Myristyl and palmityl acylation of the insulin receptor

The presence of covalently bound fatty acids in the insulin receptor has been explored in cultured human (IM-9) lymphocytes. Both alpha (Mr = 135,000) and beta (Mr = 95,000) subunits of the receptor incorporate [3H]myristic and [3H]palmitic acids in a covalent form. The effects of alkali and hydroxylamine on the labeled subunits indicate the existence of two different kinds of fatty acid linkage to the protein with chemical stabilities compatible with amide and ester bonds. The alpha subunit contains only amide-linked fatty acid while the beta subunit has both amide- and ester-linked fatty acids. Analysis by high performance liquid chromatography after acid hydrolysis of the [3H]myristate- and [3H]palmitate-labeled subunits demonstrates the fatty acid nature of the label. Furthermore, both [3H]myristic and [3H]palmitic acids are found attached to the receptor subunits regardless of which fatty acid was used for labeling. The incorporation of fatty acids into the insulin receptor is dependent on protein synthesis and is also detectable in the Mr = 190,000 proreceptor form. Fatty acylation is a newly identified post-translational modification of the insulin receptor which may have an important role in its interaction with the membrane and/or its biological function.     

Metabolism of phosphatidylcholine, phosphatidylinositol and palmityl carnitine in synaptosomes from rat brain

Abstract— Synthesis of phosphatidylcholine, phosphatidylinositol and palmityl carnitine in synaptosomes isolated from rat brain was investigated and compared with the synthesis of these compounds in microsomes and mitochondria. Electron microscopic and marker enzyme studies showed the contaminants in the synaptosomal preparation to consist of a few microsomes and almost no free mitochondria. In synaptosomes, addition of 1,2-diglyceride exerted no effect on the incorporation of [¹⁴C]choline into phosphatidylcholine or on the incorporation of [³H]myo-inositol into phosphatidylinositol, but it stimulated the incorporation of CDP[1,2-¹⁴C]choline into phosphatidylcholine by more than 50 per cent. The incorporation of the latter in intact synaptosomes, lysed synaptosomes and purified mitochondria was 15-6, 27 and 9-9 per cent, respectively, of that in the microsomes. The incorporation of [³H]myo-inositol into the phosphatidylinositol of synaptosomes and purified mitochondria was 15-8 and 11-1 per cent, respectively, of that in the microsomes. Maximal incorporation of [³H]myo-inositol occurred at pH 7–5 in a medium containing Mg²⁺ and CTP; it was linear with time and protein concentration and was inhibited by 1 mM Ca^{2 +} but unaffected by the presence of ATP. This incorporation of myo-inositol appeared to occur through the reversal of the CDP-diglyceride: inositol transferase reaction. The demonstration of carnitine palmityl transferase in synaptosomes indicated that, as in mitochondrial and erythrocyte membranes, fatty acids can be transported across the synaptosomal membrane. In contrast to mitochondria where maximal incorporation of [¹⁴C]carnitine into palmityl carnitine was observed after 20 min of incubation, the incorporation in synaptosomes increased as a function of time up to 60 min of incubation. We conclude that synaptosomes can carry on de novo synthesis of lipids, although at a limited rate. From the present data we cannot state with certainty how much of this synthesis is attributable to membranes originating from the endoplasmic reticulum.     

Mechanism of palmityl coenzyme A inhibition of liver glycogen synthase.

Palmityl-CoA inhibits free liver glycogen synthase; the concentration required for half-maximum inhibition is 3 to 4 micrometer. Almost complete inhibition was observed at 50 micrometer. Palmityl-CoA inhibition is associated with dissociation of the tetrameric enzyme into monomers, and binding of palmityl-CoA to the monomers. Glycogen-bound enzyme is also inhibited by palmityl-CoA, resulting in dissociation of the enzyme into monomers and concomitant release of the enzyme from the primer glycogen. Palmityl-CoA inhibition of the enzyme is partially reversed by the glycogen synthase activator, glucose-6-P, whereas sodium lauryl sulfate-inhibited enzyme is not reactivated by glucose-6-P. Sodium lauryl sulfate inhibition results in the dissociation of the tetramer into the monomers. Bovine serum albumin and cyclodextrin can prevent palmityl-CoA inhibition only when they are added prior to palmityl-CoA addition. The possible physiological role of palmityl-CoA in glucose homeostasis is discussed.     

Lipid conjugates of antiretroviral agents. II. Disodium palmityl phosphonoformate: anti-HIV activity, physical properties, and interaction with plasma proteins

Disodium palmityl phosphonoformate, a novel lipid phosphoester of the anti HIV agent phosphonoformate (foscarnet), inhibits HIV replication in H9 cells and syncytia formation in MOLT-3 cells as effectively as foscarnet itself, as shown by dose-response data from assays for expression of p17 and p24 viral antigens and syncytia formation. Protein binding studies indicate that in serum, the derivative exists bound to albumin and the lipoproteins, and would therefore be likely to exhibit improved serum lifetime in vivo.     

