Activation and de novo synthesis of transglutaminase in cultured glioma cells

The activity of transglutaminase (TGase) was measured in cultured C₆ glioma cells after their stimulation by either isoproterenol and isobutyl‐methylxanthine or by a serum‐containing medium. The activity fluctuated in a biphasic manner, with the peaks at 2–3 hr and 7–8 hr poststimulation. The first peak of TGase activity was affected neither by cycloheximide nor by actinomycin D, which inhibited protein synthesis. The second peak, on the other hand, was completely eliminated by cycloheximide and was reduced by actinomycin D. Immunological procedures were employed to find out whether or not the activity of TGase corresponded with the presence of the TGase antigen in the cultured cells. Indirect immunofluorescent staining and radioimmunoblot techniques suggested that unstimulated cells contained an inactive enzyme. This inactive, or cryptic, enzyme had the same molecular weight as its active counterpart. Activation of the enzyme was mediated by cell stimulation, probably by its release from the membrane. This step did not require protein synthesis, unlike the second step, which was dependent on de novo protein synthesis.