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Food Lipids: Chemistry, Nutrition, and Biotechnology

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Scheme 3 Synthesis of farnesyl pyrophosphate from the biological ‘‘isoprenyl unit.’’<br />

cholesterol. The C-14 methyl group is first oxidized to an aldehyde, <strong>and</strong> removed as<br />

formic acid. The oxidation of the C-4� methyl group leads to an intermediate, 3oxo-4�-carboxylic<br />

acid, which undergoes a decarboxylation to form 3-oxo-4�-methylsterol.<br />

This compound is then reduced by an NADPH-dependent microsomal 3oxosteroid<br />

reductase to produce 3�-hydroxy-4�-methyl sterol, which undergoes a<br />

similar series of reactions to produce a 4,4-dimethylsterol. In animal tissues, C-14<br />

demethylation <strong>and</strong> the subsequent double-bond modification are independent of the<br />

reduction of the � 24 double bond. Desmosterol (cholesta-5,24-dien-3�-ol) is found<br />

in animal tissues <strong>and</strong> can serve as a cholesterol precursor. The double-bond isomerization<br />

of 8 to 5 involves the pathway � 8 → � 7 → � 5,7 → � 5 .<br />

2. Biosynthesis of Plant Sterols<br />

In animals, 2,3-oxidosqualene is first converted to lanosterol through a concerted<br />

cyclization reaction. This reaction also occurs in yeast. However, in higher plants<br />

<strong>and</strong> algae the first cyclic product is cycloartenol (Scheme 7). The cyclization intermediate,<br />

tetracyclic protosterol cation, undergoes a different series of concerted 1,2trans<br />

migrations of hydrogen <strong>and</strong> methyl groups. Instead of the 8,9 double bond, a<br />

stabilized C-9 cation intermediate is formed. Following a trans elimination of enzyme-X<br />

� <strong>and</strong> H � from C-19, with the concomitant formation of the 9,19-cyclopropane<br />

ring, cycloartenol is formed. A nearby �-face nucleophile from the enzyme is<br />

necessary to stabilize the C-9 cation <strong>and</strong> allow the final step to be a trans elimination<br />

Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.

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