Beef liver catalase was injected intravenously into mice, and its distribution in the kidney, myocardium, and liver was studied with the electron microscope. A specific and relatively sensitive method was developed for its ultrastructural localization, based on the peroxidatic activity of catalase and employing a modified Graham and Karnovsky incubation medium. The main features of the medium were a higher concentration of diaminobenzidine, barium peroxide as the source of peroxide, and pH of 8.5. Ultrastructurally, the enzyme was seen to permeate the endothelial fenestrae and basement membranes of tubular and glomerular capillaries of the kidney. The urinary space and tubular lumina contained no reaction product. In the myocardial capillaries, the tracer filled the pinocytotic vesicles but did not diffuse across the intercellular clefts of the endothelium. In liver, uptake of catalase was seen both in hepatocytes and in Kupffer cells.

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