Malonaldehyde

Malonaldehyde, a three carbon dialdehyde, is one of the numerous carbonyl compounds associated with oxidative deterioration of food lipids. It is assumed to be formed as a product of lipid autoxidation occurring according to the generally accepted free radical mechanism. This compound is of great interest because of its potential high reactivity and its wide occurrence in autoxidized lipids. Malonaldehyde may participate in eliciting some of the adverse biological and chemical effects of in vitro and in vivo lipid autoxidation. In this investigation a micro 2-thiobarbituric acid analysis for malonaldehyde in various biological materials was developed; the acute toxicity, subacute toxicity and associated physiological responses were determined; and the reaction of malonaldehyde with glycine and protein was investigated. The single oral median lethal dose (LD₅₀) of malonaldehyde administered to rats as the ethoxy derivative, 1, 1, 3, 3-tetraethoxy propane, and the enolic sodium salt, sodium beta-oxyacrolein, was found to be 527 mg (slope function 1.67) and 632 mg (slope function 1.23) per kg body weight respectively. The LD₅₀ of malonaldehyde determined with sodium beta-oxyacrolein was the more accurate estimation of its acute toxicity. Subacute feeding of rats malonaldehyde as the ethoxy derivative, 1, 1, 3, 3-tetraethoxypropane, and as the sodium salt of the enamine derivative of glycine, sodium N-prop-2-enal amino acetate, elicited a series of similar varied and marked anatomical, physiological and/ or biochemical changes over control animals. Ingested malonaldehyde was largely metabolized or altered so that it was no longer detectable by reaction with 2-thiobarbituric acid. It was distributed throughout the body and particularly in the blood and glandular tissue. Dietary malonaldehyde was found to be concentrated in erythrocytes where it had apparently reacted in vivo with the intracellular protein, tentatively with hemoglobin. A decreased growth rate, a lowered diet utilization, varying degrees of anemia, increased organ weights, symptoms of various vitamin deficiencies, and abnormal nitrogen and tryptophan metabolism were observed. Histomorphological damage to liver tissue included a lytic necrosis of parenchymal cells, increased parenchymal cell and nuclear size, bile ductular proliferation with vacuolation of parenchymal cytoplasm with cyst formation. The response to dietary malonaldehyde probably involved both a dirept toxic action at the cellular level and an interaction with some fraction(s) of the diet altering its nutrient value. The reaction of malonaldehyde with glycine was found to yield the enamine, N-prop-2-enal amino acetic acid. The reaction was shown to conform to a S [subscript n] 2 mechanism. The rate of the reaction was shown to be highly dependent on the hydrogen ion concentration. The maximum observed reaction rate was found near pH 4.20. A postulated mechanism for the reaction involves the nucleophilic 1, 4-addition of the amino nitrogen of glycine to the end carbon atom of the alpha, beta-unsaturated carbonyl system of the enol of malonaldehyde. The reaction of malonaldehyde with bovine plasma albumin was shown to involve the e-amino lysine and N-terminal amino aspartic acid functions as judged by their loss to reaction with 1-fluoro-2, 4- dinitrobenzene. The rate of the reaction was dependent on the hydrogen ion concentration with a maximum observed reaction rate near pH 4.30. Malonaldehyde did not appear to participate in an intermolecular cross-linking reaction as judged by the lack of viscometric hardening of gelatin sols. Presumptive evidence for the reaction of malonaldehyde, derived from autoxidized lipids, and the ε-amino lysine functions of bovine plasma albumin was obtained. A postulated mechanism for the reaction of malonaldehyde with protein involves the nucleophilic 1,4-addition of the free amino functions on the protein to the end carbon atom of the alpha, beta-unsaturated carbonyl system of the free enol to form an enamine linkage.