Localization of γ‐glutamyl transpeptidase in lymphoid cells

yGlutamy1 transpeptidase catalyzes the initial step in the breakdown of glutathione. A proposed role for the enzyme in transport processes [ 1,2] is consistent with its localization in the plasma membranes of a variety of epithelial cells [3,4]. The enzyme is also present in human and rat lymphoid cells and the activity can be increased by treatment of the cells with mitogenic agents [5]. The transpeptidase activity of human lymphoblastic lines varies considerably; cell lines from patients with lymphoproliferative diseases exhibit activities significantly lower than lines obtained from normal subjects, whereas the highest activity was observed in a line derived from a multiple myeloma patient [ 51. Ultrastructural localization of the transpeptidase activity, results of which are reported here, show that the lymphoid cell plasma membrane is the primary site of enzyme activity. A largely uniform distribution of enzyme on the cell-surface is indicated from experiments with ferritin-antibody conjugates. Although short-term incubation of cells in cytochemical medium results in staining of the plasma membrane only, longer incubation reveals activity in the membranes of the endoplasmic reticulum and the Golgi. This intracellular activity may be associated with intermediate stages in the processing of the enzyme en route to its insertion in the plasma membrane and may also function in the turnover of intracellular glutathione. The similarity of the lymphoid cell enzyme to the purified kidney enzyme is evident from antibody inhibition studies and gel electrophoresis in sodium dodecyl sulfate (SDS).