Acylation of glycerol-3-phosphate by rabbit heart mitochondria and microsomes: triiodothyronine-induced increase in its activity

Effects of L-thyroxine on incorporation of (32)P into phospholipids of freshwater eels

The effects of L-thyroxine on phospholipid biosynthesis, via (32)P incorporation, were studied in gill, kidney, liver and muscle tissue of eels acclimatized at 11 degrees C. L-thyroxine treatment had no effect on tissue content of lipid, inorganic and organic acid-soluble phosphorus. Only an increase of the specific radioactivities of lipid, inorganic and organic acid-soluble phosphorus was observed in the muscle. Percentage distribution of (32)P among classes of phospholipid were significantly altered in liver and muscle, without change in phospholipid composition. A specific effect of L-thyroxine on (32)P incorporation into phosphatidic acid in muscle and liver has been shown. As expected by the higher specific radioactivity of muscle inorganic and organic acid-soluble phosphorus, the increased incorporation of (32)P into phosphatidic acid probably results from a higher specific radioactivity of muscle ATP phosphorus.

Metabolic aspects of the calorigenic effect of thyroid hormone in mammals

The increased BMR in hyperthyroidism may be accounted for by the use of chemical energy for metabolic processes and work. Major contributors are the heart work and futile cycling of FFA into triglyceride in adipose tissue, whereas gluconeogenesis in liver and the maintenance of sodium and potassium concentration gradients across the plasma membranes are unlikely to play any significant role. Information on the use of energy for protein turnover and urea production is lacking. The rate of oxygen uptake is not increased in the brain, spleen and testis. The main metabolic fuel seems to be free fatty acid. The mechanism which enables the hyperthyroid tissue to maintain normal concentrations of ATP, ADP and inorganic phosphate, (Pi) despite an increased turnover of energy-rich phosphates, is not fully elucidated. However, ultimately the increased rate of oxygen uptake in hyperthyroidism seems to rely upon an increased capacity for the transport of cytosolic ADP and Pi into mitochondria. The transport capacity is increased by an increased area of the mitochondrial membrane per g tissue and by a change in the kinetics of translocation of substrates for oxidative phosphorylation. Other transport processes across the mitochondrial membrane are also changed by hyperthyroidism, e.g. long chain fatty acid transport via carnitine acyl transferase, and oxaloacetate transport via substrate shuttles.

Effect of thyroid hormones on microsomal fatty acid chain elongation synthesis in rat liver

Evidence is presented that rat liver microsomal fatty acid chain elongation synthesis and desaturation, as well as acetyl-CoA carboxylase and fatty acid synthetase, are strongly influenced by thyroid hormone level. Conversely, the fatty acid chain elongation system in mitochondria, unlike the oxidative capacity of palmitate, NADH, succinate and malate, does not seem significantly affected by the thyrotoxic state. In triiodothyronine-induced or thyroxine-induced hyperthyroidism, rat liver acetyl-CoA carboxylase, fatty acid synthetase and microsomal chain elongation and desaturation reactions are not greatly affected after the first 10 days of treatment, while after longer intervals a respective increase in these activities is shown of up to 87, 116 and 65% after 22 days. In propylthiouracil-induced hypothyroidism, all the above synthetic activities are strongly reduced immediately after three days of drug administration and diminished no further following longer periods. Although the pattern of synthesized fatty acids in the thyrotoxic state is similar to that obtained from normal subcellular rat fractions, the esterification process of fatty acids in microsomal lipids appears to be slightly inhibited in hypothyroid rats and increased following triiodothyronine or thyroxine administration. Finally, a reduction in the hepatic cyclic AMP level of about 41% is reported after 19 days of triiodothyronine-administration to rats. On the basis of the observed insensitivity of the mitochondrial fatty acid chain elongation system to the thyrotoxic state, a tentative interpretation of its role in the hepatic cell is postulated.

Effect of bovine serum albumin on monoacyl- and diacylglycerol 3-phosphate formation in mitochondrial and microsomal fractions of rabbit hearts

The formation of monoacyl- and diacylglycerol 3-phosphate (P) by rabbit heart mitochondrial and microsomal fractions was studied by varying the concentration of acyl-CoA and that of bovine serum albumin in the assay system. The two subcellular fractions were prepared by the conventional differential centrifugation technique. The optimal concentration of acyl-CoA for both mitochondrial and microsomal acylation of glycerol 3-P was shifted to a higher range of acyl-CoA concentrations by greater amounts of albumin. A similar shift in the acyl-CoA concentration-enzyme activity relationship was observed in the acylation reaction of 1-palmitoylglycerol 3-P by the heart microsomes. The addition of albumin increased slightly the rate of diacylglycerol 3-P accumulation but increased greatly the rate of monoacylglycerol 3-P accumulation at any concentration of acyl-CoA; the effect was observed with mitochondrial or microsomal fraction as the crude enzyme source. Moreover, palmitoyl-CoA and linoleoyl-CoA served equally well as the acyl donor for the acylation reaction. However, relatively more monoacyl- than diacylglycerol 3-P was accumulated in the assays with rabbit heart mitochondrial fraction in the presence of albumin, whereas more diacyl- than monoacylglycerol 3-P was formed by the microsomal fraction. As a result, the microsomal diacyl:monoacyl-glycerol 3-P ratio was invariably greater than the mitochondrial ratio at a given concentration of acyl-CoA and albumin.

