No abstract

During the last few decades, plant protein ingredients such as soya proteins have replaced fishmeal in the diets of aquacultured species. This may affect the requirement and metabolism of methionine as soya contains less methionine compared with fishmeal. To assess whether methionine limitation affects decarboxylated S-adenosylmethionine availability and polyamine status, in the present study, juvenile Atlantic salmon were fed a methionine-deficient plant protein-based diet or the same diet supplemented with DL-methionine for 8 weeks. The test diets were compared with a fishmeal-based control diet to assess their effects on the growth performance of fish. Methionine limitation reduced growth and protein accretion, but when fish were fed the DL-methionine-supplemented diet their growth and protein accretion equalled those of fish fed the fishmeal-based control diet. Methionine limitation reduced free methionine concentrations in the plasma and muscle, while those in the liver were not affected. S-adenosylmethionine (SAM) concentrations were higher in the liver of fish fed the methionine-deficient diet, while S-adenosylhomocysteine concentrations were not affected. Putrescine concentrations were higher and spermine concentrations were lower in the liver of fish fed the methionine-deficient diet, while the gene expression of SAM decarboxylase (SAMdc) and the rate-limiting enzyme of polyamine synthesis ornithine decarboxylase (ODC) was not affected. Polyamine turnover, as assessed by spermine/spermidine acetyltransferase (SSAT) abundance, activity and gene expression, was not affected by treatment. However, the gene expression of the cytokine TNF-a increased in fish fed the methionine-deficient diet, indicative of stressful conditions in the liver. Even though taurine concentrations in the liver were not affected by treatment, methionine and taurine concentrations in muscle decreased due to methionine deficiency. Concomitantly, liver phospholipid and cholesterol concentrations were reduced, while NEFA concentrations were elevated. In conclusion, methionine deficiency did not increase polyamine turnover through depletion of hepatic SAM, as assessed by SSAT activity and abundance.

The muscle sarcoplasmic proteins from bovine M. longissimus thoracis muscle were studied using proteomics to identify possible protein markers for meat tenderness. This study included 3 experiments: A1, A2, and B. From a collection of biopsies from the bovine M. longissimus thoracis muscle, excised 4 d before slaughter from 178 Norwegian Red young bulls, 26 biopsies were studied in Exp. A1. Based on Warner-Bratzler shear force (WBSF) values at 7 d postmortem, the biopsies were separated into a tender and a tough group of 13 bulls each and analyzed by 2-dimensional gel electrophoresis (2-DE) and Western blotting. The 2-DE experiments identified 4 different proteins: stress-70 protein, protein DJ-1, peroxiredoxin-6, and malate dehydrogenase, which were different in abundance in the tender and tough groups. However, only peroxiredoxin-6 was confirmed by quantification from Western blots. Peroxiredoxin-6 is an antioxidant enzyme that plays a role in protecting cells from oxidative stress. Peroxiredoxin-6 was identified through 3 spots of the same molecular weight, but with different pI on the Western blots. Only one of the spots was more abundant in the biopsies from the tender group. In Exp. A2, samples collected 1 h postmortem from the same animals and muscles as in Exp. A1 were analyzed by Western blotting. In these postmortem samples, the same spot from peroxiredoxin-6 as in Exp. A1 was more abundant in the tender group. In addition, one of the other peroxiredoxin-6 spots was also more abundant in the tender group. To verify the results from Exp. A, biopsies from 14 additional animals were analyzed in Exp. B by Western blotting against stress-70 protein, protein DJ-1, peroxiredoxin-6, and malate dehydrogenase. No significant differences between the tough and tender groups could be observed in these biopsies. However, for peroxiredoxin-6, the tendencies pointed in the same direction as in Exp. A. In conclusion, peroxiredoxin-6 might be a potential protein marker for meat tenderness detectable in biopsies and in samples collected shortly after slaughter. However, more animals are needed to verify the findings in the present study.