The metabolism of cholesterol in the presence of liver mitochondria from normal and thyroxine-treated rats

Clearance of chylomicron triacylglycerol and cholesteryl ester from the plasma of streptozotocin-induced diabetic and hypercholesterolemic hypothyroid rats

Labeled chylomicrons in thoracic duct lymph were collected after test meals containing 14C cholesterol and 2-3H glyceryl trioleate and were given by intravenous injection to groups of control rats, rats made diabetic by treatment with streptozotocin, and rats made hypothyroid and hypercholesterolemic by a diet containing cholesterol, peanut oil, cholic acid, and thiouracil. In the diabetic rats clearances from the plasma of chylomicron triacylglycerol and cholesteryl ester were impaired. A large variability in triacylglycerol clearance in diabetic rats was ascribed to variability in plasma triacylglycerol concentrations. Adipose tissue lipoprotein lipase activity was not impaired in the female diabetic rats used in this study. In the hypothyroid hypercholesterolemic rats chylomicron cholesteryl ester clearance from the plasma was impaired but chylomicron triacylglycerol was cleared efficiently, and adipose tissue lipoprotein lipase activity was similar to or greater than activity in controls. Ten minutes after intravenous injection most plasma radioactivity was recovered in lipoproteins of density less than 1.006 g/ml in all groups of rats, but relatively more was recovered at this density in both treatment groups. We suggest that chylomicron remnants accumulate in the plasma and contribute to the development of hyperlipemia in both treatment groups, but that the remnants formed in the diabetic rat are less depleted of triacylglycerol than the remnants formed in the hypothyroid hypercholesterolemic rat. It is suggested that factors other than measured lipoprotein lipase activities of adipose tissues may be important in determining the initial extent of hydrolysis of chylomicron triacyglycerol. We propose that the hypercholesterolemic hypothyroid rat is a useful model for the experimental production of the remnants of triacylglycerol-rich primary lipoproteins.

Thyroid function and related hepatic enzymes in the germfree rat

The adult germfree rat has a low metabolic rate, and the activity of various hepatic enzymes differs substantially from values found in the conventional animal. To promote understanding of the shifts in intermediary metabolism apparent in the germfree rodent, parameters of thyroid function and the activity of three enzyme systems presumably influenced by thyroid function were determined in 40-, 60-, and 100-day-old male germfree and conventional Lobund rats of Wistar origin. Serum thyroxine concentrations generally increased with age. They were below normal levels in 40- and 60-day-old germfree rats but appeared normal in the 100-day-old germfree animal. Serum thyroxine-binding protein saturation levels were significantly reduced in germfree rats of all age groups, but the reduction was least in the 100-day-old group. Mitochondrial succinate oxidase and alpha-glycerophosphate dehydrogenase activities were lower in livers of germfree rats of all age groups, although the significance of the difference in succinic oxidase activity became less in older animals. Monoamine oxidase activity was similar in germfree and conventional animals. Although the data suggest a hypofunction of the thyroid in the immature germfree rat, no definite relation between thyroid function and the characteristic metabolic anomalies of the germfree rat could be established.

The formation of cholest-5-ene-3 ,26-diol as an intermediate in the conversion of cholesterol into bile acids by liver mitochondria

1. When [(14)C]cholesterol was incubated with rat liver mitochondria, radioactive 26-hydroxycholesterol, 3beta-hydroxychol-5-enoic acid and other bile acids were isolated from the incubation mixture. 2. In the absence of added 26-hydroxycholesterol, the specific radioactivity of the 26-hydroxycholesterol formed from [(14)C]cholesterol during the incubation was higher than that of the 3beta-hydroxychol-5-enoic acid. Addition of increasing amounts of 26-hydroxycholesterol led to a progressive fall in the specific radioactivity, and to a progressive increase in the mass, of the 3beta-hydroxychol-5-enoic acid recovered at the end of the incubation. 3. It is concluded that 26-hydroxycholesterol is an intermediate in the formation of 3beta-hydroxychol-5-enoic acid from cholesterol. 4. Comparison of the specific radioactivity of the 26-hydroxycholesterol formed in the incubation mixture with that of the added [(14)C]cholesterol indicates that endogenous cholesterol in mitochondria is accessible to cholesterol 26-hydroxylase.

The formation of lithocholic acid, chenodeoxycholic acid and alpha- and beta-muricholic acids from cholesterol incubated with rat-liver mitochondria

1. When rat-liver mitochondria were incubated with [4-(14)C]cholesterol in the presence of a soluble supernatant fraction, various steroids more polar than cholesterol were formed. These included 3beta-hydroxycholest-5-en-26-oic acid, 3beta-hydroxychol-5-enoic acid, lithocholic acid, chenodeoxycholic acid and alpha- and beta-muricholic acids. 2. All the radioactive C(24) bile acids recovered were in conjugated form, probably as taurine conjugates. 3. The formation of 3beta-hydroxychol-5-enoic acid from cholesterol shows that liver mitochondria are capable of carrying out the oxidative removal of the isopropyl unit of the side chain before any modification has occurred in the ring system.