Synthesis,processing, and secretion of hepatic very low density lipoprotein

Very low density lipoprotein (VLDL) is the major vehicle in the plasma which carries triacylglycerol synthesized in the liver to peripheral tissues for utilization. Estrogen-induced chick parenchymal liver cells (hepatocytes) synthesize and secrete large amounts of VLDL. These cells, in a primary monolayer culture system developed in this laboratory, have been employed to study the operative and regulatory aspects of VLDL synthesis, assembly, and secretion. Some 10 min are required for the translation of the principle VLDL protein constituent, apolipoprotein B, and 30–35 min are required for the two newly translated chick VLDL apolipoproteins, apolipoprotein B and apolipoprotein II, to be secreted. Apolipoprotein B is synthesized on membrane-bound polysomes as a contiguous polypeptide chain of 350K molecular weight (MW) and is not assembled posttranslationally from smaller-peptide precursors. Translocation of puromycin-discharged apolipoprotein B nascent chains into the endoplasmic reticulum lumen and their subsequent secretion are independent of both ongoing protein synthesis and the attachment of the nascent peptides to ribosomes. Apolipoprotein B nascent chains discharged by puromycin assemble with glycerolipid (mainly triacylglycerol) and are secreted as immunoprecipitable VLDL. Core oligosaccharides are added to the apolipoprotein B nascent chain co-translationally in at least two stages, at molecular weights of ～ 120K and ～ 280K. Inhibition of N-linked glycosylation of apolipoprotein B with tunicamycin affects neither the assembly of glycerolipids into VLDL nor the secretion of the VLDL particle, indicating that aglyco-apolipoprotein B can serve as a functional component for VLDL assembly and secretion. Active synthesis of the VLDL apolipoproteins is required, however, for glycerolipid assembly into VLDL and secretion from the hepatocyte. The differential kinetics with which newly synthesized apolipoproteins and glycerolipids are secreted as VLDL and the timing of the effects of protein-synthesis inhibitors on their secretion indicate that VLDL constituents are assembled sequentially in the intact liver cell. The bulk of the VLDL triacylglycerol and some VLDL phosphoglyceride is introduced early in the secretory pathway proximal, yet subsequent to apopeptide synthesis, while a significant fraction of VLDL phosphoglyceride associates with the resulting triacylglycerol-rich lipid-protein complexes just prior to their secretion as mature VLDL. Within the context of current models for VLDL structure, the late assembly of phosphoglyceride into VLDL is taken to represent a surface maturation of the nascent VLDL particle.     

New Glycerolipids from the Traditional Chinese Medicine of Syngnathus acus (Hai-Long)

Two new glycerolipids, syngaculipids A and B ( 1 and 2 ), one first naturally occurring metabolite ( 8 ), together with five known compounds ( 3 – 7 ) were isolated from the AcOEt fraction of Syngnathus acus L. (Hai-Long). Their structures were elucidated by comprehensive spectral analyses involving UV, IR, MS, 1D and 2D NMR spectra and ECD calculations. All the isolated compounds were evaluated for their cytotoxicity against A549 and HCT-116 cell lines. Compound 8 exhibited moderate cytotoxicity with IC₅₀ values of 34.5 and 38.9 μM on the A549 and HCT-116 cell lines, respectively.     

Precise identification of photosynthetic glycerolipids in microalga Tetraselmis chuii by UPLC-ESI-Q-TOF-MS

