Fatty acids, binding protein oxidation

Schnurr et al. [22] showed that rabbit 15-LOX oxidized beef heart submitochondrial particles to form phospholipid-bound hydroperoxy- and keto-polyenoic fatty acids and induced the oxidative modification of membrane proteins. It was also found that the total oxygen uptake significantly exceeded the formation of oxygenated polyenoic acids supposedly due to the formation of hydroxyl radicals by the reaction of ubiquinone with lipid 15-LOX-derived hydroperoxides. However, it is impossible to agree with this proposal because it is known for a long time [23] that quinones cannot catalyze the formation of hydroxyl radicals by the Fenton reaction. Oxidation of intracellular unsaturated acids (for example, linoleic and arachidonic acids) by lipoxygenases can be suppressed by fatty acid binding proteins [24]. 

Fatty acid utilized by muscle may arise from storage triglycerides from either adipose tissue depot or from lipid stores within the muscle itself. Lipolysis of adipose triglyceride in response to hormonal stimulation liberates free fatty acids (see Section 9.6.2) which are transported through the bloodstream to the muscle bound to albumin. Because the enzymes of fatty acid oxidation are located within subcellular organelles (peroxisomes and mitochondria), there is also need for transport of the fatty acid within the muscle cell this is achieved by fatty acid binding proteins (FABPs). Finally, the fatty acid molecules must be translocated across the mitochondrial membranes into the matrix where their catabolism occurs. To achieve this transfer, the fatty acids must first be activated by formation of a coenzyme A derivative, fatty acyl CoA, in a reaction catalysed by acyl CoA synthetase. 

Fate of fatty acids The free (unesterified) fatty acids move through the cell membrane of the adipocyte, and immediately bind to albumin in the plasma. They are transported to the tis sues, where the fatty acids enter cells, get activated to their CtA derivatives, and are oxidized for energy. [Note Active transport of fatty acids across membranes is mediated by a membrane fatty acid binding protein.] Regardless of their blood levels, plasma free fatty acids cannot be used for fuel by erythrocytes, which have no mitochondria, or by the brain because of the imperme able blood-brain barrier. rr f-... 

Two immunosensors developed by O Regan et al. [89,90] have demonstrated their usefulness for the early assessment of acute myocardial infarction (AMI). Human heart fatty-acid binding protein (H-FABP) is a biochemical marker for the early assessment of AMI. The authors constructed an amperometric immunosensor for the rapid detection of H-FABP in whole blood. The sensor is based on a one-step, direct sandwich assay in which the analyte and an alkaline phosphatase (AP) labelled antibody are simultaneously added to the immobilized primary antibody, using two distinct monoclonal mouse anti-human H-FABP antibodies. The substrate p-amino-phenyl phosphate is converted to p-aminophenol by AP, and the current generated by its subsequent oxidation at +300 mV vs. Ag/AgCl is measured. The total assay time is 50 min, and the standard curve was linear between 4 and 250 ng ml . The intra- and inter-assay coefficients of variation were below 9%. No cross-reactivity of the antibodies was found with other early cardiac markers, and endogenous substances in whole blood did not have an... 

Fatty acids are more efficiently degraded because of increased synthesis of molecules that facilitate fatty acid transport and oxidation. The concentration of citric acid cycle and (3-oxidation enzymes increases, as well as the components of the ETC. In addition, the capacity of the muscle cell to remove fatty acids from blood and to transport them into mitochondria increases. For example, increases in the synthesis of fatty acid transporter proteins and fatty acid-binding proteins, as well as carnitine and carnitine acyltransferase, have been observed. 

Kaikaus, R.M., W.K. Chan, N. Lysenko, R. Ray, P.R. Ortiz de Montellano, and N.M. Bass (1993). Induction of peroxisomal fatty acid 3-oxidation and liver fatty acid-binding protein by peroxisome proliferators Mediation via the cytochrome P450IVA1 ft)-hydroxylase pathway.,/ Biol. Chem. 268, 9593-9603. 

