Lipids nonhydrolyzable

Lipids can be classified into substances that are either hydrolyzable— i. e., able to undergo hydrolytic cleavage—or nonhydrolyzable. Only a few examples of the many lipids known can be mentioned here. The individual classes of lipids are discussed in more detail in the following pages. 

Nonhydrolyzable lipids cannot be cleaved into smaller units by aqueous hydrolysis. 

Nonhydrolyzable lipids tend to be more varied in structure. We will examine four different types fat-soluble vitamins, eicosanoids, terpenes, and steroids. 

Lipids are categorized into hydrolyzable saponifiable) and nonhydrolyzable (nonsaponifiiable) lipids. Hydrolyzable lipids contain at least one ester group, which undergoes hydrolysis in the presence of an acid, a base, or an enzyme. Hydrolysis by base is referred to as saponification. Hydrolysis cleaves a saponifiable lipid into two or more smaller molecules. Nonhydrolyzable lipids do not undergo hydrolytic cleavage into smaller molecules. 

Lipids perform a variety of physiological functions. Triacylglycerols are used for energy storage and metabolic fuel. Phospholipids, sphingolipids, and cholesterol (a steroid) are structural components of cell membranes. Nonhydrolyzable lipids perform a variety of regulatory functions (hormones, vitamins). 

Nonhydrolyzable lipids are not cleaved into smaller molecules by hydrolysis (including acid, saponification, and digestion) because of the absence of ester groups. Three classes of compounds comprise the nonsaponifiable lipids, as shown in the bottom of Fig. 19-1. Steroids contain a four-ring system of three 6-membered rings and one 5-membered ring ... 

Problem 19.28. Why are steroids, prostaglandins, leukotrienes, and terpenes classified as nonhydrolyzable lipids ... 

Problem 19.50. Which of the following are nonhydrolyzable lipids (a) triacylglycerols, ib) prostaglandins, (c) waxes, id) leukotrienes, (e) sphingomyelins, if) steroids, (g) glycolipids, ih) cerebrosides, ii) terpenes, ij) plasmalogens, ik) phosphoglycerides. 

In the absence of GTP, or in the presence of nonhydrolyzable analogues of GTP, dynamin will self-assemble onto and tubulate hposomes. This activity has been observed with liposomes of varied composition, including those formed solely using dioleoylphosphatidylserine (Sweitzer and Hinshaw, 1998), or Folch fraction I, a brain lipid extract (Takei et al, 2001). Correspondingly, dynamin s GTPase activity is stimulated 10-100-fold when measured in the presence of liposomes. We refer to this as dynamin s liposome-stimulated GTPase activity. 

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