Irreversible inhibition of human glutathione S-transferase isoenzymes by tetrachloro-1,4-benzoquinone and its glutathione conjugate.

The quinones tetrachloro-1,4-benzoquinone (1,4-TCBQ) and its glutathione conjugate (GS-1,4-TCBQ) are potent irreversible inhibitors of most human glutathione S-transferase (GST) isoenzymes. Human pi, psi, and mu are almost completely inhibited at a molar ratio 1,4-TCBQ/GST = 2/1. The isoenzyme B1B1 was inhibited up to 75%, and higher concentrations (1,4-TCBQ/GST = 6/1) were needed to reach this maximum effect. For these isoenzymes 75-85% of the maximal amount of inhibition was already reached on incubation of equimolar ratios of 1,4-TCBQ and subunit GST, while approximately 1 nmol (0.82-0.95) 1,4-[U-14C]TCBQ per nmol subunit GST could be covalently bound. These results suggest that these GST isoenzymes possess only one cysteine in or near the active site of GST, which is completely responsible for the inhibition. In agreement, human isoenzyme B2B2 which possesses no cysteine, was not inhibited and no 1,4-TCBQ was bound to it. The rate of inhibition was studied at 0 degrees: 1,4-TCBQ, trichloro-1,4-benzoquinone and GS-1,4-TCBQ all inhibit GST very fast. Especially for B1B1, the inhibition by the glutathione conjugate is significantly faster than inhibition by 1,4-TCBQ: the glutathione moiety seems to target the quinone to the enzyme. For the other isoenzymes only minor differences are observed between 1,4-TCBQ and its glutathione conjugate under the conditions used.