Laszlo Vigh

Homogeneous catalytic hydrogenation of the polar lipids of pea chloroplasts in situ and the effects on lipid polymorphism

Abstract The pattern of hydrogenation of polar lipids of pea chloroplasts incubated in the presence of the homogeneous catalyst Pd(QS) 2 , a sulphonated alizarine complex of Pd(II) has been examined. Analysis of the fatty acyl residues of the major lipid classes from chloroplast suspensions at intervals during incubation under hydrogenating conditions showed that susceptibility to hydrogenation increased in the order monogalactosyldiacylglycerol > digalactosyldiacylglycerol > sulphoquinovosyldiacylglycerol > phosphatidylglycerol. Almost 80% of the total number of double bonds in the polar lipids were removed after 2-h incubation under the conditions employed. The consequence of hydrogenation on the phase behaviour of total polar lipid extracts in aqueous dispersions were examined by freeze-fracture electron microscopy, X-ray diffraction and differential scanning calorimetry. These data indicate that progressive hydrogenation of tne lipids in situ produce a change in the organisation of the lipid when dispersed in water. Single bilayer vesicles are converted to large aggregates of planar bilayer stacks in which the hydrocarbon chains are predominantly in the gel phase configuration. Studies of lipids dispersed in 20 mM MgCl 2 suggest that cohesion between the hydrocarbon chains gradually ameliorates the repulsive effects of the charged lipids, sulphoquinovosyldiacylglycerol and phosphatidylglycerol. This results in the formation of a sheet-like lamellar phase characteristic of dispersions of saturated monogalactosyldiacylglycerols which dominates the total polar lipid extracts of pea chloroplasts.

Recovery of Dunaliella salina cells following hydrogenation of lipids in specific membranes by a homogeneous palladium catalyst

Unsaturated fatty acyl chains of Dunaliella salina membrane lipids can be catalytically reduced by the homogeneous hydrogenation catalyst palladium di(sodium alizarine monosulphonate), Pd(QS)2, under conditions permitting full recovery of the cells within 24 h. The hydrogenation is accomplished by incubation of cells with the hydride form of Pd(QS)2 under 1 atmosphere of H2 and for 2 min or less. Following this protocol, hydrogenation reduces only those fatty acids located in the plasma membrane and other membranes located near the cell surface. The limited reactivity in vivo is due to the fact the Pd(QS)2 permeates into the living cells more slowly than it does into liposomes prepared from extracted Dunaliella membrane lipids. While Dunaliella is completely unaffected by exposure to the oxygenated, inactive catalyst, hydrogenated cells cease growth for approximately 12 h, during which time the hydrogenated acyl chains are being enzymatically retroconverted to their original unsaturated form. When the lipid composition approaches its prehydrogenation values, growth resumes, presumably due to the restoration of normal membrane functions. The system shows promise for studying the metabolic regulation of membrane microviscosity.

The hydrogenation of phospholipid-bound unsaturated fatty acids by a homogeneous, water-soluble, palladium catalyst

The hydrogenation of unsaturated phospholipids by palladium di(sodium alizarine monosulphonate) activated for 5 min under H2 proceeded rapidly at 20 degrees C and 1 atm. H2. Multibilayer liposomes of dioleoyl- and dilinolenoylphosphatidylcholine were hydrogenated at similar rates while dilinoleoyl- and 1-palmitoyl-2-oleoylphosphatidylcholine were hydrogenated at slightly slower rates. The reduction of polyunsaturated fatty acids gave rise to a variety of natural and unnatural positional cis and trans isomers which were largely reduced further to saturated fatty acids as the hydrogenation continued. Dioleoylphosphatidylethanolamine was attacked by the catalyst more slowly at 20 degrees C than was the equivalent phosphatidylcholine molecular species. Experiments conducted using mixtures of phosphatidylethanolamine and phosphatidylcholine in varying proportions also suggested that phospholipids are slightly more susceptible to catalytic hydrogenation in the bilayer phase than in the hexagonalII phase. Understanding the sequence of hydrogenation reactions involving these one and two component lipid preparations is useful in interpreting the action of the palladium catalyst on living cells under the same mild conditions.

Effects of Catalytical Hydrogenation in Situ of Phosphatidylglycerol-Associated Trans-Δ3-Hexadecenoate on the Stability of the Thylakoid Light Harvesting Complex

Over the past few years steadily increasing evidence (reviewed by Dubacq and Tremolieres 1983) has pointed to a very specific role for phosphatidylglycerol (PG) containing trans- Δ3 -hexadecenoic acid (t-16:1) in stabilizing the oligomeric form of the chlorophyll-containing light harvesting complex proteins (LHCP) of chloroplast. Among the more recent findings supporting this concept are 1) the observation that LHCP of cold hardened rye had a 54% lower level of PG-bound t-16:1 and a 60% lower LHCP oligomer/monomer ratio, as measured on non-denatuing electrophoretic gels, than did LHCP from non-hardened rye (Krupa et al. 1987), 2) correlations between the lack of t-16:1 in Chlamidomonas mutants and their lack of an oligomeric form of LHCP on electrophoretic gels (Maroc et al. 1987) and 3) the finding that hydrolyzing most of the t-16:1-containing PG of tobacco thylakoids with exogenous phospholipase A2 resulted in a substantial loss of the LHCP oligomer band on gels (Remy et al. 1982). In agreement with the above observations, McCourt et al. (1985) noted a disappearance of the LHCP oligomer band from electrophoresis gels of an Arabidopsis thaliana mutant lacking t-16:1. Surprisingly, however, the mutant was no less efficient than wild type Arabidopsis in photosynthetic energy transfer, raising doubts as to the need for the apparent t-16:1 PG stabilisation.