Isozymes and the micro organization of organellar structure and function.

One of the most valuable of the wide-ranging attributes of isozymes is the novel insight they allow into some of the most complex aspects of cell biology. The microorganization of subcellular structure and function is one such area, and the present study utilizes the peroxisome as an example of these applications. The biological characteristics of the major enzymic component of this organelle, catalase, have been detailed in mammalian tissues, where the enzyme exhibits a complex heterogeneity, which is due to multiple types of epigenetic modification. The major native multiplicity, however, has been demonstrated as attributable to sialic acid attachment to the enzyme. In studying the compartmentalization of these isozymes within liver cells, the technique of differential extraction with digitonin was employed, and evidence provided which supports the presence of an appreciable proportion of catalase activity in the cytoplasmic compartment. The source of these cytoplasmic isozymes was traced to release from the peroxisome, and the mechanism of this release identified with variations in the content of lysophosphatidyl choline in the peroxisomal membrane. Studies of the association of catalase with the subcellular membranes indicated an appreciable ionic interaction which varied with membrane type and isozyme status. Activity was enhanced in the bound form, providing support for the interpretation of a general protective role of this enzyme against oxidation of membrane components within the cell. Overall, these studies are considered to contribute significantly to current knowledge of the biological role and subcellular localization of catalase in relation to organellar structure and function.