Lipid metabolism, oxidative transformations

Since ascorbate reduces photooxidation of lipid emulsions and multivitamin preparations (see Figure 4) [19], Lavoie et al. [34] studied the formation of oxidative by-products of vitamin C in multivitamins exposed to light. They found that the loss of ascorbic acid in photoexposed multivitamin preparations was associated with the generation of products other than dehydroascorbate and 2,3-diketogulonic acid, which are the usual products of vitamin C oxidation. The authors showed that hydrogen peroxide at concentrations found in TPN solutions induced the transformation of dehydroascorbate into new, biologically active compounds that had the potential to affect lipid metabolism. They believe that these species have peroxide and aldehyde functions [35]. 

The liver has a variety of functions in lipid metabolism (7.) uptake, oxidation and transformation of free fatty acids, (2.) synthesis of plasma lipoproteins, (3.) trans-... 

Many aroma compounds in fruits and plant materials are derived from lipid metabolism. Fatty acid biosynthesis and degradation and their connections with glycolysis, gluconeogenesis, TCA cycle, glyoxylate cycle and terpene metabolism have been described by Lynen (2) and Stumpf ( ). During fatty acid biosynthesis in the cytoplasm acetyl-CoA is transformed into malonyl-CoA. The de novo synthesis of palmitic acid by palmitoyl-ACP synthetase involves the sequential addition of C2-units by a series of reactions which have been well characterized. Palmitoyl-ACP is transformed into stearoyl-ACP and oleoyl-CoA in chloroplasts and plastides. During B-oxi-dation in mitochondria and microsomes the fatty acids are bound to CoASH. The B-oxidation pathway shows a similar reaction sequence compared to that of de novo synthesis. B-Oxidation and de novo synthesis possess differences in activation, coenzymes, enzymes and the intermediates (SM+)-3-hydroxyacyl-S-CoA (B-oxidation) and (R)-(-)-3-hydroxyacyl-ACP (de novo synthesis). The key enzyme for de novo synthesis (acetyl-CoA carboxylase) is inhibited by palmitoyl-S-CoA and plays an important role in fatty acid metabolism. 

Alkyl chains of many other lipids can be metabohsed by col- or co2-hydroxylation, and, for example, prostaglandins and leukotrienes may be transformed in this way. As discussed above, co-oxidation of prostaglandins is one of the metabolic prcxxsses that lead to formation of the main urinary metabolites. 

Cytochrome P-450 is a heme-containing monooxygenase that catalyzes a number of important substrate oxidations, including hydroxylation of alkanes and epoxidation of alkenes. These transformations are an important part of the metabolic processes involved in the formation of biologically active compounds from lipids and aromatics, and for the excretion of exogenous compounds in detoxification processes. The stoichiometric reaction of cytochrome P-450 is given in equation 1. 

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