Hydrogenation of biological membranes

The presence of unsaturated lipids in biological membranes confers a fluid character on the structure and this is integral to its function. Evidence for this has been established by homogeneous catalytic hydrogenation of these lipids in membrane 

Horvath, M. Droppa, T. Szito, L. A. Mustardy, L. I. Horvath, L. Vigh, Biochim. Biophys. Acta 1986, 849, 325. 

HorvAth, Z. Torok, L. Vigh, M. Kates, Biochim Biophys. Acta 1991, 1085,126. 

Demandre, L. Vigh, A. M. Justin, A. Jolliot, C. Wolf, P. Mazliak, Plant Sci. 1986, 

It is important to realize, that within a certain lipid class there is still a very large number of individual lipid molecules, which can be formed by a great variety of combinations of two fatty acids differing in their chain length (carbon number), in unsaturation (number of C=C bonds) and in their position on the glycerol backbone (Cl or C2). For simplicity, fatty acids are often denoted by the number of carbon atoms number of C=C bonds, e.g. 18 0 for stearic, 18 1 for oleic/elaidic, 18 2 for linoleic, 18 3 for linolenic acids, etc. Any chemical manipulation on lipids of natural origin usually involves simultaneous reactions of quite a few similar but not identical substrate molecules. On the other hand, individual lipids may have specific biological role which -at least in principle- could be assessed by a selective chemical transformation or removal of a lipid of particular composition. 

One such chemical transformation is the catalytic hydrogenation of C=C bonds in the fatty acyl residues of unsaturated polar lipids [308,309], However, if we want to hydrogenate lipids within an intact membrane then the reaction has to be done in an aqueous environment, since on dissolution in organic solvents the membrane structure is lost. Certain lipids, including phosphatidyl cholines form vesicles (liposomes) in water upon dispersion by ultrasound, and such liposomes are often used in studies of the basic characteristics of membrane hydrogenations. 

Catalytic hydrogenation of biological membranes has some distinct features compared to that of water soluble olefins, and the differences are not the least negligible. 

Finally, one also has to consider, that hydrogenation of live cells creates un unnatural composition and physical state (increased rigidity) of the membranes. In case the cells survive this treatment, they mobilize all their reserves to restore the original conditions, optimal for life at the given temperature, pressure, culture medium, etc. Detection of de novo synthetized unsaturated lipids during hydrogenation of protoplasts from tobacco leaf (Nicotiana plumbaginifolid) proved that such compensation of unfavourable outside effects can follow the stress very fast (immediately) and can be very effective [214], 

As can be seen from this short overview of the hydrogenation of biomembranes, one has to be very careful in the experimental work (choice of catalyst, conditions, reaction time, etc.) and even more careful in the interpretation of the changes such a manipulations bring in the properties of live cells. Nevertheless, the results outweigh the problems, and catalytic hydrogenation has become a powerful tool in membrane biochemistry [331], A selection of the various systems which have already been studied by this method is given in Table 3.11 Further details can be obtained from the references in the Table. 

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The heart has a relatively low catalase activity, which, together with the superoxide dismutase (SOD) system, acts to remove hydrogen peroxide and superoxide radicals. In addition, in man, dietary vitamin C plays an important role in the reduction of vitamin E, an intrinsic antioxidant component of biological membranes (Chen and Thacker, 1986 Niki, 1987). Both vitamins C and E can also react directly with hydroxyl and superoxide radicals (HalliwcU and Gutteridge, 1989 Meister, 1992). 

Localization of double bonds in unknown compounds has frequently been determined by ozonolysis. Unsaturated fatty acids of biological membranes are susceptible to ozone attack, but there are some important differences from autoxidation reactions. These include the fact that malonaldehyde is produced from linoleate by ozonolysis (53) but not autoxidation and also that ozonolysis does not cause double bond conjugation as judged by absorption at 233 nm (52). Reactions with the polyunsaturated fatty acids produce several possibilities for toxic reactions direct disruption of membrane integrity and toxic reactions caused by fatty acid hydroperoxides, hydrogen peroxide, and malonaldehyde. 

Lipid peroxidation of biological membranes is a destructive process, proceeding via an autocatalytic chain reaction mechanism [73]. Membrane phospholipids contain hydrogen atoms adjacent to unconjugated olefinic bonds, which make them highly susceptible to free radical oxidation. This is characterised by an initiation step, one or more propagation steps and a termination step [1], which may involve the combination of two radical species or interaction with an antioxidant molecule such as vitamin E. The products formed from such reactions include lipid peroxides, lipid alcohols and aldehydic by-products such as malondialdehyde and 4 hydroxynonenal [73]. 

The idea that chemical modification of the lipids of biological membranes could be achieved in situ was first demonstrated in 1976 [1]. The rationale underlying the work was that if the unsaturated double bonds were largely responsible for the fluid character of the membrane lipid matrix, their saturation would result in a reduction in fluidity. Although simple in concept, the practice required application of an entirely novel approach to the catalytic hydrogenation of lipids. 

The water-saturated 1-octanol phase resembles natural non-polar phases in such that its polarity is similar to that of biological membranes and the OH-function of 1-octanol is assumed to model the hydrogen-bonding donor/acceptor and dipolar effects of real macromolecular systems. 

The role of the phospholipids is primarily as constituents of the lipoprotein complexes of biological membranes. They are widely distributed, being particularly abundant in the heart, kidneys and nervous tissues. Myehn of the nerve axons, for example, contains up to 55 per cent of phospholipid. Eggs are one of the best animal sources and, among the plants, soya beans contain relatively large amounts. The phospholipids contain phosphorus in addition to carbon, hydrogen and oxygen. 

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