Some physical properties of adrenal medulla chromaffin granules isolated by a new continuous iso-osmotic density gradient method.

Centrifugation of crude preparations of chromaffin granules on iso-osmotic continuous gradients of sucrose and metrizamide produce granule fractions of high purity without pelleting. These granules have a buoyant density of 1.123 at 300 mosM, which increases to 1.222 at 1800 mosM. Exchange of water for 2H2O greatly increases the density of the granules. An apparent water space for the granules can be calculated from the difference in density in 2H2O and water. The water space of 0.63 at 300 mosM diminishes as the osmolality of the suspension medium is increased, suggesting that the granule membrane is a good osmometer and that the core contains a large osmotically active water space. Other separation methods for chromaffin granules are discussed in terms of these results.