F. W. Quackenbush

Desaturation of palmitate and stearate by cell-free fractions from soybean cotyledons

Homogenates of cotyledons from immature soybean seeds were fractionated by centrifugation. Cell-free preparations actively desaturated 1-14C-palmitate and 1-14C-stearate to produce 9,10-unsaturated acids. The 9-desaturase activity was present mainly in supernatant fractions (22,000 and 105,000 xg). These fractions also desaturated oleate to linoleate and elongated C14, C16 and C18 acids. In view of this versatility of desaturase systems in the soybean, including the 9-desaturase(s) for C14, C16 and C18 saturated acids, there would not seem to be further need to consider a separate plant pathway for biosynthesis of unsaturated fatty acids.

Influence of dietary fat on bile acid secretion of rats after portal injection of3H-cholesterol and [4-14C] cholesteryl esters

Labeled cholesterol and its esters were injected via the portal vein into bile duct-cannulated rats, subsequent to a 7 week regimen of either 5% safflower oil or 5% beef tallow in a hypercholesterolemic diet. Analysis of bile collected over a 6 hr period from the safflower group showed 8.6% higher output of bile acids, 13.6% higher conversion of3H-cholesterol to bile acids and 40% higher conversion of [4-14C]cholesteryl oleate to bile acids than bile collected from the tallow group. During the 1st hr conversion of both oleyl and linoleyl esters of14C-cholesterol to bile acids was much slower than conversion of the free3H-cholesterol to bile acids, thus eliminating these esters as preferred substrate for bile acid formation. However at 6 hr two-thirds of the injected14C of oleyl ester was recovered in the liver, and about half of this was in the form of free cholesterol. Thus cholesterol ester hydrolase, though inhibited by dietary cholesterol, evidently did not impose limitations on formation of free cholesterol for subsequent oxidation reactions. Specific radioactivities were of doubtful significance because of uncertainities as to “active” pool size. The data suggest that dietary linoleate exerts its hypocholesterolemic effect in some manner other than ester formation and that its point of action involves stimulation of cholesterol oxidation to bile acids.

Effect of dietary fat saturation of absorption and intestinal secretion of cholesterol by the hypercholesterolemic rat

The fate of an oral dose of [4-14C] cholesterol given to rats grown on diets with 20% safflower oil or 20% hydrogenated coconut oil was determined by analysis of digestive tract, feces and tissues. The pattern of isotope distribution did not support the view that rats fed a saturated fat absorb less cholesterol than those fed an unsaturated fat. Fasted animals growth on the diet with 5% of these two fats and beef fallow showed no clear difference in the amount of digitonin-peecipitable sterol in their intestines. A shorter transit time for intestinal contents was observed with the saturated fat groups. It is concluded that neither absorption of cholesterol from the gut nor secretion of β-hydroxy sterol into the gut accounts for the hypocholesterolemic effect of polyunsaturated fat.

Cholesterol absorption with different fats following thoracic duct cannulation of the rat1

Radioactive cholesterol in three unsaturated fats was absorbed into the lymph to the same extent as cholesterol in three saturated fats or in fats which had been stripped of sterols. Absorption on the first day following cannulation was less than half that on subsequent days.

Cycloheximide and chloramphenicol: Effect on rat liver acetate metabolism

Conversion of isotopic acetate to cholesterol, to fatty acids and to CO2 by rat liver homogenates is dramatically curtailed within 3 hr following injection of chloramphenicol or cycloheximide. Cholesterol synthesis from mevalonate remains unaltered, indicating that this does not imply generalized depression of all metabolic processes. This apparent shutdown of acetate metabolism suggests that caution be exercised when these drugs are employed as specific inhibitors of protein synthesis.

Mechanism of suckling-rat hypercholesteremia. II. Cholesterol biosynthesis and cholic acid turnover studies.

Cholesterol biosynthesis from acetate-2-14C by the livers of suckling rats, which is known to be relatively slow, was increased 2–3-fold within 24 hours after severing the bile duct. Cholesterol synthesis by sham-operated litter mates showed no change under similar treatment. Mevalonate biosynthesis from acetate-2-14C in vitro by recombined liver microsomal supernatant (105,000×g) fractions from suckling rats also was only 10% of that of comparable recombined fractions from normal controls (young adult rats which were consuming colony diet). Activity was not improved by combining either the microsomal or supernatant fraction from suckling rat livers with the complementary fraction from normal adult livers. On the other hand, activity was restored to 100% when microsomes from livers of duct-served suckling rats were combined with the supernatant fraction from normal controls. Likewise, recombined liver fractions prepared from adult rats fed synthetic diets exhibited low activity for mevalonate biosynthesis. Activity was restored by bile duct cannulation, but inhibited when cholic acid was infused into the cannulated animal. Therefore, surgical procedures which interrupt the enterohepatic recirculation of bile components lead to a restoration of cholesterol biosynthesis and, at least in the adult animal where cannulation studies are practicable, this effect can be reversed readily by bile acid infusion.A slow rate of fecal excretion of14C-cholic acid was observed in suckling rats and rats fed synthetic diets, apparently reflecting an efficient enterohepatic recirculation of bile salts. The data suggest that under these dietary conditions bile salt retention either directly or indirectly influences hepatic synthesis of cholesterol.

