Intracellular Localization of Acid Phosphatase a Comparative Study of Biochemical and Histochemical Methods* by G. E. Palade, M.d

In a preceding article (1), the distribution of acid phosphatase activity was studied in a number of fractions separated from rat liver homogenates by differential centrifugation. It was found that less than 5 per cent of the enzymatic activity separates with the nuclear fraction, while over 90 per cen t remains associated with the cytoplasmic components. Among the latter, the mitochondria and the microsomes showed the greatest activity. These results were found to be in sharp disagreement with the histochemical data obtained b y various authors using the test proposed by Gomori (2). According to the current interpretation, this test indicates that the acid phosphatase is predominantly a nuclear enzyme (Moog (3)). For liver cells in particular, the concentration of the enzyme in the nuclei has been repeatedly stressed (4-7). In addition, its presence in some peribiliary material, supposedly connected with the Golgi apparatus (5, 8), and in the bile capillaries (5) has been described. Because of this obvious disagreement, it was considered desirable to run in parallel biochemical determinations and histochemical tests on the same material. This was done in a series of 10 experiments on rat livers. For each liver, the activity of the respective homogenate and of the nuclear and cYtoplasmic fractions isolated therefrom was tested biochemically, while the histochemical test was performed on sections. No correlation was found between the biochemical and the histochemical results. Additional experiments showed that the histochemical test is not reliable for intracellular localization studies. The precipitation pattern it gives does not reflect, as generally assumed, differences in enzyme concentration among the various cell components, but instead differences in their lead phosphate affinity. The interpretation of the results is further hampered by artifacts produced during the acetone fixation of the tissue.