Fine structural localization of enzymes hydrolyzing nucleoside phosphates in the rat adrenal cortex has been determined, and the selective inhibition of those enzymes exhibiting intracellular localization has been effected. Glutaraldehyde-fixed adrenocortical tissue was incubated in a medium which contained a nucleoside mono-, di- or triphosphate of adenosine, inosine, guanosine, or cytidine as substrate. Intracellular enzymatic activity was exhibited when one of three nucleoside phosphate substrates was employed. When IDP was used, final product of enzymatic activity was found on membranes of the endoplasmic reticulum, Golgi cisternae and intramitochondrial microvesicles. Final product was localized on the membranes of the endoplasmic reticulum and certain mitochondria when ITP was used. With GTP as substrate, activity was primarily localized on mitochondrial microvesicles and agranular endoplasmic reticulum, with no Golgi involvement noted.
The phosphatases for which intracellular localization was determined exhibited four different sites of activity: (a) agranular endoplasmic reticulum, (b) microvesicles within mitochondria, (c) nuclear membrane, and (d) subendothelial and/or intercellular spaces with occasional involvement of the plasma membrane. When nicotinamide was added to the incubation media, intracellular phosphatase activity was inhibited. Extracellular enzymatic activity was unaffected by nicotinamide. The possible mode of action of nicotinamide in enhancing steroidogenesis and inhibiting intracellular phosphatase activity is discussed.