The cytochemical localization of Ca
2+-, Mg
2+-ATPase in peroxisomes isolated from rat renal tubules was investigated at the ultrastructural level. The reaction products were localized mainly on the cytoplasmic surface of the isolated peroxisomes, but also to some extent on the matrix, side of the limiting membrane. Neither Ca
2+-nor Mg
2+-ATPase activity was affected by treatment with levamisole, an inhibitor of alkaline phosphatase. Ca
2+-ATPase activity was inhibited by thapsigargin and myricetin, but not by PCMB. Mg
2+-ATPase activity was inhibited by PCMB. It was inhibited slightly by thapsigargin, but was not affected by myricetin.
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