SUMMARY—The protein malonaldehyde reaction was studied as a potential mechanism of protein denaturation in muscle, especially during frozen storage. Myosin, a structural protein of muscle, was reacted at pH 6.8 and ionic strength 0.5 with malonaldehyde, an oxidation product of polyunsaturated fatty acids. The rate of reaction with the s‐amino groups of myosin was greater at ‐20° than at 0° and was almost as great as that at +20°. The same relationship was observed when the decreasing malonaldehyde concentration was measured in the protein‐malonaldehyde reaction mixture. Amino acid analyses before and after reaction at 100° for 60 set showed that malonaldehyde reacted preferentially with histidine, arginine, tyrosine, and methionine. In frozen solution, malonaldehyde reacted with lysine, tyrosine, methionine, and arginine in decreasing order of intensity, but it did not react with histidine. The increased rate of reaction in the frozen system is explained as a concentration effect and as a catalytic effect involving the ic
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