Tissue animal, fatty acid synthesis

Fatty Acids Synthesis, Elongation, and Desaturation. The main objective of feeding fats to animals is to provide a concentrated energy source, not to have the fat stored in the tissues. Recognized EFA requirements are no more than several percent of dry matter at the most, but the critical roles they play in maintaining the metabolic machinery has attracted the majority of current research on dietary fat utilization. 

The RQ for the conversion of glucose to fatty acids can be calculated from the biochemical considerations presented so far. This RQ would not be expected to occur in any living tissue or in any animal, because this RQ represents an extreme case in which there is no oxidation of fatty acids or of carbohydrates in the Krebs cycle. Where the rate of fatty acid synthesis is equal to the rate of fatty acid oxidation, the RQ would be expected to be 10. Where the rate of fatty acid synthesis is twice as great as the rate of fatty acid oxidation, the RQ would be greater than 1.0. One would not, however, expect to encounter a tissue in which there is only fatty acid synthesis and no fatty acid oxidation. 

Harwood HJ, Jr., Petras SF, Shelly LD, et al. Isozyme-nonselective N-substituted bipi-peridylcarboxamide acetyl-CoA carboxylase inhibitors reduce tissue malonyl-CoA concentrations, inhibit fatty acid synthesis, and increase fatty acid oxidation in cultured cells and in experimental animals. JBiol Chem. 2003 278(39) 37099-37111. 

In 1952, Brady and Gurin reported that fatty acid synthesis from acetate by pigeon liver extracts was markedly stimulated by tricarboxylic acids 10). Subsequent work by various investigators established that acetyl-CoA carboxylase is the site of citrate activation and that acetyl-CoA carboxylase from all animal tissues requires tricarboxylic acids for activation 1, 52, 78-81, 122, 123). 

Thus, the whole process of phenol formation may be considered analogous, in an overall metabolic sense, to fatty acid synthesis in well-fed animals (with the resultant formation and storage of triacylglycerol in adipose tissue) or ketogenesis in the liver of starved or diabetic animals. These situations have effectively adapted to the demands of an imbalanced overproduction of acetyl-CoA. 

Regulation of Fatty Acid Synthesis in Animal Tissues... 

Fig. 1. Pathway of fatty acid synthesis from glucose in animal tissues. The key enzymes or enzyme systems involved are (1) pyruvate dehydrogenase, (2) pyruvate carboxylase, (3) citrate synthase, (4) citrate translocation system, (5) citrate cleavage enzyme, (6) acetyl-CoA carboxylase, (7) fatty acid synthetase, (8) 3-phosphoglyceraldehyde dehydrogenase, (9) malate dehydrogenase, (10) malic enzyme, (11) hexose monophosphate shunt.

Fig. 2. Possible control points in fatty acid synthesis from glucose in animal tissues.

It has been shown that the carboxylation of acetyl-CoA is effectivelyf the rate-determining reaction of fatty acid synthesis in animal tissues [94] and therefore has regulatory potential. In the absence of tricarboxylic acid activator, acetyl-CoA carboxylase activity is lower by nearly two orders of magnitude than in the fully activated state where its catalytic capacity is nearly kinetically matched to those of the citrate cleavage enzyme and fatty acid synthetase [76,77,94,150]. Therefore, changes in the level of tricarboxylic acid effector presumably could control the rate of the carboxylase reaction, and thus regulate fatty acid synthesis. 

Because cholesterol and phospholipid synthesis is likely to be necessary for the maintenance of the integrity of the structural lipoproteins in nervous tissue, especially in the Schwann cells and the oligodendrocytes, and also because cholesterol and fatty acid synthesis is decreased in livers of diabetics, cholesterol and fatty acid synthesis has been measured in the central and peripheral nervous tissue of animals made diabetic with alloxan. The synthesis of both compounds is decreased in diabetic animals in the central but not in the peripheral nervous system. Whether the extension of such studies will someday lead to the elucidation of the pathogenesis of diabetic neuropathy remains to be seen. 

