JIA-2018-09

2123 ZHANG Yi-min et al. Journal of Integrative Agriculture 2018, 17(9): 2118–2125 dioxygenase family, catalyzes the formation of formate and 2-keto-4-methylthiobutyrate (KMTB) from 1,2-dihydroxy- 3-keto-5-methylthiopentene and O 2 , indicating its function in oxidoreduction. PDHB, the pyruvate dehydrogenase complex catalyzes the overall conversion of pyruvate to acetyl-CoAand CO 2 , and thereby links the glycolytic pathway to the tricarboxylic cycle pyruvate dehydrogenase (NAD + ) activity, which promotes the production of NADH; it was found positively correlated ( P <0.05) with L*, a*, b* values and negatively correlated with MetMb content in our study. Similar with our results, previous studies also found some oxidoreductases were over abundant in beef samples during early storage, and reported that they are positively correlated with a* values (Yu et al . 2017). The over expression of these oxidoreductive enzymes plays a role in minimizing lipid oxidation-induced myoglobin oxidation, leading to an improved beef color and color stability; or benefiting the production of NADH, which further promotes the conversion of MetMb to Mb. HSPB6 belongs to the family of small heat shock protein (HSP20), sharing significant sequence similarity within the “alpha-crystallin domain”, but exhibiting different patterns of function. It seems to have versatile functions in various biological processes, such as to mitigate protein misfolding and denaturation triggered by noxious environmental stimuli such as decreased pH (Taylor and Benjamin 2005). In the current study, HSPB6 was probably triggered by the fast pH decline resulting from step-chilling, which is supported by previous studies, showing its response to metabolic activity and pH decline, to stabilize myofibrillar proteins (Golenhofen et al . 2000; Gagaoua et al . 2015). It was also reported that another small HSP of the same family, αB-crystallin, was discovered to bind to the myofibril, induced by a slight pH decrease in postmortemmuscle (Lomiwes et al . 2014). The direct relationship between HSPB6 and meat color is not clear yet, its function to maintain denatured proteins in a folding-competent state, may facilitate color development. RNH is a family member of proteinaceous cytoplasmic proteins, and this inhibitor may have a role in the regulation of angiogenin. When cells are stressed, RNH1 is associated with angiogenin to inhibit enzymatic activity so no unnecessary rRNA is produced to save anabolic energy (Pizzo et al . 2013). No previous color-related proteomics study was found to detect this protein, and its relationship with meat color study needs further exploration. Both adenosine kinase (ADK) and adenylate kinase 1 (AK1) were over abundant in RC-treated steaks. They are involved in purine metabolism, while AK1 displays broad nucleoside diphosphate kinase activity. AK1 catalyzes the reversible conversion of ATP to ADP, playing an important role in energy homeostasis, and one of its molecular functions is defined as ATP-binding. Consistent with the current study, Canto et al . (2015) found AK1 was abundant in color-labile beef steaks and Mahmood et al . (2018) found Table 4  The variance (%) of color L*, a*, b* values and MetMb as explained by each protein marker and treatment Spot Protein L* a* b* MetMb 1309 PDHB 75.4 * (+) 64.7 * (+) 78.1 * (+) 86.8 ** (–) 1714 FKBP4 74.8 * (–) 66.4 * (–) 75.7 * (–) 85.8 ** (+) 9208 PRDX1 72.2 * (–) 71.4 * (–) 73.0 * (–) 85.5 ** (+) ** , P <0.01; * , P <0.05. (+) means positive correlation between the expression of protein marker and color trait. (–) means negative correlation between the expression of protein marker and color trait. MW (kDa) 100 10 pH 3 SC RC pH 3 10 10 Fig. 1 Two-dimensional gel images of the sarcoplasmic proteome extracted from beef M. longissimus lumborum muscle treated with step-chilling (SC) or routine chilling (RC). Seven protein spots abundant in SC and four in RC, are numbered. MW, molecular weight. The identified proteins were listed in Table 3.