Fibre‐type distribution and subcellular localisation of α and β enolase in mouse striated muscle

Abstract Enolase is a dimeric glycolytic enzyme exhibiting tissue specific isoforms. During ontogenesis, a transition occurs from the embryonic αα towards the specific αβ, and ββ isoforms in striated muscle. Immunocytochemical analyses on transverse sections of adult mouse gastrocnemius muscle, allowed us to compare the expression of α and β subunits to that of myosin heavy chain (MHC) isoforms. Levels of β immunoreactivity followed the order IIB > IIX > IIA > I. This gradient parallels the ATPase activity associated to MHC isoforms, indicating that the expression of β enolase in myofibres is finely regulated as a function of energetic requirements. By contrast, variations in α immunolabelling intensity appeared independent of fibre types. Longitudinal muscle sections exhibited a striated pattern of α immunoreactivity. Confocal microscopy analyses demonstrated that α was localised at the M band. Most β immunoreactivity was diffuse all over the sarcoplasm. However, some β immunoreactivity was striated and localized at both Z and M bands. Thus, ββ enolase could participate to multi-enzyme complexes present at the I band, and involved with local ATP production. Our results support the notion that isozymes differ in their ability to interact with other macromolecules, thus segregating to different subcellular sites where they would respond to specific functional demands.