Effects of palmityl coenzyme A and palmitylcarnitine on phosphorylating respiration in heart mitochondria

Glutamate-supported respiration in mitochondria is inhibited by palmityl-CoA in the presence of carnitine. Palmityl-CoA-induced lag phase and depressed state 3 rates increase with increasing ADP. Palmityl-CoA inhibition of state 3 respiration with glutamate shows an increased I₅₀ for palmityl-CoA (three to fourfold) when ADP increases and carnitine is present. ADP alone has a small effect. Glutamate-supported respiration is more profoundly inhibited by palmityl-CoA (+carnitine) than palmityl-CoA oxidation. With palmityl-CoA (+ carnitine) alone, the I₅₀ for palmityl-CoA is two-to threefold greater than when glutamate is also present. Active respiration with palmityl-CoA as substrate demonstrates a 2.5-fold greater apparent affinity for ADP than when glutamate is also present. The kinetics are competitive in both cases. Palmitylcarnitine, above 30 μm, produces inhibition of glutamate-supported respiration, concomitant with mitochondrial swelling and eventual lysis. At 15 μm palmitylcarnitine (minimal swelling), succinate (+ rotenone)-supported respiration decreases with a decrease in K_app for ADP; no effect of 15–20 μm palmitylcarnitine on glutamate-supported respiration is observed. However, palmityl-CoA (+ carnitine)-inhibited respiration with glutamate is further decreased with 15 and 20 μm palmitylcarnitine, i.e., by 13 and 29%, respectively. Inhibition is competitive with ADP. With 3 μm palmitylCoA and 20 μm palmitylcarnitine, a decrease in carnitine (1.5 to 0.25 mm) decreases the apparent K_i for palmityl-CoA from 2.6 to 1.8 μm. The results suggest that glutamate increases the palmityl-CoA available to inhibit adenine nucleotide transport. Inhibition may take place external to the inner membrane. Competition of carnitine and palmitylcarnitine for substrate sites may explain the decreased apparent K_i for palmityl-CoA as carnitine decreases.     

Myristyl and palmityl acylation of pI 5.1 carboxylesterase from porcine intestine and liver.

Immunoblotting analyses revealed the presence of carboxylesterase in the porcine small intestine, liver, submaxillary and parotid glands, kidney cortex, lungs and cerebral cortex. In the intestinal mucosa, the pI 5.1 enzyme was detected in several subcellular fractions including the microvillar fraction. Both fatty monoacylated and diacylated monomeric (F1), trimeric (F3) and tetrameric (F4) forms of the intestinal protein were purified here for the first time by performing hydrophobic chromatography and gel filtration. The molecular mass of these three enzymatic forms was estimated to be 60, 180 and 240 kDa, respectively, based on size-exclusion chromatography and SDS/PAGE analysis. The existence of a covalent attachment linking palmitate and myristate to porcine intestinal carboxylesterase (PICE), which was suggested by the results of gas-liquid chromatography (GLC) experiments in which the fatty acids resulting from alkali treatment of the protein forms were isolated, was confirmed here by the fact that [3H]palmitic and [3H]myristic acids were incorporated into porcine enterocytes and hepatocytes in cell primary cultures. Besides these two main fatty acids, the presence of oleic, stearic, and arachidonic acids was also detected by GLC and further confirmed by performing radioactivity counts on the 3H-labelled PICE forms after an immunoprecipitation procedure using specific polyclonal antibodies, followed by a SDS/PAGE separation step. Unlike the F1 and F4 forms, which were both myristoylated and palmitoylated, the F3 form was only palmitoylated. The monomeric, trimeric and tetrameric forms of PICE were all able to hydrolyse short chain fatty acids containing glycerides, as well as phorbol esters. The broad specificity of fatty acylated carboxylesterase is discussed in terms of its possible involvement in the metabolism of ester-containing xenobiotics and signal transduction.     

The palmityl binding sites of fatty acid synthetase from yeast.

Fatty acid synthetase was covalently labelled with [14C]palmitic acid from [14C]palmityl-CoA. Tryptic and peptic digestion of the [14C]palmityl enzyme resulted in the formation of radioactive palmityl peptides carrying the long-chain acyl residue both in oxygen-ester and thio-ester linkage. The lipophilic palmityl peptides were purified by column and thin-layer chromatography using organic lolvent systems. Peptides arising from the acyl carrier protein, the condensing enzyme and the palmityl transferase were identified and characterized. The amino acid sequence of a 4'-phosphopant-etheine-containing peptide was established. It comprises 13 residues and shows a high degree of homology with the acyl carrier protein from Escherichia coli. A heptapeptide and an octapeptide from the palmityl transferase active site were partially sequenced. The identical amino acid composition of palmityl transferase and malonyl transferase core peptides is briefly discussed.     

Inhibition of palmityl carnitine oxidation in rat liver mitochondria by tert-butyl hydroperoxide

Mitochondria as an energy generating cell device are very sensitive to oxidative damage. Our previous findings obtained in hepatocytes demonstrated that Complex I of the respiratory chain is more sensitive to oxidative damage than other respiratory chain complexes. We present additional data on isolated mitochondria showing that palmityl carnitine oxidation is strongly depressed at a low (200 microM) tert-butyl hydroperoxide (tBHP) concentration, while oxidation of the flavoprotein-dependent substrate - succinate is not affected and neither is ATP synthesis inhibited by tBHP. In the presence of tBHP, the respiratory control index for palmityl carnitine oxidation is strongly depressed, but when succinate is oxidized the respiratory control index remains unaffected. Our findings thus indicate that flavoprotein-dependent substrates could be an important nutritional factor for the regeneration process in the necrotic liver damaged by oxidative stress.