Turnover of bile acids in the hypercholesterolemic rat as influenced by saturation of dietary fat

Injections of [24-14C] chenodeoxycholate and3H-cholate were made by heart puncture into 300 g male rats that bore T-cannulas in their bile ducts. The animals had been raised on diet A, containing glucose, cholesterol and cholate, or diet B, containing sucrose and cholesterol; each of the diets contained 5% safflower oil or 5% beef tallow as variables. From analysis of bile samples collected from the T at intervals over a 5 day period, it was observed that the safflower oil group fed diet B had a 17% shorter cholate half-life, a 29% larger cholate pool size and 52% higher rate of cholate synthesis than those fed beef tallow in the same diet. The safflower group fed diet A also had a larger cholate pool size, but synthesis and half-life were obscured by cholate feeding. Chenodeoxycholate turnover data were not obtainable because the decay curves were bimodal for all treatments and hence did not conform to a simple pool model. It is concluded that dietary safflower oil causes more rapid formation of cholate than does dietary beef tallow in the cholesterol-fed rat.

Reduction of atherogenicity of natural fats by small additions of ethyl linoleate in the diet of the rat.

Ethyl linoleate was substituted in part for the 20% of butterfat, hydrogenated coconut oil, lard, or tallow in an atherogenic diet fed to rats throughout a 40-week experimental period. Aortic degeneration, evidenced by lipid infiltration of the intima, was observed in the control groups but not in the linoleate-fed groups. Groups that received butterfat or hydrogenated coconut oil showed reduced plasma and hepatic cholesterol levels when fed 2% of ethyl linoleate; groups that received lard or tallow showed no significant change in plasma and hepatic cholesterol levels when fed 2% of ethyl linoleate; and groups that received a fat-free diet with 2% of ethyl linoleate showed lower plasma and hepatic cholesterol levels and more complete aortic protection than groups that were fed 20% of corn oil or cottonseed oil. The data suggest that, in the cholesterol-fed rat, the kind and amount of dietary fatty esters may influence aortic condition via some route(s) other than control of plasma and hepatic cholesterol levels.

Arylsulfonate esters as hypocholesteremic agents: III. Mechanism of action studies

The mechanism responsible for the hypocholestermic action of arylsulfonate esters of long chain fatty alcohols has been studied with rats fed either normocholesteremic or hypercholesteremic (1% cholesterol plus 0.5% glycocholate) diets. Linoleyl tosylate is more effective in lowering plasma and liver cholesterol levels of rats on the hypercholesteremic diet than several other hypocholesteremic agents tested. Linoleyl tosylate does not redistribute cholesterol to extrahepatic tissues nor inhibit hepatic cholesterol biosynthesis. Linoleyl tosylate is not effective in counteracting Tritoninduced hypercholesteremia nor in lowering plasma cholesterol levels of the suckling rat. Linoleyl tosylate increases the fecal elimination of dietary [4-14C] cholesterol and prevents its accumulation in blood and liver. Oleyl p-(n-decyl) benzene sulfonate also prevents the apparent absorption of [26-14C] cholesterol from the gastrointestinal tract. Linoleyl tosylate increases the fecal excretion of neurtal sterols but not of bile acids. The results indicate that the arylsulfonate esters of long chain fatty alcohols lower body cholesterol levels by inhibiting cholesterol absorption from the gastrointestinal tract. Exactly how absorption is inhibited is not clear, but linoleyl tosylate was found to stimulate the activity of cholesteryl esterase prepared from the intestinal mucosa.

2-Monoglyceride as an aid to the absorption of cholesterol into the thoracic lymph

Absorption of radioactive cholesterol given in triolein was nearly three-fold that of a similar dose given in ethyl oleate or oleic acid. The difference appears to reflect a need for monoglyceride, since addition of 2-monoolein to ethyl oleate improved cholesterol absorption.