12-[(5-iodo-4-azido-2-hydroxybenzoyl)amino]dodecanoic acid: biological recognition by cholesterol esterase and acyl-CoA:cholesterol O-acyltransferase

Design, synthesis and pharmacological characterization of a potent radioiodinated and photoactivatable peptidic oxytocin antagonist

Using a segment strategy, we have synthesized four iodinated photoactivatable cyclic peptidic ligands of oxytocin, bearing a beta-mercapto-betabeta-cyclopentamethylene propionic group (Pmp) on their N-terminus. All the syntheses were RP-HPLC monitored, and the compounds were HPLC purified. They were characterized by 1H NMR, MALDI-TOF, or FAB mass spectrometries. The affinities of Pmp-Tyr(Me)-Ile-Thr-Asn-Cys-Gly-Orn-Phe(3I,4N3)-NH2 (20), Pmp-Tyr-Ile-Thr-Asn-Cys-Gly-Orn-Phe(3I,4N3)-NH2 (21), Pmp-Tyr(Me)-Ile-Thr-Asn-Cys-Pro-Orn-Phe(3I,4N3)-NH2 (22), and Pmp-Tyr-Ile-Thr-Asn-Cys-Pro-Orn-Phe(3I,4N3)-NH2 (23) were evaluated as inhibition constants (K(i), in nM) for the human oxytocin receptor expressed in Chinese hamster ovary cells by displacement of a radioiodinated disulfide-cyclized antagonist (Elands et al. Eur. J. Pharmacol. 1987, 147, 197-207). The most potent of them, compound 22, was synthesized by another method in order to allow its radiolabeling by 125I. Its dissociation constant (K(d)) for the human oxytocin receptor, directly measured in saturation studies, was 0.25 +/- 0.04 nM, and its antagonist properties were determined by inactivation of phospholipase C, thus obtaining an inactivation constant (K(inact)) of 0.18 +/- 0.02 nM, evaluated by inositol phosphate accumulation. This compound is a very good tool for the mapping of peptidic antagonist binding sites in the human oxytocin receptor.

Acyl-coenzyme A:cholesterol acyltransferase

Due to its presumed role in regulating cellular cholesterol homeostasis, and in various pathophysiological conditions, acyl-coenzyme A:cholesterol acyltransferase (ACAT) has attracted much attention. Cloning the ACAT gene provides the necessary tool to advance molecular studies of this enzyme. The topics reviewed in this chapter include the pathophysiological roles of ACAT, the biochemistry and molecular biology of the ACAT protein and the ACAT gene, and the mode of regulation by sterol or nonsterol agents in mammalian cells. In addition, we present a working model linking the presumed allosteric property of ACAT with cholesterol trafficking into and out of the endoplasmic reticulum.

Photolabeling acyl CoA binding proteins in microsome preparations

To identify key enzymes that participate in acylglycerol metabolism, 12-[(5-iodo-4-azidosalicyl)amino]dodecanoyl-coenzyme A (ASACoA) was employed as a photoaffinity label for those enzymes that use fatty acyl CoA as a co-substrate. ASACoA inhibited diacylglycerol acyltransferase activity in liver microsomes and was incorporated into triacylglycerol in a microsome dependent reaction. When photoactivated, ASACoA labeled four proteins in rat liver (75, 54, 50 and 37 kDa) and in epididymal fat cell (75, 64, 54 and 37 kDa) microsomes. Photolabeling was sensitive to palmitoyl CoA. After solubilization in Triton X-114, all four proteins were concentrated into the detergent phase, indicating that they are integral membrane proteins.

Synthesis of the cholesteryl ester prodrugs cholesteryl ibuprofen and cholesteryl flufenamate and their formulation into phospholipid microemulsions

Phospholipid micoremulsions have been suggested as a drug-delivery system for hydrophobic compounds. In this study hydrophobicity was achieved by derivatizing with cholesterol. Cholesteryl ibuprofen (3) and cholesteryl flufenamate (4) were synthesized. 3 was isolated as an amorphous, white solid with a melting range of 114-120 degrees C. 4 was isolated as a crystalline, white solid with a melting range of 145-148 degrees C. The proposed structures of 3 and 4 were supported by IR, NMR, MS, and organic microanalysis. Phospholipid:cholesteryl ester microemulsions were prepared by the addition of a 1-propanol solution of the cholesteryl ester, other lipids, and phospholipid to a rapidly mixing KCl/KBr solution. The hydrophobic phase was modified by the addition of cholesteryl oleate or triolein to study the effect of the fluidity of the hydrophobic core on the formation of the microemulsions. The results indicated that a molar ratio of 75:25 and a total lipid concentration of 60 mg/mL consistently gave microemulsions with a mean size of 100-150 nm. In addition, the formation of eutectic mixtures of 3 and 4 with cholesteryl oleate were determined to be 16% (w/w) for 3 and 12% (w/w) for 4; melting points were 35.2 and 45.2 degrees C, respectively. The solubilities of 3 and 4 in triolein were determined to be 13.2% (w/w) and 11.5% (w/w), respectively. Other investigators have shown that if the core of a phospholipid:cholesteryl estermicroemulsion exists in a liquid state at physiologic temperature, the turnover of the cholesteryl esters from these microemulsions occurs at a faster rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Photolabeling of soybean microsomal membrane proteins with photoreactive acyl-CoA analogs

