Abstract
It was hypothesized that the longissimus thoracis (LT) muscle proteome, phosphoproteome, and metabolome could explain postmortem metabolism and tenderness differences in muscle from cattle supplemented zinc (Zn) and/or ractopamine hydrochloride (RH). High percentage Angus steers (N = 20) were fed in a 2 × 2 factorial assigned to Zn and RH treatments: control (CON; n = 10; analyzed 36 mg Zn/kg dry matter [DM]) or supranutritional Zn supplementation (SUPZN; n = 10; control diet + 60 mg Zn/kg DM [from ZnSO4] + 60 mg Zn/kg DM [from Zn-amino acid complex]) for the entire 89-d trial. During the 28 d before harvest, steers were blocked by body weight within Zn treatments to RH treatments of 0 (NO; n = 10) or 300 mg (RAC; n = 10) per steer per day. Steers were harvested at the Iowa State Meat Laboratory, where pH decline (1, 3, 6, and 24 h postmortem) was measured. At 24 h postmortem, LT muscle sections were removed from carcasses, and steaks were analyzed for Warner-Bratzler shear force (WBSF) values at 1, 3, 7, and 14 d postmortem. Muscle samples were taken at 1 h, 1, 3, 7, and 14 d postmortem for the following analysis: troponin-T degradation (1, 3, 7, and 14 d postmortem), myosin heavy chain analysis (1 h postmortem), sarcoplasmic proteome analysis through tandem mass tagging analysis (1 h and 1 d postmortem), metabolome analysis (1 h and 1 d postmortem), and phosphoproteome analysis (1 h postmortem). SUPZN-NO tended to have a lower (P = 0.06) pH at 6 h postmortem and a lower WBSF value (P = 0.06) at 1 d postmortem. CON-RAC had a higher (P = 0.04) pH at 6 h postmortem and WBSF value (P < 0.01) at 1 d postmortem. A lower pH at 6 h postmortem and lower WBSF value at 1 d postmortem in the SUPZN-NO treatment was accompanied by more sorbitol and fructose at 1 d postmortem, and less myosin regulatory light chain 2 at 1 h postmortem, and less adenosine monophosphate deaminase 1 (AMPD1) at 1 d postmortem than all other treatments. A higher pH at 6 h postmortem and higher WBSF value at 1 d postmortem in CON-RAC and SUPZN-RAC was accompanied by more soluble structural proteins (troponin-T and myosin-7) at 1 h postmortem than CON-NO. At 1 h postmortem, CON-RAC had more glyceraldehyde-3-phosphate dehydrogenase than CON-NO or SUPZN-RAC. Differences in energy metabolism enzymes, metabolites, and structural proteins may affect ATP production, rigor development, and lactate buildup which may explain the differences in postmortem metabolism and tenderness development at 1 d postmortem.