Why is meat red?
The typical answer can be found on http://www.articlesbase.com/home-improvement-articles/understanding-meats-why-red-meat-is-red-and-why-white-meat-is-white-63616.html
The problem with this standard answer is that it is incorrect. It is true that meat is red because muscle tissue of land vertebrates has a haeme-containing protein myoglobin that colors it red. However, contrary to what a lot of people believe and the textbooks taught, the function of this protein in land animals remains a mystery.
...THE ROLE OF MYOGLOBIN as an oxygen storage protein in the muscles of diving birds and mammals is well-established. In contrast, the function of the lower myoglobin content in striated muscles of humans and other terrestrial vertebrates has long been a subject of debate. In 1939, Millikan identified three potential functional roles for myoglobin in the aerobic muscles of terrestrial animals: first, "as an agent in oxygen transport"; second, "as an intracellular catalyst"; and third, "as an oxygen store." Largely by elimination of the first two possibilities, Millikan concluded that myoglobin acted primarily as "a short term oxygen store, tiding the muscle over from one contraction to the next." This conclusion is consistent with the observation that myoglobin content is higher in more aerobic muscles and is highest in aerobic muscles with long contraction duration and hence more prolonged impedance of blood flow during contractions.
..In the 1960s, it became apparent that it is precisely the fast, near-equilibrium nature of myoglobin's binding kinetics, along with its oxygen dissociation constant near the PO2 of active muscle and its high concentration compared with dissolved oxygen, that enables myoglobin to facilitate oxygen diffusion. Subsequent experiments demonstrated that inhibition of myoglobin oxygen binding decreased maximum oxygen consumption (O2 max) in isolated muscles, and by 1990, the importance of myoglobin to oxygen transport in aerobic muscle was widely accepted. This consensus suffered an apparent setback in 1998, when it was reported that genetic knockout of myoglobin expression in mice had no effect on exercise performance.
...However, Godecke et al. (using an independently developed knockout line, reported cardiac adaptations (e.g., increased capillarity and blood flow) that could have compensated for the loss of myoglobin's transport function. Using this knockout as a control, the same group showed that the effects of CO poisoning on performance and O2 in cardiac muscle of wild-type mice were specific to its effect on myoglobin. Furthermore, recent studies of cardiac muscle from the same myoglobin knockout mouse line have resurrected catalytic roles for myoglobin, first as an NO scavenger, and more generally, as an antioxidant.
...Of course, the three functions of myoglobin are not mutually exclusive. In particular, the properties of myoglobin that suit it for a role in oxygen transport (high concentration, half-saturation near the in vivo PO2, and fast kinetics) suit it equally well for short-term oxygen storage. Thus, just as for the creatine kinase system, the spatial and temporal buffering functions of myoglobin are inseparable, and sorting out their relative importance in a single physiological context can be difficult. Defining the role of myoglobin in human skeletal muscle during exercise has been particularly difficult. Despite the well-known correlation between muscle myoglobin content and aerobic capacity across species, little correlation was reported across human individuals, and aerobic training appeared to have little effect. In 2004, a good correlation across individuals between myoglobin content and muscle-specific functional aerobic capacity, estimated from the kinetics of phosphocreatine recovery after exercise was demonstrated. The association between myoglobin content and indexes of aerobic function in human muscle now seems established.
...Does this result prove that myoglobin plays a role in oxygen transport in human muscle? Alas, no, because the temporal and spatial aspects of oxygen buffering by myoglobin are equally relevant in the context of aerobic exercise. http://ajpregu.physiology.org/cgi/content/full/287/6/R1304#R2
It is one thing to pontificate on the crucial importance of myoglobin for muscle function; it is a different thing to produce a myoglobin-free animal and discover that the muscle performance is not affected:
...The myoglobin knockout mice showed no apparent phenotype other than depigmentation of the heart and
soleus muscles. They grew normally, were able to perform exhaustive treadmill exercise and responded normally to a hypoxic challenge. In addition, skeletal muscles and hearts isolated from the knockout mice were equal to those from their wild-type counterparts in studies of contractile function in the presence or absence of O2.
http://jeb.biologists.org/cgi/reprint/207/20/3441.pdf
http://ajpcell.physiology.org/cgi/content/abstract/281/5/C1487
There was increased embryonic lethality, but the mice that survived lived happily ever after. It is painfully clear that myoglobin is not as hugely important as was believed all these years. It does something, but that "something" can be done otherwise, without apparent detriment. It is hardly a secret that birds have a lot of white meat despite their strenous flying, that pigs are "the other white meat," etc. The concentration of myoglobin varies significantly, yet the animals having less of it do fine. Which brings us back to the original question:
Why is meat red?