Precise structural identification of photosynthetic polar glycerolipids in microalga Tetraselmis chuii has been established using Ultra Performance Liquid Chromatography–Electrospray ionization-Quadrupole-Time of Flight Mass Spectrometry (UPLC-ESI-Q-TOF-MS) by direct analysis of the total lipids extract. The mass spectrometry was performed in reflective time-of-flight using electron spraying ionization in both positive and negative modes. The structural determination was based on the char-acteristic product ions yielded by different glycerolipids under ESI-MS/MS mode, and confirmed the molecular species by the carboxylate anions produced by glycerolipids in the negative mode. As a result, more than 40 lipid molecular species, including 11 monogalactosyldiacylglycerols (MGDG), 7 digalactosyldiacylglycerols (DGDG), 16 sulfoquinovosyldiacylglycerols (SQDG), and 9 phosphatidyl-glycerols (PG), were detected in Tetraselmis chuii, which had never been identified before in this mi-croalga. Furthermore, some intact lipid molecules with hydroxylated fatty acids that could not be de-tected by the traditional GC-MS method were found this time, providing novel information for the pho-tosynthetic lipidome of Tetraselmis chuii. Comparative studies on fatty acids at the sn-2 position showed that SQDG and MGDG are dominantly biosynthesized through the prokaryotic pathway, PG is a typically mixed biosynthetic pathway, while DGDG is somewhat peculiar with C14:0 and C16:0 at its sn-2 position. This method could provide a full structural profile of intact photosynthetic lipid molecular species, which may be applied to study the physiological and ecological functions of lipid by monitor-ing their individual changes.     

Precise identification of photosynthetic glycerolipids in microalga Tetraselmis chuii by UPLC-ESI-Q-TOF-MS

Precise structural identification of photosynthetic polar glycerolipids in microalga Tetraselmis chuii has been established using Ultra Performance Liquid Chromatography-Electrospray ionization-Quadrupole-Time of Flight Mass Spectrometry (UPLC-ESI-Q-TOF-MS) by direct analysis of the total lipids extract. The mass spectrometry was performed in reflective time-of-flight using electron spraying ionization in both positive and negative modes. The structural determination was based on the characteristic product ions yielded by different glycerolipids under ESI-MS/MS mode, and confirmed the molecular species by the carboxylate anions produced by glycerolipids in the negative mode. As a result, more than 40 lipid molecular species, including 11 monogalactosyldiacylglycerols (MGDG), 7 digalactosyldiacylglycerols (DGDG), 16 sulfoquinovosyldiacylglycerols (SQDG), and 9 phosphatidylglycerols (PG), were detected in Tetraselmis chuii, which had never been identified before in this microalga. Furthermore, some intact lipid molecules with hydroxylated fatty acids that could not be detected by the traditional GC-MS method were found this time, providing novel information for the photosynthetic lipidome of Tetraselmis chuii. Comparative studies on fatty acids at the sn-2 position showed that SQDG and MGDG are dominantly biosynthesized through the prokaryotic pathway, PG is a typically mixed biosynthetic pathway, while DGDG is somewhat peculiar with C14:0 and C16:0 at its sn-2 position. This method could provide a full structural profile of intact photosynthetic lipid molecular species, which may be applied to study the physiological and ecological functions of lipid by monitoring their individual changes. Supported by the Program for Changjiang Scholars and Innovative Research Team in Universities (PCSIRT) (Grant No. IRT0734), Project of National Ocean Bureau (Grant No. 200805067), Project of Ministry of Science and Technology (Grant No. 2007BAD43B09), and K. C. Wong Magna Fund in Ningbo University     

Lipid metabolism in ageing leaves of dark-grown barley seedlings

The rapid senescence of the etiolated leaves of dark-grown barley seedlings in the dark is accompanied by the loss of those lipids associated with the plastids. The linolenate content of the plastid glycerolipids rapidly decreased whereas it tended to increase in the extraplastidic phospholipids. Kinetin treatment slowed down the loss of the plastid lipids and their constituent fatty acids. The hormone treatment brought about increased linolenate, particularly in phosphatidylcholine and monogalactosyldiacylglycerol. The senescing leaf attempts to adapt to ageing by increased membrane synthesis and/or membrane repair. Kinetin appears to control the sequential desaturation of oleate to linolenate.     

Synthesis of Alkyl Glycerolipids with Functional Groups in Their Polar Heads

A number of alkyl glycerolipids with short-chain substituents at the C2 atom of glycerol and functional groups (carboxy and amino) in the polar head were synthesized. Cationic alkyl glycerolipids with a hydroxyl function in the hydrophilic moiety were also obtained.     

The Synthesis of Cationic Glycerolipid Acetals Containing Aliphatic and Heterocyclic Bases

1,3-Dioxolane series cationic lipids containing residues of aliphatic or heterocyclic nitrogenous bases were synthesized. The bases were attached to the glycerol backbone either directly (piperazine) or via a spacer group through a thioether bond.