A Remark about the Determination of the Water Content of Mitochondria

Glas and Bahr (1) have proposed a method for the determination of the water content of subcellular particles which can be schematically described as follows: A suspension of particles in 0.44 M sucrose is layered in a centrifuge tube over a silicone oil denser than the sucrose solution but less dense than the particles. The particles are sedimented through the silicone oil by high speed centrifugation, and they form a pellet which is supposedly free of extraparticulate aqueous phase. The water content of this pellet is measured directly by distillation. The method is based on the implicit assumption that gravitation and buoyancy are the only factors at play. No account is taken of the interfacial tension which opposes the penetration of hydrophilic particles into the oil phase. We shall show in the present paper that the centrifugal field is far from being strong enough to force individual particles through the interface between the two phases. The pellet is actually formed by aggregates of particles which carry down a substantial amount of extraparticulate aqueous phase. Since the subcellular particles are obviously very hydrophilic, we may assume as a first approximation that the surface tension between the particles and the sucrose solution in which they are suspended is small. When the particles are in contact with silicone oil, the interfacial tension may thus be assumed to be about the same as the surface Laboratoire de Chimie Physiologique, University of