The problem with this standard answer is that it is incorrect. It is true that meat is red because muscle tissue of land vertebrates has a haeme-containing protein myoglobin that colors it red. However, contrary to what a lot of people believe and the textbooks taught, the function of this protein in land animals remains a mystery.
...THE ROLE OF MYOGLOBIN as an oxygen storage protein in the muscles of diving birds and mammals is well-established. In contrast, the function of the lower myoglobin content in striated muscles of humans and other terrestrial vertebrates has long been a subject of debate. In 1939, Millikan identified three potential functional roles for myoglobin in the aerobic muscles of terrestrial animals: first, "as an agent in oxygen transport"; second, "as an intracellular catalyst"; and third, "as an oxygen store." Largely by elimination of the first two possibilities, Millikan concluded that myoglobin acted primarily as "a short term oxygen store, tiding the muscle over from one contraction to the next." This conclusion is consistent with the observation that myoglobin content is higher in more aerobic muscles and is highest in aerobic muscles with long contraction duration and hence more prolonged impedance of blood flow during contractions.
..In the 1960s, it became apparent that it is precisely the fast, near-equilibrium nature of myoglobin's binding kinetics, along with its oxygen dissociation constant near the PO2 of active muscle and its high concentration compared with dissolved oxygen, that enables myoglobin to facilitate oxygen diffusion. Subsequent experiments demonstrated that inhibition of myoglobin oxygen binding decreased maximum oxygen consumption (O2 max) in isolated muscles, and by 1990, the importance of myoglobin to oxygen transport in aerobic muscle was widely accepted. This consensus suffered an apparent setback in 1998, when it was reported that genetic knockout of myoglobin expression in mice had no effect on exercise performance.
...However, Godecke et al. (using an independently developed knockout line, reported cardiac adaptations (e.g., increased capillarity and blood flow) that could have compensated for the loss of myoglobin's transport function. Using this knockout as a control, the same group showed that the effects of CO poisoning on performance and O2 in cardiac muscle of wild-type mice were specific to its effect on myoglobin. Furthermore, recent studies of cardiac muscle from the same myoglobin knockout mouse line have resurrected catalytic roles for myoglobin, first as an NO scavenger, and more generally, as an antioxidant.
...Of course, the three functions of myoglobin are not mutually exclusive. In particular, the properties of myoglobin that suit it for a role in oxygen transport (high concentration, half-saturation near the in vivo PO2, and fast kinetics) suit it equally well for short-term oxygen storage. Thus, just as for the creatine kinase system, the spatial and temporal buffering functions of myoglobin are inseparable, and sorting out their relative importance in a single physiological context can be difficult. Defining the role of myoglobin in human skeletal muscle during exercise has been particularly difficult. Despite the well-known correlation between muscle myoglobin content and aerobic capacity across species, little correlation was reported across human individuals, and aerobic training appeared to have little effect. In 2004, a good correlation across individuals between myoglobin content and muscle-specific functional aerobic capacity, estimated from the kinetics of phosphocreatine recovery after exercise was demonstrated. The association between myoglobin content and indexes of aerobic function in human muscle now seems established.
...Does this result prove that myoglobin plays a role in oxygen transport in human muscle? Alas, no, because the temporal and spatial aspects of oxygen buffering by myoglobin are equally relevant in the context of aerobic exercise. http://ajpregu.physiology.org/cgi/content/full/287/6/R1304#R2
It is one thing to pontificate on the crucial importance of myoglobin for muscle function; it is a different thing to produce a myoglobin-free animal and discover that the muscle performance is not affected:
...The myoglobin knockout mice showed no apparent phenotype other than depigmentation of the heart and
soleus muscles. They grew normally, were able to perform exhaustive treadmill exercise and responded normally to a hypoxic challenge. In addition, skeletal muscles and hearts isolated from the knockout mice were equal to those from their wild-type counterparts in studies of contractile function in the presence or absence of O2.
http://jeb.biologists.org/cgi/reprint/207/20/3441.pdf
http://ajpcell.physiology.org/cgi/content/abstract/281/5/C1487
There was increased embryonic lethality, but the mice that survived lived happily ever after. It is painfully clear that myoglobin is not as hugely important as was believed all these years. It does something, but that "something" can be done otherwise, without apparent detriment. It is hardly a secret that birds have a lot of white meat despite their strenous flying, that pigs are "the other white meat," etc. The concentration of myoglobin varies significantly, yet the animals having less of it do fine. Which brings us back to the original question:
Why is meat red?