A radiochemical assay for acetoacetyl-CoA synthetase

HepatoNet1: a comprehensive metabolic reconstruction of the human hepatocyte for the analysis of liver physiology

We present HepatoNet1, the first reconstruction of a comprehensive metabolic network of the human hepatocyte that is shown to accomplish a large canon of known metabolic liver functions. The network comprises 777 metabolites in six intracellular and two extracellular compartments and 2539 reactions, including 1466 transport reactions. It is based on the manual evaluation of >1500 original scientific research publications to warrant a high-quality evidence-based model. The final network is the result of an iterative process of data compilation and rigorous computational testing of network functionality by means of constraint-based modeling techniques. Taking the hepatic detoxification of ammonia as an example, we show how the availability of nutrients and oxygen may modulate the interplay of various metabolic pathways to allow an efficient response of the liver to perturbations of the homeostasis of blood compounds.

Efficient synthesis of functional isoprenoids from acetoacetate through metabolic pathway-engineered Escherichia coli

We show here an efficient synthesis system of isoprenoids from acetoacetate as the main substrate. We expressed in Escherichia coli a Streptomyces mevalonate pathway gene cluster starting from HMG-CoA synthase and including isopentenyl diphosphate isomerase (idi) type 2 gene and the yeast idi type 1 and rat acetoacetate-CoA ligase (Aacl) genes. When the alpha-humulene synthase (ZSS1) gene of shampoo ginger was expressed in this transformant, the resultant E. coli produced 958 mug/mL culture of alpha-humulene with a lithium acetoacetate (LAA) supplement, which was a 13.6-fold increase compared with a control E. coli strain expressing only ZSS1. Next, we investigated if this E. coli strain engineered to utilize acetoacetate can synthesize carotenoids effectively. When the crtE, crtB, and crtI genes required for lycopene synthesis were expressed in the transformant, lycopene amounts reached 12.5 mg/g dry cell weight with addition of LAA, an 11.8-fold increase compared with a control expressing only the three crt genes. As for astaxanthin production with the E. coli transformant, in which the crtE, crtB, crtI, crtY, crtZ, and crtW genes were expressed, the total amount of carotenoids produced (astaxanthin, lycopene, and phytoene) was significantly increased to 7.5 times that of a control expressing only the six crt genes.

Fatty acid oxidation and ketogenesis by astrocytes in primary culture

The oxidation of the fatty acids octanoate and palmitate to CO2 and the ketone bodies acetoacetate and D-(-)-3-hydroxybutyrate was examined in astrocytes that were prepared from cortex of 2-day-old rat brain and grown in primary culture to confluence. Accumulation of acetoacetate (by mass) in the culture medium of astrocytes incubated with octanoate (0.3-0.5 mM) was 50-90 nmol C2 units h-1 mg of protein-1. A similar rate was obtained using radiolabeled tracer methodology with [1-14C]octanoate as labeled substrate. The results from the radiolabeled tracer studies using [1-14C]- and [7-14C]octanoate and [1-14C]-, [13-14C]-, and [15-14C]palmitate indicated that a substantial proportion of the omega-terminal four-carbon unit of these fatty acids bypassed the beta-ketothiolase step of the beta-oxidation pathway and the 3-hydroxy-3-methylglutaryl (HMG)-CoA cycle of the classic ketogenic pathway. The [14C]acetoacetate formed from the 1-14C-labeled fatty acids, obligated to pass through the acetyl-CoA pool, contained 50% of the label at carbon 3 and 50% at carbon 1. By contrast, the [14C]acetoacetate formed from (omega-1)-labeled fatty acids contained 90% of the label at carbon 3 and 10% at carbon 1, whereas that formed from the (omega-3)-labeled fatty acid contained 20% of the label at carbon 3 and 80% at carbon 1. These results indicate that acetoacetate is primarily formed either by the action of 3-oxo-acid-CoA transferase (EC 2.8.3.5) or acetoacetyl-CoA deacylase (EC 3.1.2.11) or both on acetoacetyl-CoA and not by the action of the mitochondrial HMG-CoA cycle involving HMG-CoA lyase (EC 4.1.3.4), which was readily detected, and HMG-CoA synthase (EC 4.1.3.5), which was barely measurable.

Milk-substitutes comparable to rat's milk; their preparation, composition and impact on development and metabolism in the artificially reared rat

1. Procedures are described to prepare nutritionally adequate rat milk-substitutes by modifying commercially available processed cow's milk, rich in carbohydrate and low in protein and fat compared with rat's milk. 2. Premilk formulas, prepared as intermediates in the preparation of rat milk-substitutes, are rich in protein but low in their concentration of fat, carbohydrate, and minerals when compared with rat's milk. 3. Premilks were supplemented with lactose, vitamins, minerals, fat as oil mixtures, certain amino acids and other constituents to yield rat milk-substitutes which resemble the known composition of rat's milk in their properties and composition. 4. Detailed analyses of the milk-substitutes show them to be comparable to rat's milk in energy content, pH, osmolarity, the concentration of the macronutrients, fat, protein and carbohydrate, and the major minerals. 5. Rat pups were artificially reared from postnatal day 4 or 5 until days 16-18 by fitting them with gastric cannulas through which the milk-substitutes could be infused automatically. 6. The nutritional impact of the milk-substitutes was assessed by a comparison of growth and metabolic characteristics for artificially reared rats with age-matched sucking rats reared by their mother. 7. Indices which were taken to be appropriate included (a) body-weight gain; (b) the concentration in blood of protein, amino acids, ketone bodies, carnitine, glucose, galactose, lactate, insulin, and the electrolytes calcium, sodium, potassium and chloride; (c) the turnover of glucose and 3-hydroxybutyrate; (d) the concentration in brain of protein, cholesterol, cerebroside sulphate and the activities of the enzymes pyruvate dehydrogenase (EC 1.2.4.1), 3-oxo-acid-CoA transferase (EC 2.8.3.5) and acetoacetyl-CoA ligase (EC 6.2.1.16). 8. The studies suggest that milk-substitutes approximating to rat's milk in composition promote acceptable metabolism in the artificially reared rat pup.

Capacity for substrate utilization in oxidative metabolism by neurons, astrocytes, and oligodendrocytes from developing brain in primary culture

Neuron, astrocyte, and oligodendrocyte cultures which were established from developing rat brain were examined for their utilization of glucose, ketone bodies, and free fatty acids by oxidative processes. 14CO2 production was measured in these cells from [1-14C] or [6-14C]glucose; [1-14C]octanoate and [1-14C], [6-14C], or [16-14C]palmitate; and [3-14C]acetoacetate and D(-)-3-hydroxy[3-14C]butyrate. Pyruvate dehydrogenase (EC 1.2.4.1.) and 3-oxoacid-CoA transferase (EC 2.8.3.5) activities were found at high levels in each of the cell populations. Astrocytes and oligodendrocytes produced much more 14CO2 from [1-14C]glucose than from [6-14C]glucose, indicating substantial hexose monophosphate shunt activity. This process was not as active in neurons. All three cell populations readily utilized the ketone bodies for oxidative metabolism at rates 7-9 times greater than they utilized glucose. Only astrocytes were able to utilize fatty acids for 14CO2 production, and the rate of utilization was greater than that of the ketone bodies. We found that the metabolic patterns of these brain cells which were derived from the developing brain complement the nature of the diet of the suckling animal which is rich in fat and low in carbohydrate. They readily utilized the ketone bodies or fatty acids and spared glucose for processes that metabolites of fat cannot fulfill.