Glutathione transferase (GST) activity and GST
isoenzyme composition have been determined for 24 human
neoplasms and 6 human tumor cell lines. Substantial activity (40-1010 milliunits/mg
protein) was identified in all
tumor specimens examined and three of the tumor cell lines. Three tumor cell lines, the human
small cell carcinoma line SW2-10S, the
Burkitt's lymphoma derived cell line Raji, and the human
breast carcinoma cell line MCF-7, contained minimal GST activity. Although the small size of the
tumor samples precluded
isoenzyme analysis by substrate specificities, analysis of GST activity following sample separation by isoelectric focusing indicated that the predominant (comprising at least 70% of the
1-chloro-2,4-dinitrobenzene-conjugating activity) GST
isoenzyme in each of these primary
tumor (17 of 17) and tumor cell line (3 of 3) extracts was anionic (isoelectric point, 4.5-4.8). In three
tumor samples,
adenocarcinomas of the lung, colon, and stomach, analysis by isoelectric focusing identified minor but detectable (10-20% of total) cationic GST. The anionic form of GST has been purified to homogeneity from three primary human
tumors: a
malignant melanoma; a
mesothelioma; and a
breast carcinoma. GST from these
tumors consists of two subunits each of Mr 25,200. On Western blot analysis,
antibodies raised against the anionic GST purified from
mesothelioma detect
protein of Mr approximately 25,000 in extracts of both normal kidney and
tumors containing anionic GST activity but not in extracts of human liver that did not contain detectable anionic activity. The
amino acid compositions of these
proteins were quite similar to that previously described for GST-pi and the amino-terminal amino acid sequences for these
tumor-derived
isoenzymes are identical to one another and to that previously described for GST-pi from human placenta. GST is a major enzymatic activity in many human
malignancies, comprising as much as 3% of the cytosolic
protein of some
tumors. Anionic GST is the predominant form of
glutathione transferase activity in many human
tumors and human tumor cell lines. In selected
tumor samples the predominant anionic GST
isoenzyme has been identified as a member of the pi class of this
enzyme family. In addition, at least 3 of 17
tumor samples contained lesser but detectable amounts of cationic GST, probably of the alpha class. By conjugating
glutathione with electrophilic anticancer drugs, the substantial levels of GST in human
tumors may have a role in the innate or acquired resistance of these
neoplasms to anticancer
therapy.