Turnover of cholesterol-4-14-C and cholic acid-24-14-C by rabbits fed a diet containing lactose

Rabbits fed 0.35% of cholesterol in diets containing either 29.35% of lactose or sucrose were studied for 14 weeks. The rabbits fed lactose had higher plasma and liver cholesterol concentrations than those fed sucrose. The half-life of cholesterol was 19.0 days and 35.0 days for rabbits fed sucrose and lactose, respectively. The half-life, pool size, and daily production of deoxycholic acid were 9.7 days, 1.29 g, and 74.1 mg for rabbits fed sucrose; and 14.2 days, 1.40 g, and 49.1 mg, for those fed lactose. Cholesterol was the major neutral sterol in the feces of the rabbits fed lactose, whereas coprostanol (5 Beta-cholestan-3 Beta-ol) dominated the corresponding fraction in those fed sucrose. The fecal steroid composition did not vary between day and night collections. No sterol esters were detected in the feces. Urinary elimination of radioactivity was less than 10% of that injected. The "lactose effect" seems to be due to enhanced retention of steroids, the mechanism of which has not been elucidated.     

In vitro incorporation of cholesterol-14C into very low density lipoprotein cholesteryl esters

The cholesteryl esters of very low density lipoproteins become labeled when human plasma is incubated with cholesterol-(14)C. The relative order of magnitude of the specific activity of the cholesteryl esters of the major lipoprotein fractions is: high density lipoproteins > very low density lipoproteins > low density lipoproteins. This pattern of labeling is similar to that found by others in experiments performed in vivo. Very low density lipoprotein cholesteryl esters are probably not formed by direct action of the plasma lecithin:cholesteryl acyltransferase, since significant esterification of cholesterol does not occur when very low density lipoproteins are incubated separately with the enzyme. Instead, labeled cholesteryl esters formed in the other lipoprotein fractions transfer to the very low density lipoproteins, the relative amount of monounsaturated esters transferred being slightly greater than that of saturated and polyunsaturated esters. The results support the possibility that the acyltransferase indirectly increases the concentration of very low density lipoprotein cholesteryl esters in vivo.     

Placental transfer of cholesterol-4-14C into rabbit and guinea pig fetus

A tracer dose of cholesterol-4-(14)C was given daily in the diet of six pregnant guinea pigs to establish an isotopic steady state. At the time of parturition, maternal and fetal blood and fetal tissues were collected and analyzed for cholesterol content and cholesterol specific activity. A comparison of these specific activities in neonatal and maternal serum indicated that about 22% of the fetal serum cholesterol was transferred from maternal blood. In the newborn, tissues generally had the same cholesterol specific activity as serum. Brain tissue was an exception in having a specific activity only 8.4% of that of serum. Dietary cholesterol did not increase serum cholesterol levels in the newborn but did increase the percentage of fetal cholesterol derived from the maternal circulation. The rapid transfer of cholesterol-4-(14)C across the placenta was indicated by the appearance of this isotope in the newborn 2 days after its administration to pregnant rabbits. A considerable amount of the cholesterol content of newborn guinea pigs and rabbits originated from the maternal blood.