Induced formation of dimethylamine and formaldehyde by lizardfish (Saurida micropectoralis) kidney trimethylamine-N-oxide demethylase

The distribution of trimethylamine-N-oxide demethylase (TMAOase) in various internal organs of lizardfish (Saurida micropectoralis) and its effect on the solubility of myofibrillar proteins were studied. Among the internal organs tested, kidney contained the highest TMAOase activity, followed by spleen, bile sac, intestine and liver, respectively. TMAOase from the kidney showed a higher activity at pH 7.0 than at pH 5.0. The molecular mass was estimated to be 128 kDa, based on activity staining. In the presence of trimethylamine-N-oxide (TMAO) and cofactors (FeCl2, ascorbate and cysteine), addition of partially purified kidney TMAOase resulted in an increase of dimethylamine (DMA) and formaldehyde in washed mince from both red seabream (Pagrus major), white muscle fish, and skipjack (Katsuwonus pelamis), red muscle fish, during iced storage, especially in the first 6 days. TMAOase addition led to a loss in protein solubility, which is associated with increased cross-linking of protein. Therefore, contamination of muscle by TMAOase from internal organs, especially kidney, possibly causes the changes in fish texture.