activity of beta-hydroxybutyrate dehydrogenase measured for newborn lung.(ABSTRACT TRUNCATED AT 250 WORDS)