Fuel metabolism metabolic fuels

Oxidation-reduction reactions always involve a pair of chemicals an electron donor, which is oxidized in the reactions, and an electron acceptor, which is reduced in the reaction. In fuel metabolism, the fuel donates electrons, and is oxidized, and NAD and FAD accept electrons, and are reduced. 

Many metabolic fuels are oxidized in the mitochondrial matrix. Pyruvate is oxidatively decarboxylated to acetyl-CoA by the pyruvate dehydrogenase complex (PDH)... 

In the liver, its major function is to ptovide glucose for extrahepatic tissues. In muscle, it setves mainly as a ready source of metabolic fuel fot use in muscle. 

A SUPPLY OF METABOLIC FUELS IS PROVIDED IN BOTH THE FED STARVING STATES (Figure 27-1)... 

In the Fed State, Metabolic Fuel Reserves Are Laid Down... 

For several hours after a meal, while the products of digestion are being absorbed, there is an abundant supply of metabolic fuels. Under these conditions, glucose is the major fuel for oxidation in most tissues this is observed as an increase in the respiratory quotient (the ratio of carbon dioxide produced to oxygen consumed) from about 0.8 in the starved state to near 1 (Table 27-1). 

Metabolic Fuel Reserves Are Mobilized in the Starving State... 

Figure 27-1. Metabolic interrelationships between adipose tissue, the liver, and extrahepatic tissues. In extrahepatic tissues such as heart, metabolic fuels are oxidized in the following order of preference (1) ketone bodies, (2) fatty acids, (3) glucose. (LPL, lipoprotein lipase FFA, free fatty acids VLDL, very low density lipoproteins.)...

Table 27-1. Energy yields, oxygen consumption, and carbon dioxide production in the oxidation of metabolic fuels.

In adipose tissue, the effect of the decrease in insulin and increase in glucagon results in inhibition of lipo-genesis, inactivation of lipoprotein lipase, and activation of hormone-sensitive lipase (Chapter 25). This leads to release of increased amounts of glycerol (a substrate for gluconeogenesis in the liver) and free fatty acids, which are used by skeletal muscle and liver as their preferred metabolic fuels, so sparing glucose. 

Table 27-2. Plasma concentrations of metabolic fuels (mmol/L) in the fed and starving states.

Caprio S et al Oxidative fuel metabolism during mild hypoglycemia critical role of free fatty acids. Am J Physiol... 

Besides water, the diet must provide metabolic fuels (mainly carbohydrates and lipids), protein (for growth and turnover of tissue proteins), fiber (for roughage), minerals (elements with specific metabolic functions), and vitamins and essential fatty acids (organic compounds needed in small amounts for essential metabolic and physiologic functions). The polysaccharides, tri-acylglycerols, and proteins that make up the bulk of the diet must be hydrolyzed to their constituent monosaccharides, fatty acids, and amino acids, respectively, before absorption and utilization. Minerals and vitamins must be released from the complex matrix of food before they can be absorbed and utifized. 

Cortisol is an important component of the body s response to physical and psychological stress. Nervous signals regarding stress are transmitted to the hypothalamus and the release of CRH is stimulated. The resulting increase in cortisol increases levels of glucose, free fatty acids, and amino acids in the blood, providing the metabolic fuels that enable the individual to cope with the stress. A potent inhibitor of this system is cortisol itself. This hormone exerts a negative-feedback effect on the hypothalamus and the adenohypophysis and inhibits the secretion of CRH and ACTH, respectively. 

Tabernero, A., Vicario, C. and Medina, J. M. Lactate spares glucose as a metabolic fuel in neurons and astrocytes from primary culture. Neurosci. Res. 26 369-376,1996. 

The classical endocrine system is composed of a series of glands that secrete hormones directly into the blood where they are carried to act on cells in the body often quite distant from the place of secretion. Insulin, for example, secreted from (3 pancreatic islet cells has actions on fuel metabolism in most tissues of the body. Compare this with a radio or television broadcast originating in one place but arriving at multiple sites. 

Where two enzymes compete for the same substrate, we expect to see some form of metabolic control and in this case the concentrations of NADH and acetyl-CoA are the key controlling factors (Figure 6.44). When glucose is not available as a fuel, metabolism switches to 3- oxidation of fatty acids, which generates more than sufficient quantities of both NADH and acetyl-CoA to drive the TCA cycle and to maintain oxidative phosphorylation. Pyruvate dehydrogenase activity is suppressed and pyruvate carboxylase is stimulated by ATP, NADH and acetyl-CoA (strictly speaking by low mitochondrial ratios of ADP/ATP, NAD+/NADH and coenzyme A/acetyl-CoA), so... 

---