Reaction of cysteine and methionine with malonaldehyde

Abstract

Abstract 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 × 10 4 . 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 pK a 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 pK a 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.