Cholesterol radioactive, synthesis

Wolfe has presented an excellent description of the systematic application of stable and radioactive isotope tracers in determining the kinetics of intestinal fat absorption, hepatic triglyceride synthesis, lipid mobilization, triglyceride-fatty acid recycling, and cholesterol turnover. 

Mosbach et al, (35) have used cholestyramine to confirm that the la-hydroxylation of cholesterol is the rate-limiting step in bile acid synthesis. Because of the loss of cholesterol from the enterohepatic circulation, there is a marked increase in cholesterol synthesis during administration of cholestyramine to rats (27) or man (36). Mosbach et aL, using perfused rabbit liver, showed that the biliary content of glycocholic acid rose from 0.34 to 3.3 mg, while the content of glycodeoxycholic acid fell from 7.4 to 3.7 mg. The conversion of radioactive acetate, mevalonate, or cholesterol to bile acids was increased from five- to twentyfold, but the conversion rate of 7a-hy-droxycholesterol to cholic acid was unchanged. The formation of 7a-hy-droxycholesterol from cholesterol is enhanced by treatment with cholestyramine (37,38). 

The synthesis of radioactive cholesterol and fatty acids in vitro. J. biol. Chem. 189,... 

It was shown that considerably more radioactivity was present as the fatty acids than as sphingosine. A small fraction of the radioactivity appeared in cholesterol, reflecting metabolism of the lipid to radioactive acetate and subsequent synthesis of cholesterol. This suggests that part of the radioactivity recovered in... 

The rate of cholesterol synthesis is influenced by the amount of cholesterol in the diet. The incorporation of radioactivity from acetate was markedly diminished in the liver, adrenals, intestine, and skin, when dogs and rabbits were maintained on a high intake of cholesterol. - Blood... 

These findings explain the increased body cholesterol storage in obesity but what of the accelerated cholesterol synthesis To answer this question we obtained adipose tissue from obese patients and measured the incorporation of radioactive glucose into cholesterol. The results showed very little cholesterol synthesis under a wide variety of conditions. These included addition of insulin to the medium, and varying the diets or reducing the patients weights. 

Another approach to the synthesis of labeled cholesteryl esters is to use radioactive fatty acids instead of the isotopically labeled sterol. Borgstrbm (1952) prepared this type of ester from stearic acid 1-C, cholesterol, and com oil in the presence of CO2 at 200 C. The mixture was taken up in light petroleum, washed, and dried. The products of this reaction were cholesteryl esters and unreacted cholesterol. 

The possible contribution of this pathway to phytanic acid synthesis in Refsum s disease was investigated in a Norwegian patient who was given 25 juc mevadonic acid-2- " C intravenously. Although there was normal uptake of radioactivity into plasma cholesterol there was little radioactivity in the plasma fatty acids, suggesting that if any biosynthesis of phytanic acid occurred, it must be at a very slow rate. Confirmation was provided by extended studies of the uptake of deuterium oxide over four months in the s lme patient. Although after this time 91% of the expected equilibration occurred between body water and cholesterol, there was minimal incorporation of deuterium in the phytanic acid, and this occurred unevenly, mainly concentrated in the first three carbon atoms of the molecule [121, 126, 127, 128]. 

---