Analysis of the mobility of a soluble enzyme in squid axoplasm

The solubility of phosphoglucoisomerase (EC 5.3.1.9) in the giant axon of Loligo pealei was studied. Axoplasm was separated from its surrounding membrane by extrusion into a 0.7 ml buffer that simulates the solution conditions in the axon. Soluble proteins elute out of the axoplasm into the bath and equal samples were removed at selected times. The rates of both components differ by as much as an order of magnitude. The rate of the first component can be explained by free diffusion of monomeric PGI. Isokinetic sucrose gradients of fractions of both components as well as from whole axoplasm suggest that PGI is not complexed with other proteins as it elutes from axoplasm. The authors propose that elution of some of the PGI may be delayed due to associations with non-diffusing structures in the axoplasmic cylinder. Electron microscopic and axonal transport studies that demonstrate non-random movements of soluble enzymes in association with cytoskeletal structures support this hypothesis.