Localization of two distinct acid phosphatases in secretory ameloblasts of rat molar tooth germs

Acid phosphatases were examined histochemically at the light- and electron-microscopic levels using para-nitrophenyl phosphate (pNPP) and β-glycerophosphate (β-GP) as substrates. By light microscopy, there was intense activity with pNPP in supranuclear and distal regions of the secretory ameloblast, and moderate or slight activity respectively in those regions with β-GP. These enzyme activities were less at the late secretory stage of amelogenesis and disappeared at the transitional stage. By electron microscopy, acid phosphatase activity was seen in the trans side cisternae of the Golgi apparatus, in lysosome-like granules, and in small vesicles in the Tomes' processes. The activity with pNPP but not β-GP was also localized at the plasma membrane (proximal, lateral and distal surface). Activity with β-GP was completely inhibited by 1 mM sodium tartrate and by 1 mM NaF; activity with pNPP was inhibited by 1 mM NaF and 10 mM sodium tartrate, but not by 1 mM sodium tartrate. Thus there are at least two different acid phosphatases, one tartrate-sensitive and the other 1 mM tartrate-resistant, in the secretory ameloblast; the tartrate-resistant enzyme is plasma-membrane bound.