Synthesis of 32P-labeled 12-azidooleoyl-CoA and 125I-labeled 12-[(azidosalicyl)amino]dodecanoyl-CoA (ASD-CoA) was achieved. The synthesized radioactive, photoreactive reagents were tested as photoaffinity labels for acyl-CoA:lysophosphatidylcholine acyltransferase from the microsomal membranes of developing soybean cotyledons. When a mixture of microsomal membranes and the azidooleoyl-CoA or ASD-CoA were incubated in the dark, the analogs were recognized as substrate and competitive inhibitor, respectively. The enzyme preferentially utilizes unsaturated acyl-CoAs rather than saturated acyl-CoAs. Incubation of microsomal membranes with acyl-CoA analogs and immediately followed by photolysis resulted in an irreversible inhibition of lysophosphatidylcholine acyltransferase activity. Analysis of photolyzed microsomal membranes by SDS/PAGE and autoradiography revealed that azidooleoyl-CoA preferentially labeled eight acyl-CoA binding proteins, but ASD-CoA labeled only three polypeptides with molecular masses of 110, 90 and 32 kDa that are commonly labeled by both the analogs. Oleoyl-CoA and dodecanoyl-CoA protect the enzyme against photoinactivation by azidooleoyl-CoA and ASD-CoA, respectively. The protection was profound in 110-kDa polypeptide indicating that this protein could be lysophosphatidylcholine acyltransferase. These results demonstrate that the photoaffinity of acyl-CoA analogs makes them potential probes to identify and characterize lipid biosynthetic enzymes.

Photoaffinity labeling of acyl-CoA oxidase with 12-azidooleoyl-CoA and 12-[(4-azidosalicyl)amino]dodecanoyl-CoA

Synthesis of 32P-labeled CoA of high specific activity was achieved using partially purified dephospho-CoA kinase (EC 2.7.1.24) from pig liver with [gamma-32P]ATP as donor and dephospho-CoA as acceptor. A photoaffinity dodecanoic acid analog, 12-[(4-azidosalicyl)amino]dodecanoic acid was synthesized, as were its CoA derivative (ASD-CoA) and the CoA derivative of 12-azidooleic acid. The CoA derivatives were synthesized from azido fatty acid analogs by acyl-CoA synthetase. The synthesized photolabile reagents were tested as photoaffinity labels for acyl-CoA oxidase (EC 1.3.99.3) from Arthrobacter species. When a mixture of oxidase and the acyl-CoA analogs were incubated in the absence of ultraviolet light, the analogs were recognized as substrate. Acyl-CoA oxidase was incubated in the presence of acyl-CoA analogs and immediately photolyzed, which resulted in irreversible inhibition. Oleoyl-CoA and dodecanoyl-CoA protect the enzyme from photoactivated inhibition by 12-azidooleoyl-CoA and ASD-CoA, respectively. Analysis of photolyzed enzyme preparations by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography revealed that both analogs preferentially labeled a 54,000 molecular weight protein. These results demonstrate that the photoaffinity acyl-CoA analogs have potential application as probes to identify and characterize lipid biosynthetic enzymes and to identify the active site of these proteins.

Photoreactive fatty acid analogues that bind to the rat liver fatty-acid binding protein: 11-(5'-azido-salicylamido)-undecanoic acid derivatives

Photoreactive probes for the hydrophobic pocket of the liver fatty acid-binding protein, 11-(5'-azido-salicylamido)-undecanoic acid (5' ASU) and its acetyl ester (Ac5' ASU), were synthesized and their interaction with the protein was assessed. Fatty acid-binding proteins are closely related proteins which are abundantly expressed in tissues with active lipid metabolism. A simple model that assumes that the protein possesses a single kind of sites fitted the binding of radioiodinated 5' ASU to L-FABP satisfactorily. The apparent dissociation constant, 1.34 x 10(-7) M, evidenced a slightly higher affinity than that reported for C16-C20 fatty acids. Consistent with the binding curve, 5' ASU effectively competed with palmitic acid for the hydrophobic sites and the effect was nearly complete for concentrations of 1 microM; oleic acid, in turn, displaced the radiolabelled probe. Irradiation at 366 nm of 125I-5' ASU bound to L-FABP caused the covalent cross-linking of the reagent. The amount of radioactivity covalently bound reached a maximum after 2 min thus agreeing with the photo-activation kinetics of the unlabelled compound that evidenced a t1/2 of 31.1 sec. The yield with which probes bound to L-FABP became covalently linked to the protein, appraised after SDS-PAGE of irradiated samples, was estimated as 23 and 26 per cent for 5' ASU and Ac5' ASU respectively. In turn, irradiation of L-FABP incubated with 5' ASU or Ac5' ASU resulted in the irreversible loss of about one fourth its ability to bind palmitic acid.(ABSTRACT TRUNCATED AT 250 WORDS)