Variable transglutaminase activity in human diploid fibroblasts during in vitro senescence

The specific activity of transglutaminase (TGase) was followed in human diploid fibroblasts (HDF) as a function of in vitro age. It was determined that at least 90% of the TGase activity was found in a soluble fraction at all in vitro ages; but the activity was variable with age. It was high in cells that had completed less than 50% of their lifespan (%LSC), declined to a minimum between 60 and 85% LSC, and again became elevated at more than 90% LSC. These age related variations in TGase activity could not be attributed to cellular growth characteristics, enzyme amount, or clonal selection processes. It is postulated that the variable TGase activity observed during in vitro senescence of HDF may reflect a change in affinity of the enzyme for a particular molecule; possibly fibronectin.