Existence of cysteic acid-related compound in the dark muscle of common mackerel [Scomber japonicus]

In a previous paper it was observed that the high peak of an unidentified substance appeared just before taurine peak on the chromatogram of amino acid analysis from dark muscle of fishes. After hydrolyzing the dark muscle extracts of common mackerel Scomber japonicus, this peak almost disappeared and a new peak was observed in an earlier position on the chromatogram. The peak was not considered to be that of a peptide because other amino compounds were produced only in trace amounts after the hydrolysis, except glycine which was produced from nucleotide and its relat-ed compounds such as inosinic acid in the muscle extracts. Because crystallization could not be performed successfully before the hydrolysis, the substance was isolated from the hydrolyzed dark muscle extracts by the 2-step column chromatography using AG 50 W×8 (H+ form, 200-400 mesh, 2.6×40cm column) and AG 2×8(OH- form, 20-50 mesh, 2.6×40cm column). The molecular weight of the isolated compound was estimated to be 169 from the results of ultimate analysis and 13C-NMR spectra. An IR spectrum indicated peak characteristics of -COOH, -NH2 and -SO3H at 1750cm-1, 1600cm-1 and 1230cm-1, respectively. From these results, the isolated compound was identified as cysteic acid. It was assumed from the chromatographic behaviour of the substance before and after the hydrolysis that cysteic acid-related compound existed in the dark muscle of fish.