Glutathione S-transferase-π is secreted as a monomer into human plasma by platelets and tumor cells

Abstract By employing an ELISA for detection of glutathione S -transferase-π (GST-π) established in our laboratory, gel filtration profiles of GST-π in the plasma of normal subjects and patients with malignant tumors were investigated. The results showed that the plasma GST-π for both of these groups was approximately half the molecular size of placental GST-π used as a standard control. Similar analyses were performed on GST-π of platelets and cultured cancer cells, which are considered to be the main sources of the GST-π in the plasma of normal subjects and cancer patients, respectively. The results indicated that the GST-π in both the centrifuged supernatants of aggregated platelets and in the culture medium of cancer cells was about half of the molecular size of intact GST-π. Morover, the GST-π in the culture medium was shown to have an N-terminus and a C-terminus, by analysis with specific ELISA. Western blot analysis of the GST-π in the culture medium detected a single band migrating at 23 kDa, confirming that the extracellular GST-π was the monomer, not a cleaved form of intact GST-π. The release of GST-π from cancer cells was suppressed at 4°C, or by sodium azide, but not suppressed by colchicine or cytochalasin B. These findings suggest that the GST-π may be released by an energy-dependent, active process, and not by a secretion mechanism.