Reversible changes in conformation of mitochondria of constant volume.

When potato mitochondria are prepared in a suitable isotonic or hypertonic medium, their morphology depends upon whether or not a macromolecular solute is present in the final suspending medium. Any of several chemically disparate polymers or macromolecules, above a certain critical concentration, will produce filamentous or rod-like mitochondria, which morphologically resemble mitochondria in the living cell. Below this critical level, or in the absence of a suitable macromolecular solute, mitochondria are spherical, even in hypertonic solution. The transition from rod-like to spherical morphology is reversible, rapid and independent of metabolism. Swelling of the mitochondria is not involved, since there is no change in mitochondrial fresh weight, nor in the optical density of mitochondrial suspensions when mitochondria are transformed from one form to the other. Alternative mechanisms, based either on screening interaction between macromolecules and the mitochondrial membrane, or on the reduction of mitochondrial interfacial tension, are proposed to account for the reversible changes in morphology. The reversible sphere to rod transition ostensibly depends upon the maintenance of mitochondrial resilience or elasticity, which in turn is considered to depend upon the presence of an appropriate chelator and sulfhydryl agent during the isolation process. In the absence of the latter, mitochondria are subsequently irreversibly spherical. Mercaptobenzothiazole effectively substitutes for a mixture of EDTA and cysteine.