REACTION OF CYSTEINE AND METHIONINE WITH MALONALDEHYDE

Malonaldehyde (M), a product of polyunsaturated fatty acid oxidation, reacted with the sulfhydryl as well as with the amino groups of cysteine (cys). The cys-M product had an absorption maximum at 310 mμ, and the extinction coefficient at pH 7.0 was 2.3 × 104. Elementary analysis of the cys-M product agreed with a structure in which 2 moles of cysteine had reacted with 3 moles of malonaldehyde. The molecular weight of cys-M preparations increased on storage and the UV absorption changed from 310 to 315-320 mμ, with a consequent shift to longer wavelength in the visible. Methionine (meth) reacted with malonaldehyde under the same reaction conditions only at the α-amino group, similar to glycine (gly). The apparent pKa of the carboxyl group of gly-M increased to 3.36 and that of meth-M to 3.19, representing an increase of about one pKa unit over the natural amino acids. For gly-M and meth-M the respective absorption maxima were 272 and 282 mμ. The spectral shifts from 267 to 315 mμ of the amino acid-M products with respect to β-oxyacrolein were explained in terms of increasing substitution at the β-carbon of the α,β-unsaturated carbonyl system. When the α-amino-malonaldehyde condensation products of methionine and glycine reacted with semicarbazide the original amino acids and disemicarbazone of malonaldehyde were formed. © 1969 AOCS Press.