Studies in rats on the nutritional value of hydrogen peroxide-treated fish protein and the utilization of oxidized sulphur-amino acids

1. The nutritional value of fish protein oxidized with different levels of hydrogen peroxide, and the utilization of oxidized sulphur-amino acids in the rat were investigated.

2. The results showed that the biological value (bv) and protein efficiency ratio (per) of the fish protein were significantly decreased by the treatment with H₂O₂, and although there were no significant differences between samples treated with 20, 40 or 80 g H₂O₂/kg, there was a gradual decrease of BV at higher concentrations. Digestibility was not affected by the treatment.

3. Supplementation with 1 g L-methionine/kg or 1 g L-cystine/kg significantly increased the BV for all H₂O₂-treated samples. With the exception of the sample treated with 80 g H₂O₂/kg, the BV was increased by L-methionine supplementation to the level for the untreated fish protein.

4. Treatment with H₂O₂led to the formation of methionine sulphoxide, methionine sulphone and cysteic acid. The availability of these compounds was studied as supplements to the untreated fish protein as well as to the H₂O₂-treated fish protein. The results showed that L-methionine DL-sulphoxide was as available as methionine, but that L-methionine sulphone and cysteic acid had no supplementary effect.

5. The availability of lysine in fish protein was not affected by treatment with 20 g H₂O₂/kg as judged by its effect as a supplement to wheat flour.

6. The plasma amino acid pattern for rats given a diet with H₂O₂-treated fish protein was identical to that for rats given an untreated fish protein, with the exception of the presence of the two isomers of L-methionine DL-sulphoxide and L-methionine sulphone. The oxidized S-amino acids were also found in liver, kidney and gastrocnemius muscle from the rat.

7. Only traces of L-methionine DL-sulphoxide were found in the urine. About half the ingested L-methionine sulphone was found in the urine and almost all was present as an acid-labile conjugate.