Myoglobin: What Is It for?

A. D. Vinogradov

Department of Biochemistry, School of Biology, Lomonosov Moscow State University, Moscow, 119899 Russia; fax: (095) 939-3955; E-mail: adv@biochem.bio.msu.su

Received December 11, 1998

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The longer the tail, the smarter the mouse,
And capable in all endeavors.
This rule is believed by every mouse.
Whether or not this is the truth, we won't discuss it now.
I. A. Krylov
"Mouse Council" (fable)

Everyone who teaches general biochemistry is certainly familiar with this question: what example should I choose to clearly illustrate the link between details of protein physical chemistry and a well-known physiological function? A number of such examples can easily be found in any biochemistry textbook. Some of these, such as the hormonal control of glycogen breakdown, the reversible denaturation of ribonuclease, or the sigmoid curve for hemoglobin saturation by oxygen are classic, and it is somewhat boring for the teacher to repeat these examples each year. The last example is concerned with the physiology of the supply of oxygen to cells, and it is usually discussed as a comparison between binding of oxygen by tetrameric hemoglobin and by monomeric myoglobin [1, 2]. It is generally accepted that the function of myoglobin is to facilitate the intracellular diffusion of oxygen to the consumers (mitochondria). Also, myoglobin is believed to be an oxygen "buffer" [3]. Indeed, the myoglobin content in muscle tissues of animals which are often exposed to hypoxic conditions, such as aquatic diving mammals (whales and seals), is many times higher than that in tissues of terrestrial animals. "Usual" terrestrial habitants contain myoglobin only in cardiac myocytes and "red" skeletal muscles (that is why these muscles are red). Many people are certainly familiar with difference in color (and taste!) of "white" meat of chicken or hazel grouse (Tetrastes bonasia), who are typical "sprinters", and the dark meet of wild duck (Anas platyrhynchos) or black grouse (Lururus tetrix), who are typical "stayers".

In the October issue of Nature, a paper by two American research groups (from the University of Texas Southwestern Medical Center, Dallas, and from the University of Cincinnati, Ohio) appeared [4] which is certainly sensational for biochemists. The authors succeeded in knocking out the myoglobin expressing gene in mice. No myoglobin was detected by polyclonal antibodies in ventricular myocytes or in skeletal myofibrils of knocked-out animals. The whitening of the heart and skeletal muscle was also evident visually. What happened to the behavior of the mice? The answer is very simple--nothing! More precisely, nothing that could be detected by the standard physiological tests. Newborn control (myoglobin^+,+) mice, heterozygous (myoglobin^+,-) mice, and homozygous (myoglobin^-,-) mice gained weight at the same rate and reached sexual maturity at the same age. The three groups were fertile in mating and showed no obvious phenotypic abnormalities during their usual activities. Moreover, the three groups were indistinguishable when subjected to a hard exercise protocol. Ventilatory rate and heart stroke work were the same. When challenged by exposure to hypoxia (13.5% oxygen, which is equivalent to an altitude of 3700 m), myoglobin-lacking mice survived and exhibited ventilatory responses during and after exposure that were comparable to those of normal animals. The muscles from myoglobin-lacking mice did not exhibit changes in histological appearance.

Do these results mean that myoglobin (a classic textbook example) is not needed for the normal life of mice? I must be fair--the authors interpret their findings with great caution. First, they are aware that more detailed studies might reveal abnormalities that were not detected by the standard physiological tests. (Could it be that mice lacking myoglobin cannot dream in color?) The question then arises: what worth are the standard physiological tests? Another possibility is that myoglobin is needed, but not for what are considered to be "textbook" facts. What might that unknown function be? It seems very unlikely that myoglobin has been conserved during evolutionary pressure while having no particular function.

In my opinion, this paper is a perfect example of studies which formally fall into the category of "negative results". However, this negative result seem much more significant than many studies with a "positive" findings because it clearly demands reevaluation of a dogma. But what is a stronger drive for progress in science than the continuous reevaluation of dogmas?

Biochemistry teachers, let us take great precaution when classic examples are presented!

REFERENCES

1. Voet, D., and Voet, J. G. (1990) Biochemistry, John Wiley, N. Y.
2. Lehninger, A. L., Nelson, D. L., and Cox, M. M. (1993) Principles of Biochemistry, 2nd ed., Worth Publishers, N. Y.
3. Oxford Dictionary of Biochemistry and Molecular Biology (1997), Oxford University Press, Oxford, N. Y.
4. Garry, D. J., Ordway, G. A., Lorenz, J. N., Radford, N. B., Chin, E. R., Gange, R. W., Bassel-Duby, R., and Williams, R. S. (1998) Nature, 395, 905-908.

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