Lipid catabolism

Figure 16-2. The citric acid cycle the major catabolic pathway for acetyl-CoA in aerobic organisms. Acetyl-CoA, the product of carbohydrate, protein, and lipid catabolism, is taken into the cycle, together with HjO, and oxidized to CO2 with the release of reducing equivalents (2H). Subsequent oxidation of 2H in the respiratory chain leads to coupled phosphorylation of ADP to ATP. For one turn of the cycle, 11 are generated via oxidative phosphorylation and one arises at substrate level from the conversion of succinyl-CoA to succinate.

In die physiological system, niacin and related substances maintain nicotinamide adenine diiuicleotide (NAD) and nicotinamide adenine ciinucleotide phosphate (NADP). Niacin also acts as a hydrogen and electron transfer agent in carbohydrate metabolism and furnishes coenzymes for dehydrogenase systems. A niacin coenzyme participates in lipid catabolism, oxidative deamination, and photo synthesis,... 

In catabolic role furnishes coenzymes for lipid catabolism, oxidative deamination and key reactions in TCA cycle. 

This volume contains eight beautifully written, well-illustrated chapters that present both new and reviewed information fundamental to a clear understanding of lipid catabolism and transport at the molecular level. Three-dimensional structures of important serum lipoproteins, apoli-poproteins, and lipases, utilizing X-ray data when available, are emphasized, and an attempt is made to relate structures to functions. 

Poliak, J.K. and W. Harsas. Effects of organochlorine compounds on lipid catabolism of foetal rat liver mitochondria and microsomes. Bull. Environ. Contam. Toxicol. 28 313-318, 1982. 

The adverse effects of trans fatty acids on serum lipids and lipoproteins are thought to be mediated by alterations in lipid catabolism and metabolism. Trans fatty acids increase the catabolism rates of apolipoprotein A-I and decrease apolipoprotein B catabolism rates (Matthan et al., 2004), reduce LDL-C particle size (Mauger et al., 2003), and can increase cholesteryl ester transfer protein (CETP) activity (van Tol et al., 1995). CETP mediates the transfer of cholesterol esters from HDL- to LDL- and very-low-density lipoprotein (VLDL)-C, thereby offering a potential explanation for the LDL-C-raising and HDL-C-lowering effect of trans fatty acids. 

Ketones are produced when there is an imbalance in lipid catabolism, compared with carbohydrate catabolism. If fatty acids are being p-oxidized to produce acetyl-CoA, but there is insufficient oxaloacetate because it is being drawn off for gluconeogenesis, the acetyl-CoA molecules combine to form ketone bodies. 

Lipid catabolism is often associated in mammals with an elevation of ketone bodies in the plasma. In birds, the only ketone body detectable in plasma is )8-hydroxybutyrate, and although its concentration increases in fasting as fat stores are mobilised, glucagon infusions do not promote a detectable increase in the plasma level (Epple Brinn, 1987). 

Our knowledge of lipolytic enzymes in plants is meager. This is surprising, perhaps, when the commercial importance of seed oils and other plant lipids is considered. In addition to the obvious importance of lipid metabolism in oil seed crops, the involvement of storage and membrane lipids in plant biochemistry and its applications should be backed by a much better understanding of lipid catabolism. The surface lipids of all plants have been almost totally neglected in this respect until the recent developments described by Kolattukudy (this volume. Chapter 18). 

Effect on Lipid Metabolism. Our knowledge of the effect of cortisone on lipid metabolism is still fragmentary. Interpretation of the results is complicated by the fact that the effect of the hormone seems to vary depending upon the source of the adipose tissue. Adrenalectomy stimulates and corticoid injections decrease lipogenesis in the adipose tissue of the mesentery. The decreased lipogenesis induced by corticosteroids is accompanied by release of free fatty acids. When corticosterone and hydrocortisone are added to epididymal adipose tissue incubated in vitro, the hormones fail to stimulate lipogenesis from [ " Cjpyruvate, but they accelerate fatty acid release, and the lipolytic effect is completetly blocked by actinomycin D. Consequently, one effect of glucocorticoids on some of the adipose tissues seems to be to accentuate lipid catabolism. 

Murase T, Nagasawa A, Suzuki J, Hase T, Tokimitsu I. Beneficial effects of tea catechins on diet-induced obesity stimulation of lipid catabolism in the liver. Int J Obes. 2002 26(11) 1459—1464. 

Water stress provokes a decrease in leaf polar lipid content (Pham Thi et al.1982).This decrease is due to an inhibition of the biosynthesis from C-acetate (Pham Thi et al.l985), but certainly also to an acceleration of degradative phenomena. The enzymes responsible for polar lipid catabolism in plants have received little attention (Galliard et al.l974). This paper describes the enzymatic systems acting on MGDG and PC breakdown and their evolution under water stress. ... 

The conversion of ACC to ethylene is believed to be a free radical associated process, which in microsomes is attributed to hydroperoxides generated by membrane-associated lipoxygenase. In this work, we will attempt to establish the hypothesis that the growth of sunflower cotyledons could be linked to a boron mediated lipid catabolic process which is geared to an ethylene evolving system. 

---