DIFFERENCES IN STEREOSELECTIVITY AND CATALYTIC EFFICIENCY OF 3 HUMAN GLUTATHIONE TRANSFERASES IN THE CONJUGATION OF GLUTATHIONE WITH 7-BETA,8-ALPHA-DIHYDROXY-9-ALPHA,10-ALPHA-OXY-7,8,9,10-TETRAHYDROBENZO(A)PYRENE
- 1 May 1986
- journal article
- research article
- Vol. 46 (5) , 2220-2224
Abstract
The kinetics of the enzyme-catalyzed conjugation of glutathione with (.+-.)-7.beta.,8.alpha.-dihydroxy-9.alpha.-oxy-7,8,9,10-tetrahydrobenzoa(a)pyrene [(.+-.)-anti-BPDE] have been studied with the following human cytosolic glutathione transferases: the basic (.alpha.-.epsilon.) and near-neutral (.mu.) isoenzymes from liver, and the acidic (.pi.) isoenzyme from placenta. When the BPDE concentration was varied (using 5 mM glutathione) the apparent Vmax values for transferases .alpha.-.epsilon., .mu., and .pi. were 38, 570, and 825 nmol .cntdot. mg-1 .cntdot. min-1, respectively, with corresponding apparent Km values of 88, 27, and 54 .mu.M. The apparent Km values for glutathione [using 80 .mu.M (.+-.)-anti-BPDE] were 0.4, 0.7, and 0.1 mM for transferases .alpha.-.epsilon., .mu., and .pi., respectively. The glutathione conjugates formed with the two enantiomers of (.+-.)-anti-BPDE were resolved by high performance liquid chromatography. The percentages of conjugates derived from the highly carcinogenic (+)-enantiomer were 59, 60, and .gtoreq. 90% for transferases .alpha.-.epsilon., .mu., and .pi., respectively. The separate enantiomers of anti BPDE were assayed in experiments with transferases .mu. and .pi.. Both enantiomers were substrates for transferase .mu., but only the (+)-enantiomer gave measurable activity with transferase .pi.. A 3-fold increase in Vmax and Km values for transferase .pi. was obtained with (+)-anti-BPDE as compared with the racemic substrate and could be quantitatively accounted for by the finding that (-)-anti-BPDE serves as a competitive inhibitor for transferase .pi.This publication has 31 references indexed in Scilit:
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