3.2. Sarcoplasmic proteome analyses

3.2. Sarcoplasmic proteome analyses
The gel image of ISM is presented in Fig. 4, where 186 protein spots
were identified by image analyses. Fifteen spots were differentially
abundant in ISM and OSM 48 h post-mortem, and they are encircled
and numbered. These spots exhibited 1.5-fold difference (P b 0.01)
between ISM and OSM. Among the differentially abundant spots, 10
were over-abundant in OSM, whereas 5 spots were over-abundant in
ISM. Tandemmass spectrometry determined the identity of differentially
abundant spots, and all proteins were matched to the bovine family
(Bos taurus) in the NCBI database (Table 2). Six proteinswere identified
to be differentially abundant between ISM and OSM (Table 2). Of
these, triosephosphate isomerase and creatine kinase M-type were
over-abundant in OSM, whereas beta-enolase, fructose-bisphosphate
aldolase A, phosphoglycerate mutase 2, and phosphatidylethanolaminebinding
protein 1 were over-abundant in ISM (Table 2). Several spots
with similar molecular weight, but different isoelectric point (pI),
were identified as the same protein; therefore, only 6 proteins were
identified to be differentially abundant. Detection of protein isoforms
in 2-DE gels have been reported previously (Joseph et al., 2012) and
could be attributed to post-translational modification such as phosphorylation
(Anderson et al., 2014). Phosphorylation could lead to
acidic shift in pI of proteins (Maurides, Akkaraju, & Jagus, 1989; Zhu,
Zhao, Lubman, Miller, & Barder, 2005). Nonetheless, phosphorylation
was not confirmed in the present study. Previous research on
intermuscular variation in beef color stability reported differential abundance
of sixteen proteins between color-stable (longissimus lumborum)
and color-labile (psoas major) muscles (Joseph et al., 2012). However,
the present study is the first one to investigate the proteomic basis for
the intramuscular variation in beef color stability.
3.3. Correlation of differentially abundant proteins with color traits
The correlation between differentially abundant sarcoplasmic
proteins and meat color attributes is presented in Table 3. Creatine
kinaseM-type and triosephosphate isomerase exhibited positive correlation
(P b 0.05) with MRA and R630/580 (color stability), whereas
beta-enolase and phosphoglycerate mutase 2 exhibited negative
correlation (P b 0.05) with MRA and R630/580. In addition, fructosebisphosphate
aldolase A, and phosphatidylethanolamine-binding
protein 1 exhibited negative correlation (P b 0.05) with MRA.
3.4. Functional roles of differentially abundant proteins
The functional roles of differentially abundant proteins are listed in
Table 4. Beta-enolase, fructose-bisphosphate aldolase A, phosphoglycerate
mutase 2, and triosephosphate isomerase are involved in glycolysis,
whereas creatine kinase M-type and phosphatidylethanolaminebinding
protein is involved in ATP regeneration and ATP binding,
respectively. Among the glycolytic enzymes, fructose-bisphosphate
aldolase A, phosphoglycerate mutase 2, and beta-enolase were overabundant
(P b 0.01) in color-labile ISM whereas triosephosphate
isomerase was over-abundant (P b 0.01) in color-stable OSM.
Fructose-bisphosphate aldolase A, also known asmuscle-type aldolase
(www.uniprot.org) is an enzyme catalyzing the cleavage of fructose
1, 6-bisphosphate to glyceraldehyde 3-phosphate and dihydroxyacetone
phosphate; it exhibited a negative correlation (P b 0.05) with
MRA. On their investigations on pork quality, Zelechowska, Przybylski,
Jaworska, and Santé-Lhoutellier (2012) reported that aldolase A present
in the drip exhibited a positive correlation to b* values of longissimus
chops. Furthermore, a greater level of aldolase activity was observed
in rabbit meat with higher glycolytic characteristics compared with
the control group (Ramirez et al., 2004).
Triosephosphate isomerase is also a glycolytic enzyme, which
catalyzes the interconversion of glyceraldehyde 3-phosphate and
dihydroxyacetone phosphate (Albery & Knowles, 1976). Five spots
(spots 51–55 in Fig. 4) were identified as triosephosphate isomerase,
were of over-abundance (P b 0.01) in color-stable OSM, and exhibited
positive correlation (P b 0.05) with MRA and R630/580. In support,
Joseph et al. (2012) also reported over-abundance of triosephosphate
isomerase in color-stable beef longissimus lumborum in comparison
with color-labile psoas major.

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