Glutathione conjugation of the fluorophotometric epoxide substrate, 7-glycidoxycoumarin (GOC), by rat liver glutathione transferase isoenzymes.
The fluorophotometric substrate, 7-glycidoxycoumarin (GOC), was examined for the assay of epoxide-glutathione (GSH)-conjugating activities of seven major GSH transferases (GSTs) isolated from rat liver cytosols. GST 7-7 (GST-P), isolated from the liver cytosol of rats bearing hepatic hyperplastic nodules, catalysed the GSH conjugation of GOC at a higher rate than any other examined GST isolated from the normal rat liver cytosol. GSTs 3-3, 3-4 and 4-4 (group 3-4 enzymes) had specific activities towards GOC by one fifth to one third of that of GST 7-7. GSTs 1-1, 1-2 and 2-2 (group 1-2 enzymes) had very low activities towards this epoxide. A kinetic study indicated that GST 7-7 showed the largest kappa cat/Km value for the catalytic reaction of GOC-GSH conjugation among the GSTs. In spite of their much smaller kappa cat values, group 3-4 enzymes showed much larger kappa cat/Km values for GOC than the group 1-2 enzymes, because GOC had a much higher affinity for group 3-4 enzymes than for group 1-2 enzymes. A comparative study was also done with GSH conjugations of styrene 7,8-oxide (STO) and 1-chloro-2,4-dinitrobenzene by the GSTs. Unlike GOC, the conjugation of STO was mediated at rates about twice as high by group 3-4 enzymes than by GST 7-7. STO was also a very poor substrate for group 1-2 enzymes.
Hiratsuka
A
A
Department of Hygienic Chemistry, Tokyo College of Pharmacy, Japan.
Yokoi
A
A
Sebata
N
N
Watabe
T
T
Satoh
K
K
Hatayama
I
I
Sato
K
K
D000818:Animals; D003374:Coumarins / Q000378:metabolism*; D004137:Dinitrochlorobenzene / Q000378:metabolism; D004852:Epoxy Compounds / Q000378:metabolism; D005978:Glutathione / Q000378:metabolism*; D005982:Glutathione Transferase / Q000502:physiology*; D066298:In Vitro Techniques; D007527:Isoenzymes / Q000502:physiology; D008099:Liver / Q000201:enzymology*; D008297:Male; D051381:Rats; D011919:Rats, Inbred Strains; D013379:Substrate Specificity
D003374:Coumarins; D004137:Dinitrochlorobenzene; D004852:Epoxy Compounds; D007527:Isoenzymes; C041461:7-glycidoxycoumarin; C013690:styrene oxide; D005982:Glutathione Transferase; D005978:Glutathione