ULTRASTRUCTURAL STUDY OF MEMBRANE-BOUND ENZYMES IN THYMOCYTES

Calf thymocytes in suspension, as well as isolated calf thymocyte nuclei, were incubated in the presence of several phosphorylated substrates. 5'-Nucleotidase was easily detected on the plasma membrane of thymocytes (external side), but could be demonstrated on isolated nuclei only to a small extent. No other substrates were detectably hydrolyzed by isolated nuclei except adenosine triphosphate and 3'-thymidine monophosphate. The surface of whole cells was found to be much more reactive. A 3'-nucleotidase activity was shown to occur on the plasma membrane of a number of thymocytes and produced large lead phosphate deposits, some of them protruding into the cytoplasm. Enzymic activities splitting β-nicotinamide adenine dinucleotide phosphate and uridine diphosphate glucose were also readily detectable on the surface of cells. Since the pattern of the lead phosphate deposits and the number of reactive cells varied with the added substrate, and since cells were compared with their isolated nuclei, the positive reactions were considered to indicate the presence (on the exposed membranes) of the corresponding enzymes on the exposed membranes.