Reactivity of plasma glutathione peroxidase with hydroperoxide substrates and glutathione.
We studied enzyme kinetics parameters of plasma glutathione peroxidase (GSHPx-P) and the major cellular enzyme, GSHPx-1, for the substrates, H2O2, linoleic acid hydroperoxide (LinOOH), and glutathione (GSH). The major objectives were to determine whether the relatively slow GSHPx-P enzyme had a lower reactivity with hydroperoxides or with GSH and to identify favored hydroperoxide substrates. The rate constants describing the reactivity of human GSHPx-P and human GSHPx-1 with LinOOH and H2O2 are in the same range; GSHPx-P is more reactive with LinOOH and GSHPx-1 is more reactive with H2O2. GSHPx-P also has a low level of reducing activity toward cholesterol 7 alpha-OOH and no detectable activity with the 5 alpha-OOH isomer in contrast to phospholipid hydroperoxide glutathione peroxidase (PHGPx) which readily reduced both isomers. GSHPx-P catalytic activity toward phospholipid hydroperoxides is demonstrable in the absence of detergents, enhanced at low concentrations by deoxycholate, and strongly inhibited by Triton X-100 and incorporation into liposomes. These properties are the opposite of PHGPx. These results suggest that GSHPx-P largely lacks the membrane interfacial properties of PHGPx. GSHPx-P exhibits a smaller GSH rate constant than GSHPx-1. This property partially explains the slower turnover of GSHPx-P with several hydroperoxide substrates; the low reactivity with GSH is not consistent with efficient GSHPx function in the bulk plasma volume. GSHPx-P kinetic properties suggest that it would function best as a free fatty acid hydroperoxidase in GSH-rich microenvironments. Minimally, the secretion of reduced enzyme would permit it to scavenge free fatty acid hydroperoxides.
Esworthy
R S
RS
Department of Medical Oncology and Therapeutics Research, City of Hope National Medical Center, Duarte, California 91010.
Chu
F F
FF
Geiger
P
P
Girotti
A W
AW
Doroshow
J H
JH
D000818:Animals; D005979:Glutathione Peroxidase / Q000097:blood* / Q000378:metabolism*; D006801:Humans; D006861:Hydrogen Peroxide / Q000378:metabolism*; D007527:Isoenzymes / Q000097:blood / Q000378:metabolism; D007700:Kinetics; D008041:Linoleic Acids / Q000378:metabolism*; D008054:Lipid Peroxides / Q000378:metabolism*; D008297:Male; D051381:Rats; D017207:Rats, Sprague-Dawley; D013379:Substrate Specificity; D013737:Testis / Q000201:enzymology
D007527:Isoenzymes; D008041:Linoleic Acids; D008054:Lipid Peroxides; C008285:linoleic acid hydroperoxide; D006861:Hydrogen Peroxide; D005979:Glutathione Peroxidase