Search

The polymerization of malonyldialdehyde (MDA) formed after the acid hydrolysis of 1,1,3,3-tetraethoxypropane (TEP) and 1,1,3,3-tetramethoxypropane (TMP) has been studied using techniques of gas-liquid, thin-layer, and gel-filtration chromatography. The results show that, even while hydrolysis is still in progress, low-molecular-weight polymers of MDA begin to form. After hydrolysis at least 13 different polymers could be detected. Several of these had absorption peaks in both the visible and the ultraviolet region. With the thiobarbituric acid reagent (TBA) at room temperature the various polymers produced different colour reactions; but heating in 28% trichloroacetic acid leads to depolymerisation and the formation of the characteristic 532 nm MDA TBA coloured complex. MDA polymers could not be shown to account for any of the multiple TBA-reactive compounds produced by autoxidised linolenic acid. In experimental work designed to study the biological activity of MDA in its own right, the use of TEP or TMP as the source material and the strong tendency of MDA to polymerise must be taken into account.

Summary The peroxidation of ox-brain phospholipid liposomes catalysed by oxidised and reduced iron and copper ions have been studied by the measurement of fluorescent non-polar MDA-amino complexes. The inhibitory activity of superoxide dismutase when added to the system implicated superoxide radicals as important intermediates in both reduced and oxidised metal-ion catalysed peroxidation of lipids.

The effect of calcium ions on the peroxidation of ox-brain phospholipid liposomes in different free-radical catalysing systems has been assessed. Using thiobarbituric acid-reactivity (TBA) as a measure of lipid peroxidation, calcium ions both inhibited and enhanced peroxidation in the different systems. Changing the composition of the ox-brain phospholipid liposome with synthetic non TBA-reactive phosphatidylcholine, significantly altered its susceptibility to peroxidation both in the presence and absence of calcium ions. The results are discussed with reference to the possibility that calcium ions induce conformational changes in membrane phospholipids. Susceptibility to peroxidation is then influenced by a complex interrelationship between the qualitative lipid composition of the membrane, the pro-oxidant catalyst and the presence of calcium or other active ions.

The specificity of the thiobarbituric acid reaction (TBA) has been investigated using techniques of high-performance thin-layer chromatography and spectrofluorimetry. It was found that malondialdehyde (MDA) derived from different lipid and nonlipid origins formed the same MDA-TBA complex. This complex could be separated from other TBA-reactive compounds by both chromatography and spectrofluorimetry. Normal human plasma and urine both formed an MDA-TBA complex along with other TBA-reactive compounds. In plasma this was associated mainly with phosphatidylcholine and appeared to be peroxidic in reaction. Urine, however, contained polar MDA-forming compounds probably resulting from the oxidation of 2-deoxyaldoses during the acid-heating stage of the TBA test.