The specificity of the histochemical reaction for adenosine triphosphatase (ATPase) has been a subject of debate. The study by Glick and Fischer introduced a method to localize ATPase, but its validity was questioned due to the potential interference from other phosphatases. However, Glick maintained that the reaction was valid when ATP staining occurred in sites that did not show reaction with phosphomonoesters. This claim was supported by subsequent work by Wang and Naidoo and Pratt. The specificity of ATPase was further explored by Maengwyn-Davies, Friedenwald, and White, who used a variety of inhibitors and activators. They concluded that specific myosin-ATPase activity, dependent on -SH groups, was detectable at pH 8.25, while a different alkaline ATPase or pyrophosphatase was responsible for ATP dephosphorylation at pH 9.9. They also suggested that 5-nucleotidase contributed to the dephosphorylation of ATP. In this study, the authors performed the enzymic reaction in a medium similar to that described by Gomori, using sulphydryl compounds (BAL and cysteine) to control the interference of alkaline phosphatase. They demonstrated that true ATPase activity is -SH dependent and can be inhibited and reversed by these compounds. The staining of certain structures, such as mitochondria and smooth muscle, was unaffected by -SH inhibitors, suggesting the presence of a different type of phosphatase. The study also compared the localization of ATPase activity in various tissues, including kidney, muscle, and large arteries, using ATP, ADP, and A-5-P as substrates. The results showed that the staining patterns differed significantly between these substrates, indicating that 5-nucleotidase does not contribute to the staining when ATP is the substrate. The authors concluded that three categories of phosphatase activity toward ATP at pH 9.4 have been isolated histochemically: (1) phosphatase requiring -SH groups, (2) phosphatase inhibited by -SH groups, and (3) phosphatase indifferent to -SH groups. True ATPase was separated histochemically by its -SH dependence and was demonstrated in cardiac and skeletal muscle fibers and the mitochondria of kidney tubules. The staining of these structures can be prevented and restored by -SH inhibitors and compounds, respectively. The staining of certain structures was unaffected by -SH inhibitors, suggesting the presence of a different type of phosphatase.The specificity of the histochemical reaction for adenosine triphosphatase (ATPase) has been a subject of debate. The study by Glick and Fischer introduced a method to localize ATPase, but its validity was questioned due to the potential interference from other phosphatases. However, Glick maintained that the reaction was valid when ATP staining occurred in sites that did not show reaction with phosphomonoesters. This claim was supported by subsequent work by Wang and Naidoo and Pratt. The specificity of ATPase was further explored by Maengwyn-Davies, Friedenwald, and White, who used a variety of inhibitors and activators. They concluded that specific myosin-ATPase activity, dependent on -SH groups, was detectable at pH 8.25, while a different alkaline ATPase or pyrophosphatase was responsible for ATP dephosphorylation at pH 9.9. They also suggested that 5-nucleotidase contributed to the dephosphorylation of ATP. In this study, the authors performed the enzymic reaction in a medium similar to that described by Gomori, using sulphydryl compounds (BAL and cysteine) to control the interference of alkaline phosphatase. They demonstrated that true ATPase activity is -SH dependent and can be inhibited and reversed by these compounds. The staining of certain structures, such as mitochondria and smooth muscle, was unaffected by -SH inhibitors, suggesting the presence of a different type of phosphatase. The study also compared the localization of ATPase activity in various tissues, including kidney, muscle, and large arteries, using ATP, ADP, and A-5-P as substrates. The results showed that the staining patterns differed significantly between these substrates, indicating that 5-nucleotidase does not contribute to the staining when ATP is the substrate. The authors concluded that three categories of phosphatase activity toward ATP at pH 9.4 have been isolated histochemically: (1) phosphatase requiring -SH groups, (2) phosphatase inhibited by -SH groups, and (3) phosphatase indifferent to -SH groups. True ATPase was separated histochemically by its -SH dependence and was demonstrated in cardiac and skeletal muscle fibers and the mitochondria of kidney tubules. The staining of these structures can be prevented and restored by -SH inhibitors and compounds, respectively. The staining of certain structures was unaffected by -SH inhibitors, suggesting the presence of a different type of phosphatase.