Abstract
Fluorophosphate-reactive (FPR) sites in the adult male rat liver, tentatively identified as esterase active centers, were localized and measured using the combined techniques of quantitative electron microscope radioautography and morphometric analysis with the light and electron microscope. The FPR sites were measured in liver which had been prefixed by perfusion with 1.5% glutaraldehyde and reacted with 10(-4) M tritium-labeled diisopropyl fluorophosphate (3H-DFP). Under the experimental conditions 64-67% of the esterase activity in fresh liver was retained for reaction with the 3H-DFP, which is known to bind irreversibly to the active sites of certain esterases. In light and electron microscope radioautographs the developed silver grains were concentrated over the cytoplasm of hepatocytes. A low concentration occurred over erythrocytes. All other areas in the liver had a concentration of grains resembling the background concentration. Quantitative measurements of grain density in the electron microscope radioautographs revealed the highest density, after correcting for radiation spread, in cytoplasmic granules (mainly cytolysomes). The grain densities over the rough and smooth endoplasmic reticulum and associated structures were also equal to or above the average hepatocyte grain density. Due to the large fractional volume of endoplasmic reticulum per hepatocyte (58% of cell volume) and the fraction of the liver occupied by hepatocytes (79% of liver volume) the majority of FPR sites in the liver occurred in the rough and smooth endoplasmic reticulum and associated structures. The average numbers of FPR sites were calculated per micrometer3 of hepatocyte (5.0 x 10(5) sites/micrometer3) and per unit volume of each significantly labeled organelle. In addition, the number of FPR sites per hepatocyte (2.5 X 10(9) sites/cell), per cm3 liver (4.1 X 10(17) sites/cm3) and in the total liver of an average 100 g male rate (2.2 X 10(18) sites/total liver) were also calculated.