1038 CHAPTER TWENTY-SIX Lipids GOOD CHOLESTEROL7 BAD CHOLESTEROL? WHAT'S THE DIFFERENCE? holesterol is biosynthesized in the liver, trans- altered. Regular exercise increases HDL and reduces orted throughout the body to be used in a va- LDL cholesterol, as does limiting the amount of satu- ety of ways, and returned to the liver where it rated fat in the diet. Much progress has been made in serves as the biosynthetic precursor to other steroids. developing new drugs to lower cholesterol. The But cholesterol is a lipid and isn't soluble in water. statin class, beginning with lovastatin in 1988 fol- How can it move through the blood if it doesn' t dis- lowed by simvastatin in 1991 have proven especially part of a lipoprotein(lipid+ protein= lipoprotelyy is instead carried through the blood and tissues HOA The proteins that carry cholesterol from the liver are called low-density lipoproteins, or LDLs those that return it to the liver are the high-density apoproteins, or HDLs. If too much cholesterol is being transported by LDl, or too little by HDl, the extra /CH3 cholesterol builds up on the walls of the arteries caus- ing atherosclerosis. a thorough physical examination nowadays measures not only total cholesterol con- H3C centration but also the distribution between ldl and HDL cholesterol. An elevated level of ldl cholesterol is a risk factor for heart disease. LDL cholesterol is The statins lower cholesterol by inhibiting the en- bad"cholesterol HDLS, on the other hand, remove zyme 3-hydroxy-3-methylglutaryl coenzyme a reduc excess cholesterol and are protective HDL cholesterol tase, which is required for the biosynthesis of meva onic acid (see Section 26. 10). Mevalonic acid is an The distribution between LDL and HDL choles- obligatory precursor to cholesterol, so less mevalonic terol depends mainly on genetic factors, but can be acid translates into less cholesterol 26 12 VITAMIN D A steroid very closely related structurally to cholesterol is its 7-dehydro derivative 7-Dehydrocholesterol is formed by enzymic oxidation of cholesterol and has a conju- it in its B in the tissues of the skin where it is transformed to vitamin D3 by a sunlight-induced photochemical reaction. C CH3 7-Dehydrocholestero Vitamin D3 itamin D, is a key compound in the process by which Ca- is absorbed from the intes- tine. Low levels of vitamin D3 lead to Ca concentrations in the body that are insuffi cient to support proper bone growth, resulting in the bone disease called rickets Back Forward Main MenuToc Study Guide ToC Student o MHHE Website1038 CHAPTER TWENTY-SIX Lipids 26.12 VITAMIN D A steroid very closely related structurally to cholesterol is its 7-dehydro derivative. 7-Dehydrocholesterol is formed by enzymic oxidation of cholesterol and has a conju￾gated diene unit in its B ring. 7-Dehydrocholesterol is present in the tissues of the skin, where it is transformed to vitamin D3 by a sunlight-induced photochemical reaction. Vitamin D3 is a key compound in the process by which Ca2 is absorbed from the intes￾tine. Low levels of vitamin D3 lead to Ca2 concentrations in the body that are insuffi- cient to support proper bone growth, resulting in the bone disease called rickets. sunlight HO H H3C H H3C H3C CH3 CH3 7-Dehydrocholesterol HO H H3C H3C CH3 CH3 Vitamin D3 GOOD CHOLESTEROL? BAD CHOLESTEROL? WHAT’S THE DIFFERENCE? Cholesterol is biosynthesized in the liver, trans￾ported throughout the body to be used in a va￾riety of ways, and returned to the liver where it serves as the biosynthetic precursor to other steroids. But cholesterol is a lipid and isn’t soluble in water. How can it move through the blood if it doesn’t dis￾solve in it? The answer is that it doesn’t dissolve, but is instead carried through the blood and tissues as part of a lipoprotein (lipid protein  lipoprotein). The proteins that carry cholesterol from the liver are called low-density lipoproteins, or LDLs; those that return it to the liver are the high-density lipoproteins, or HDLs. If too much cholesterol is being transported by LDL, or too little by HDL, the extra cholesterol builds up on the walls of the arteries caus￾ing atherosclerosis. A thorough physical examination nowadays measures not only total cholesterol con￾centration but also the distribution between LDL and HDL cholesterol. An elevated level of LDL cholesterol is a risk factor for heart disease. LDL cholesterol is “bad” cholesterol. HDLs, on the other hand, remove excess cholesterol and are protective. HDL cholesterol is “good” cholesterol. The distribution between LDL and HDL choles￾terol depends mainly on genetic factors, but can be altered. Regular exercise increases HDL and reduces LDL cholesterol, as does limiting the amount of satu￾rated fat in the diet. Much progress has been made in developing new drugs to lower cholesterol. The statin class, beginning with lovastatin in 1988 fol￾lowed by simvastatin in 1991 have proven especially effective. The statins lower cholesterol by inhibiting the en￾zyme 3-hydroxy-3-methylglutaryl coenzyme A reduc￾tase, which is required for the biosynthesis of meva￾lonic acid (see Section 26.10). Mevalonic acid is an obligatory precursor to cholesterol, so less mevalonic acid translates into less cholesterol. HO O O O O CH3 H3C H3C CH3 CH3CH2 Simvastatin Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website