The glycerophosphateacyltransferases and their function in the metabolism of fatty acids

1. From different studies on the cellular localization, postional specificity, and regulatory properties of acyl-CoA: glycerophosphate acyltransferase (EC 2,3,1.15) AND ACYL-CoA: 1-ACYLGLYCEROPHOSPHATE ACYLTRANSFERASE (EC 2,3,1....) the following conclusions can be drawn: The glycerophosphate acyltransferase is localized in the endoplasmatic reticulum (microsomes) and in the outer membrane of the mitochondria of the animal cell. Its reaction product is 1-acylglycerophosphate (1-lysophosphatidic acid). The mitochondrial enzyme shows a high preference for saturated fatty acids while the microsomal enzyme is less specific (alternatively the microsomes contain more than one glycerophsophate acyltransferase). 2. The 1-acylglycerphosphate acyltransferase is localized in the endoplasmatic reticulum (microsomes) in the animal cell. Possibly a minor fraction of this enzyme is localized to the outer membrane of the mitochondria. This enzyme shows a strong preference for unsaturated fatty acids. 3. Both the microsomal and the mitochondrial dihydroxyacetonephosphate acyltransferase show similar fatty acid specificity as the corresponding glycerophosphate acyltransferases. It cannot be excluded that dihydroxy-acetonephosphate and glycerophosphate are acylated by the same enzymes. 4. The activity of the glycerophosphate acyltransferase(s) in the liver decreases in fasting or fat feeding and increases upon feeding of carbohydrate. The activity of carnitine palmityltransferase varies exacty opposit. These enzymes do not show dietary variations in heart and adipose tissue. 5. Under the otherwise identical conditions the rate of carnitine acylation in isolated mitochondria decreases more than the rate of glycerophosphate acylation when the concentration of palmityl-CoA is reduced. 6. In isolated liver cells (which has lost most of their carnitine) addition of carnitine increases the rate of fatty acid oxidation and decreases the rate of triglyceride formation. 7. Glycerol and fructose lower the rate of fatty acid oxidation, probably by lowering the levels of acyl-CoA and acyl-carnitine in the cells. 8. It is concluded that the relative activities of glycerophosphate acyltranse and carnitine palmityltransferase probably influence the fate of fatty acids in the cell.

Positional and fatty acid specificity of monoacyl- and diacylglycerol 3-phosphate formation by rabbit heart microsomes

Fatty acid selectivity of acyl-CoA:glycerol-3-phosphate acyltransferase (EC 2.3.1.15) and acyl-CoA:monoacylglycerol-3-phosphate acyltransferase (EC 2.3.1.52) of the microsomal fraction prepared from rabbit heart was studied. 1. The rate of acylation of glycerol 3-phosphate was increased proportionally with the concentration of acyl-CoA. The maximum rate was reached at 0.3 mumol acyl-CoA per ml. Palmitoyl-, oleoyl- and linoleoyl-CoA all served equally well as acyl donors, and produced approximately equal amounts of mono- and diacylglycerol 3-phosphate. The rate of reaction measured in the presence of two acyl-CoA esters was similar to that in the presence of a single acyl-CoA. 2. Treatment of synthesized monoacylglycerol 3-phosphate with Crotalus adamanteus venom phospholipase A2 (EC 3.1.1.4) and that with phosphatidate phosphatase (EC 3.1.3.4) indicated that the heart enzyme synthesized almost exclusively 1-acylglycerol 3-phosphate regardless of the kind of acyl donor. 3. Hydrolysis of diacylglycerol 3-phosphate with phospholipase A2 revealed that there was a slight preference for linoleoyl-CoA at position 2 and a slight discrimination against palmitoyl- and stearoyl-CoA at position 2, when diacylglycerol 3-phosphate was synthesized in the presence of a mixture of acyl-CoA esters. When diacylglycerol 3-phosphate was formed in the presence of a single acyl-CoA, the fatty acid distribution was random. 4. The rate of acylation of 1-palmitoylglycerol 3-phosphate by rabbit heart microsomal fraction was increased proportionally to the increasing concentrations of 1-palmitoylglycerol 3-phosphate up to 50 nmol per ml; higher concentrations were inhibitory. Differences in the activities measured with palmitoyl-, oleoyl- and linoleoyl-CoA as acyl donors were negligible. When stearoyl-, arachidonoyl- and erucoyl-CoA acted as acyl donors, the rates of reaction were low. 5. The acyl-CoA:1-palmitoylglycerol-3-phosphate acyltransferase activity increased proportionally to the increasing concentrations of acyl-CoA up to 10 nmol per ml; acyl donor specificity was similar to that found above [4]. The acyltransferase showed