The liver is the main origin of ketones in laboratory animals, where the long chain fatty acids are released from plasma albumin and bound to fatty acid-binding proteins in the hepatocytes. The long chain fatty acids react with CoA and then can be used to synthesize triacylglycerol or undergo beta-oxidation to acetyl CoA. When the levels of plasma fatty acids are elevated, acetyl CoA can be metabolized to form acetoacetate and 3-hydroxybutyrate or enter the tricarboxylic acid cycle. In ketosis, the levels of acetone, acetoacetate, and 3-hydroxybutyrate (also known as beta-hydroxybutyrate) are increased in both plasma and urine these three compounds historically were collectively called ketone bodies. Urine test strips can be used to test for ketonuria, and there are several enzymatic assays for 3-hydroxybutyrate and acetoacetate. 

Fig. 23.1. Overview of mitochondrial long-chain fatty acid metabolism. (1) Fatty acid binding proteins (FaBP) transport fatty acids across the plasma membrane and bind them in the cytosol. (2) Fatty acyl CoA synthetase activates fatty acids to fatly acyl CoAs. (3) Carnitine transports the activated fatty acyl group into mitochondria. (4) p-oxidation generates NADH, FAD(2H), and acetyl CoA (5) In the liver, acetyl CoA is converted to ketone bodies...

Within the liver, they bind to fatty acid-binding proteins and are then activated on the outer mitochondrial membrane, the peroxisomal membrane, and the smooth endoplasmic reticulum by fatty acyl CoA synthetases. The fatty acyl group is transferred from CoA to carnitine for transport through the inner mitochondrial membrane, where it is reconverted back into fatty acyl CoA and oxidized to acetyl CoA in the (3-oxidation spiral (see Chapter 23). 

Fatty acid uptake by muscle requires the participation of fatty acid-binding proteins and the usual enzymes of fatty acid oxidation. Fatty acyl-CoA uptake into the mitochondria is controlled by malonyl-CoA, which is produced by an isozyme of acetyl-coA carboxylase (ACC-2 the ACC-1 isozyme is found in liver and adipose tissue and is used for fatty acid biosynthesis). ACC-2 is inhibited by phosphorylation by the AMP-activated protein kinase (AMP-PK) such that when energy levels are low the levels of malonyl CoA will drop, allowing fatty acid oxidation by the mitochondria. In addition, muscle cells also contain the enzyme malonyl CoA decarboxylase, which is activated by phosphorylation by the AMP-PK. Malonyl CoA decarboxylase converts malonyl CoA to acetyl CoA, thereby relieving the inhibition of carnitine palmitoyl transferase I (CPT-I) and stimulating fatty acid oxidation (Fig. 47.5). Muscle cells do not synthesize fatty acids the presence of acetyl CoA carboxylase in muscle is exclusively for regulatory purposes. 

Fatty acid uptake into cardiac muscle is similar to that for other muscle cell types and requires fatty acid-binding proteins and carnitine palmitoyl transferase I for transfer into the mitochondria. Fatty acid oxidation in cardiac muscle cells is regulated by altering the activities of ACC-2 and malonyl CoA decarboxylase. 

Fig. 5. Diacylglycerol (DAG) stimulates mRNA production of enzymes for p-oxidation in the small intestine of C57BL/6J mice. C57BL/6J male mice, 7 wk old, were fed 5% triacylglyc-erols (TAG low-fat control), 30% TAG + 13% sucrose (high-fat control), 15% DAG + 15% TAG -I- 13% sucrose for 10 d (before obesity development). Values are means SEM, n = 6. P< 0.01, p< 0.001. Abbreviations AGO, acyl-CoA oxidase MCAD, medium-chain acyl-CoA dehydrogenase UCP, uncoupling protein-2 FAT, fatty acid translocase L-FABP, liver fatty acid binding protein.

FABPpm - Plasma Membrane Fatty Acid-Binding Protein FAT/CD36 - Fatty Acid Translocase/Cluster of Differentiation FAO - Fatty Acid Oxidation ... 

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