Enzymes of the gamma-glutamyl cycle in 'aging' WI-38 fibroblasts and in HeLa S3 cells.

gamma-Glutamyltransferase ((5-glutamyl)-peptide:amino-acid 5-glutamyltransferase, EC 2.3.2.2) activity of WI-38 fibroblasts decreased only slightly in relation to a constant amount of cell-associated protein as the cells were carried in culture serially from middle to late passage numbers leading toward senescence, e.g., from population doubling level 27 through 41. Also, when the enzyme activity was expressed on the basis of a unit number of cells or unit amount of DNA, little change occurred over that range of PDLs. As the culture approached 'phase-out', the transferase activity rose sharply regardless of how the activity was expressed. The possibility is considered that the large increase in activity could be a reflection of a significant increase in size of cells and therefore changes in the membranes where the transferase is located. The occurrence of other enzymes of the 'gamma-glutamyl cycle' in WI-38 and HeLa S3 cells also was demonstrated. These included gamma-glutamylcyclotransferase ((gamma-L-glutamyl)-L-amino-acid gamma-glutamyltransferase (cyclizing), EC 2.3.2.4) and 5-oxoprolinase, whose activities showed no large increase comparable to that of the gamma-glutamyltransferase, as the culture approached 'phase-out'.