Abstract
The ultrastructural localization of catalase and L-alpha-hydroxy acid oxidase (LalphaHAO) was studied in two species of Hydra. Diaminobenzidine reaction product of catalase activity was present in small round or elongated bodies resembling microperoxisomes in the epitheliomuscular, digestive and gland cells. They were closely related to the endoplasmic reticulum, and were often found in proximity to deposits of lipid and glycogen. Reaction product of LalphaHAO activity was also associated with the microperoxisomes. With rapidly oxidized substrates, such as L-lactic acid, reaction product diffused into the cytoplasm around the microperoxisomes. With slowly oxidized substrates, such as DL-alpha-hydroxyisovaleric acid, reaction product was restricted to the matrix of the microperoxisomes. No reaction product was present in the microperoxisomes in the absence of substrate or with D-lactic acid. The rate of substrate oxidation measured biochemically roughly paralleled the amount of cytochemical reaction product deposited with different substrates. Microperoxisome-like bodies reactive for LalphaHAO were also found in the epidermal cnidoblasts; however, catalase could not be demonstrated in them. This study provides the first cytochemical evidence for the presence of an H2O2-producing oxidase in microperoxisomes.