A globin in every cell?

A crucial requirement of metabolically active aerobic cells is a steady supply of oxygen. The red pigment of vertebrate skeletal muscle, oxygen-binding myoglobin (Mb), serves this function by facilitating the delivery of O 2 from the plasma membrane to the energy-producing mitochondria (1). The delivery of O2 from lungs or gills to muscles is also very efficient because of the cooperative loading and unloading of O2 by the hemoglobin of the red cells. Since Ray Lankester’s (2) first identification, vertebrate Mb has been believed to occur solely in cardiac and skeletal muscle. Only one exception was noted: the smooth muscle sphincter cells of the human rectum, since confirmed (3). Mb can act as a store of O 2 to help maintain a constant supply of O 2 during rapidly fluctuating demands of contraction (4). This helps explain why the concentration of Mb is highest in the skeletal muscles of diving mammals, and why Mb increases in animals, including humans, after chronic muscular activity or hypoxia (4, 5). In human skeletal muscle, Mb in mitochondria-rich oxidative myofibers shows elevated synthesis in response to exposure to high altitudes (5) or intense endurance training under reduced oxygen pressures (4–6). Fraser et al. (7) now report, on page 2977 in this issue of PNAS, that the hypoxia-tolerant common carp (Cyprinus carpio) has Mb not only in muscle but also in other metabolically active tissues that include liver, brain, and gills. What is Mb doing …