However, these observations provide only indirect evidence for a role of NADPH in the development of ketosis in diabetes. When attempts were made to measure directly the ratio of NAD to NADH in tissues of diabetic and normal animals, no significant differences were found. This old theory must be evaluated in the light of new information on fatty acid synthesis. Wakil [140] has clearly established that the carboxylation of acetyl CoA is the rate-limiting reaction in fatty acid synthesis. This is a reaction in which acetyl CoA is converted to malonyl CoA in the presence of acetyl CoA carboxylase, a biotin enzyme. 

The elongation of long-chain fatty acids in mammals has certain aspects of similarity with the synthesis of acetoacetate. Zabin has studied this process and has found that 2-carbon units are added to the carboxyl group of the homologous fatty acid with 2 fewer carbon atoms in a manner precisely analogous to the short-chain fatty acid synthesis described by Barker, Kamen, and Bornstein. Although no direct evidence is at hand for the reaction in animal tissues, the following reaction sequence seems likely ... 

Acetyl-CoA carboxylase catalyzes the formation of malonyl-CoA from acetyl-CoA and is involved in fatty acid synthesis (Eq. 4). The mechanism, the different isoforms, and the regulation of the activity of this enzyme from animal tissues have been reviewed (45). 

In animal cells and yeasts, multienzyme complexes localised in cytosol, referred to as type I fatty acid synthase (FAS I), carry out the bulk of the de novo fatty acid synthesis. In animals, it occurs primarily in the liver, adipose tissue, central nervous system and lactating mammary gland. FAS I contains seven distinct catalytic centres and is arranged around a central acyl carrier protein (ACP) containing bound pantothenic acid (see Section 5.9.1). In prokaryotes and plants, distinct soluble enzymes localised in mitochondria and plastids, referred to as type II fatty acid synthase (FAS II), carry out the reactions. 

One of our first observations was on the subcellular distribution of ACP In spinach mesophyll cells. In 1979 we found that when protoplasts were gently lysed and their organelles seperated on sucrose gradients, essentially all of the ACP present could be attributed to the chloroplast fraction (2). This result revealed that plant mesophyll cells differ from animal and fungal cells by the absence of fatty acid synthesis In the cytoplasm. It Is now known that plastlds are also a major site of fatty acid synthesis In many non-green plant tissues although It has not been established whether other subcellular sites In these tissues also contribute to FAS. 

The results are of comparative interest but may have little bearing on the present discussion because the regulatory mechanisms for microbial fatty acid synthesis and for fatty acid synthesis in animal tissues appear to operate at quite different sites of control. Apart from the obvious absence of primary hormonal signals in bacteria, the following differences stand out. Only animal tissue acetyl-CoA carboxylase is activated by citric acid bacterial, plant and yeast carboxylase do not respond to this type of allosteric modulation. Similarly, microbial acetyl-CoA carboxylases are much more resistant to inhibition by palmitoyl-CoA at least at the concentration which inhibit the hepatic enzyme. 

Konrad Bloch, in the opening lecture, emphasized the features unique to fatty acid synthesis in animal tissues and discussed the importance of comparative biochemical research in the study of control mechanisms of lipid biosynthesis. 

The questions of when and how much the actual level of acetyl-CoA carboxylase present in a tissue regulates the rate of fetty acid synthesis in that tissue has only recently been satisfectorily answered. It had been observed by Allmann et al. (1965) that the stimulation of fatty acid synthesis that occurs upon refeeding fasted animals is prevented by puromycin and actinomycin injection at the time of refeeding. The latter agents prevented new protein (i.e., enzyme) synthesis. Later, Butterworth et al. (1966) reported that the quantity of purified enzyme protein (fatty acid synthetase complex) was reduced in the livers of fasted pigeons and returned to near control levels following refeeding. 

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