New freeze-dry and vapor fixation method for immunohistochemistry of soluble proteins: subcellular location of the progesterone receptor.

We describe a new application of freeze-drying and vapor fixation for immunohistochemical location of soluble proteins. The method avoids the liquid phase, which eliminates the possible diffusion of soluble proteins. Two vapor fixatives, paraformaldehyde and p-benzoquinone, were tested and p-benzoquinone was found to preserve antigenicity of progesterone receptor (PR) and ovalbumin better than paraformaldehyde. The method proved to be highly sensitive, since higher concentrations of antigen were found in some tissues and some tissues found to be antigen negative by earlier liquid fixation methods proved to contain antigen. The location of PR as a highly soluble protein was studied. With the present method, both unoccupied and occupied PR were located in the nuclei, a similar finding as with the earlier liquid fixation method. The results further support the concept that PR is an intranuclear protein independent of its ligand occupation. PR was detected in a few cells inside the follicles of the bursa of Fabricius and in the smooth muscle cell nuclei of the small intestine, observations not previously made owing to the insensitivity of the earlier methods.