Unsaturated fatty acyl-CoA inhibition of cholesterol synthesis in vitro.

1. The influence of saturated and unsaturated fatty acids and fatty acyl coenzyme A thioesters on cholesterol synthesis in vitro has been studied in a rat liver post-mitochondrial supernatant system. 100 micronM free fatty acids do not influence in vitro cholesterol synthesis. Various fatty acyl-CoA thioesters at 10--100 microntm inhibit [14C]acetate incorporation into digitonin-precipitable sterols, the more unsaturated derivatives causing the greatest inhibition. 10 micronM arachidonoyl-CoA inhibits [14C]acetate incorporation into sterols 17% and 50 micronM inhibits 55%. [14C]Acetyl-CoA incorporation into sterols is similarly inhibited but [14C]mevalonate incorporation is not inhibited. Thus, the inhibition may be on the rate-controlling step of cholesterol synthesis, the conversion of beta-hydroxy-beta-methylglutaryl-CoA to mevalonate. Unsaturated fatty acyl-CoA thioesters may be important in regulating cholesterol synthesis. 2. Studies were undertaken to determine if the previously observed inhibition of cholesterol synthesis by thyroxine in vitro may relate to the thyroxine stimulation of fatty acid desaturation. 50 micronM thyroxine causes a preferential incorporation of [14C]acetate into unsaturated fatty acids while inhibiting acetate incorporation into sterols. However, a sufficient increase in unsaturated fatty acyl-CoA thioesters to account for the thyroxine inhibition of cholesterol synthesis was not demonstrated.

Fatty acyl-CoA inhibition of beta-hydroxy-beta-methylglutaryl-CoA reductase activity.

The influence of the fatty acyl-CoA thioesters on rat liver microsomal hydroxymethylglutaryl-CoA reductase activity was tested in vitro to determine if the previously demonstrated inhibition of [14C]acetate incorporation into cholesterol is due to inhibition of this rate limiting step in cholesterol synthesis. The polyunsaturated fatty acyl-CoA thioesters caused the greatest inhibition of enzyme activity, 50 micron arachidonoyl-CoA inhibiting 67% and 5 micron inhibiting 22%. 50 micron linoleoyl-CoA inhibited 56% with the more saturated thioesters causing less inhibition. 50--100 micron free fatty acids, free CoA, cholesterol esters, phospholipids, carnitine derivatives, prostaglandins and non-specific detergents caused little or no inhibition of enzyme activity. Kinetic studies revealed the inhibition to be noncompetitive with respect to hydroxymethylglutaryl-CoA with a Ki for arachidonoyl CoA of 3.10 micron. Fatty acyl-CoA inhibition of in vitro cholesterol synthesis is due to inhibition of hydroxymethylglutaryl-CoA reductase activity. Variation in intracellular concentrations of fatty acyl-CoA thioesters may signficantly alter cholesterol synthesis.

Fatty acid desaturation in experimental diabetes mellitus.

Microsomal fatty acid desaturation is defective in streptozotocin-induced experimental diabetes. This defect is correctable by insulin treatment. The electron transport chain needed for microsomal fatty acid desaturation was studied in liver microsomes of streptozotocin diabetic rats, and the defect was localized to the terminal desaturase enzyme. Cytochrome b5 levels were elevated in the face of decreased fatty acid desaturation and returned to normal after 48 h of insulin treatment; 2 U of regular insulin every 6 h for 24 h repaired the fatty acid desaturation defect, while 0.5 U failed to correct the defect. Both the delta 6 and delta 9 desaturase defects (linoleic acid and stearoyl-CoA desaturation) required similar amounts of insulin and periods of time for correction, although these are different enzymes. This is consistent with the desaturation defect being due to a protein synthetic effect. Diabetic rats treated twice daily with injections of 4 U of NPH insulin showed a "super" repair of their desaturase defect by 48 h: delta 9 desaturase activity increased eight times over control activity, while delta 6 desaturase activity increased two and one-half times over control activity. This, together with the fact that delta 6 desaturase activity in diabetes (64% of control) is altered less than is delta 9 desaturase activity (22% of control), indicates that delta 6 desaturase enzyme activity is less responsive to insulin than is delta 9 desaturase enzyme activity. The physiologic significance of altered fatty acid desaturation in diabetes mellitus is unknown.

Effect of thyroid hormone on metabolic adaptation to fasting.

The acute effect of triiodothyronine (T3) on mobilization of fat and protein energy stores has been measured in five fasting, normal men. Fasting subjects were chosen for this study to amplify catabolic effects occurring during brief thyroid hormone treatment. Subjects were fasted for 72 hr on two occasions with admintration of T3, 150 mug every 12 hr, for 72 hr before and during the second fast. Plasma beta hydroxybutyrate, acetoacetate, and free fatty acid levels as well as ketone, creatine, and urea excretion were measured during control and T3 fasts. T3 enhances catabolism of protein stores as indicated by the doubling of urea excretion during the T3 fasts. Likewise, creatine excretion is increased six to ninefold during the T3 fasts. Catabolism of fat stores is enhanced during the T3 fasts as shown by increased plasma free fatty acid and ketone levels, and increased ketone excretion. Brief T3 treatment for 3 days augments the expected protein and fat catabolism of starvation without causing subjective changes of hyperthyroidism. Much of the catabolic expression of hyperthyroidism may simply reflect inadequate caloric intake to fuel energy requiring processes stimulated by thyroid hormone such as cell membrane sodium pumping and protein synthesis.

Stimulation of peptide elongation by thyroxine.

This study suggests that thyroxine stimulates peptide elongation in a cell-free rat liver polyribosome system. The thyroxine effect persists in the presence of sufficient aurintricarboxylic acid to prevent polyuridylic acid-stimulated peptide initiation. In addition, thyroxine stimulates elongation of pre-existing polyphenylalanine chains providing conclusive evidence that the effect does not depend on peptide initiation. Thyroxine does not stimulate release of nascent peptides from ribosomes into the supernatant phase of the reaction mixture. Therefore in this protein-synthesis system the thyroxine effect is expected to occur at one or more of the reactions of peptide chain elongation, which include aminoacyl-tRNA binding, peptide bond synthesis and translocation.