462 ANIMAL BIOCHEMISTRY 



pears in cholesterol is introduced. The enzyme system required for 

 the cyclization and hydroxylation reactions is referred to as squalene 

 oxidocyclase. These reactions also appear to depend on the presence 

 of either TPNH or DPNH. The conversion of lanosterol to zymos- 

 terol requires TPNH and oxygen and has been demonstrated using 

 rat liver homogenate. Both in vivo and in vitro studies show the pos- 

 sibility of conversion of zymosterol to cholesterol. 



Cholesterol biosynthesis varies with the amount of cholesterol in 

 the diet. Thus cholesterol biosynthesis is at a maximvmi when choles- 

 terol-free diets are fed. On the other hand, cholesterol synthesis is 

 sharply reduced during fasting and is drastically increased when the 

 animal is fed a diet rich in carbohydrates. 



Pathological Cholesterol Concentrations 



About 80 per cent of the cholesterol metabolized by the body is 

 converted into various bile acids (pages 94, 436). Under certain con- 

 ditions, cholesterol crystals separate from the bile and produce the 

 calculi commonly referred to as gallstones. The formation of calculi 

 becomes clinically important if they descend into and block the biliary 

 tract, preventing flow of bile. 



Cholesterol-containing plaques sometimes form and attach them- 

 selves to the walls of the arteries, producing a characteristic lesion 

 referred to as atheromatosis. At the present time, the mechanism and 

 the cause of the deposition of such cholesterol plaques in man are 

 quite controversial. Although evidence is increasing which suggests 

 that inclusion of saturated fatty acids in the diet encourages formation 

 of atheroma, there is no positive proof that substitution of the essen- 

 tial fatty acids for these saturated homologues will in fact alleviate or 

 prevent atheroma formation. In some animals, pyridoxine deficiency 

 results in the formation of atheromas. Since pyridoxine is required 

 for the formation of the essential fatty acid, arachidonic acid, it is 

 conceivable that faulty cholesterol metabolism can be related to a 

 vitamin deficiency. 



REFERENCES 



The Lipids; Their Chemistry and Biocheiuistry. H. J. Dciiel. Interscience Pub- 

 lishing Co., New York, 1951. 



Lipid Metabolisin. R. T. Williams. Cambridge University Press, Cambridge, Eng- 

 land, 1952. 



