Our study shows that an increase in the proportion of arginine in relation to different Met levels could affect the yellowness of the meat. Some studies have shown that changes in the yellowness (b* parameter) appear in the case of defects in poultry meat. Pekel et al. [
26] showed lower yellowness in the case of white striping in broiler chickens. In another study, Bowker and Zhuang [
27] showed lower yellowness in broiler breast filets with higher water-holding capacity. The relationship between yellowness and drip loss of breast muscle was also confirmed by Rammouz et al. [
23]. Cai et al. [
28] and Zhang et al. [
29] showed higher b* values in meat with wooden breasts of broilers in comparison to normal meat. Tasoniero et al. [
30] also showed higher b* values in spaghetti meat in comparison to normal meat. Baldi et al. [
31] observed that an increase in the yellowness of muscle is related to adipose tissue accumulation among muscle fibers occurring in spaghetti meat. Dalle Zotte et al. [
32] stated that fat possesses a yellowish color and might be directly contributed to the increased yellowness of meat. Moreover, Hocquette et al. [
33] stated that an increase in yellow color is associated with fat deposition in meat. On the other hand, Lee and Choi [
34] found no difference in the yellowness of PSE and white-stripping muscles compared to normal-quality chicken meat. According to Jankowski et al. [
4], arginine as a precursor in the synthesis of various metabolites could influence muscle metabolism and then, in consequence, could influence the meat color b* parameter. The obtained results in relation to pH and other parameters of color are in agreement with the results of Zampiga et al. [
35] and Jankowski et al. [
4], who also did not note the effect of increasing arginine levels in feed on meat quality parameters, such as pH or color a* parameter in chickens and turkeys.
In addition, those authors did not observe the effect of Arg in a diet on the protein:DNA ratio, which demonstrates that Arg does not affect the mitotic activity of the satellite cells. On the other hand, Castro et al. [
3] showed that dietary supplementation with Arg leads to overall body growth with increased lean deposition. The authors of these studies argued that the increased protein content in turkey meat is likely related to increased creatine levels, an endogenous Arg metabolite involved in protein metabolism. This result partly corresponds to the results of the current authors’ own research, which showed that the groups with the highest share of Arg were characterized by a slightly different profile of myofibrillar proteins, regardless of the level of Met in the diet. In another study, Zhai et al. [
2] showed that supplementing the diet of chickens with methionine (Met) resulted in an increase in muscle protein deposition, which, however, was more the result of an increase in the amount of sarcoplasmic proteins than myofibrils. Additionally, Zhai et al. [
2] stated that myofibrillar hypertrophy is associated with a quantity increase of actin and myosin, which increases the strength of muscle contraction and is associated with physical activity and exercise. This myofibrillar hypertrophy could worsen the tenderness of meat [
2]. This statement is in agreement with our results in relation to actin. The level of actin increased with a higher level of Arg (100% and 110%) independently of the level of Met. The opposite effect was found for myosin HC, LC1, and LC2. Bowker and Zhuang [
25] found a lower level of myosin HC in broiler breast meat, with severe degrees of white striping in relation to normal meat. The results presented in
Table 6 and
Table 7 show the impact of the studied nutritional factors, i.e., Arg and Met level, on the amount of myosin HC. These results show that the increase in the share of Arg and Met in the diet reduces the share of myosin HC in the myofibrillar protein profile in turkey muscle. Huffman et al. [
11], based on a study on muscle myosin HC growth in turkeys, put forward the hypothesis that nutritional factors such as feed restriction could alter the molecular mechanisms controlling muscle growth in poultry. However, other results obtained by Li et al. [
36] did not show any differences in the level of myosin and actin between the muscles of chickens with PSE defects and normal ones. Only the effect of Met as the main factor was found for the level of myosin LC2. A study by Cai et al. [
28] showed that myosin LC2 was overabundant in the muscle of woody broiler breast meat. A higher level of myosin LC2 may lead to a greater concentration of fast glycolytic muscle fibers and decreased pH. This statement is partly in agreement with the results presented in
Table 5 for the parameter of b* color because Rammouz et al. [
23] showed a significant relationship between muscle glycolytic potential and b* value. However, these observations are not confirmed by the results of the current study for ultimate pH. Another study by Soglia et al. [
37] showed that the muscle of chickens with wooden breast defects as well as white striping exhibited a lower abundance of myosin LC1 in relation to normal samples. Additionally, Mudalal et al. [
38] showed that the absolute concentrations of myofilament proteins such as actin LC1 and LC3 myosin were decreased in chicken breasts with white striping defects and may indicate the degeneration process of myofilament proteins. The results presented in
Table 5 showed a lower abundance of myosin LC1 as an effect of the increasing Arg level, independent of the Met level. In conclusion, the current results showed that different levels of Arg and Met in the turkey diet influenced the quantitative distribution of myofibrillar proteins. The obtained differences in the levels of different myofibrillar proteins may be the results not only of their synthesis but also of degradation during post-mortem proteolysis [
39]. It has been shown that the calpain proteolytic system is responsible for this post-mortem proteolysis, and other proteases are also involved [
39,
40]. The activity of these enzymes is related to many different factors, i.e., Ca
2+ concentrations, pH of muscle, temperature, stress before slaughter, age of the animal, nutrition, genetics, and many other factors [
41]. Research studies have demonstrated that, within duck muscles, the calpain system significantly influences the degradation of desmin and troponin-T [
41]. However, the outcomes of our results indicate no discernible disparities in troponin-T degradation among the analyzed groups. This suggests that the effects of arginine (Arg) and methionine (Met) need further exploration. In the case of desmin, the results showed a significant effect of Arg and Met, and an increase in the level of Arg in the diet was associated with an increase in the amount of desmin, while an increase in Met caused its decrease. However, it should be recalled that desmin performs a variety of functions in the cell, such as ensuring the correct position of cellular organelles by creating a “lattice skeleton” connecting to the Z line, maintaining the shape and tension of the cell walls and intracellular elements, maintaining proper communication between the cell and the matrix extracellular, ensuring mechanical integration during muscle contraction and relaxation, supporting the work of tubulin and actin, sending information within cell elements, and regulating intracellular signaling and gene expression [
42,
43]. It seems that, in this case, the increase in Arg in the diet had a positive effect on the increase in desmin levels and was related to its demand in the cell due to its multiple functions. However, based on the results concerning proteins with lower molecular weights (33 kDa, 32 kDa, 28 kDa, 26 kDa, 14 kDa) and the share of tropomyosin, it can be concluded that the increase in the share of Arg in the diet of turkeys had a negative impact on the processes of muscle protein degradation and, thus, post-mortem tenderization since their amounts decreased with increasing Arg in the diet. These proteins are often considered indicators of post-slaughter meat tenderization [
40]. Thus, referring to the previous considerations and these results, it can be concluded that the increase in the share of Arg and Met in the diet of turkeys increases the content of some myofibrillar proteins (actinin, desmin, actin) and reduces degradation during the post-slaughter proteolysis of proteins that are considered tenderization indicators. This may affect the functional properties during processing and the sensory quality of turkey meat. The cluster analysis findings revealed that, when Arg is present at a high concentration of 110%, variations in Met levels at 35% or 45% appear to have diminished significance. However, at the Arg levels of 100% or 90%, there were no significant differences regardless of the Met level (30% or 45%).