ONLY THE GLUTATHIONE-DEPENDENT ANTIOXIDANT ENZYMES ARE INHIBITED BY HEMATOTOXIC HYDROXYLAMINES

Hydroxylamine and some of its derivatives are known to cause oxidative effects both in vitro and in vivo. In the current study we investigat ed the effects of hydroxylamines on the enzymatic antioxidant defense system in human erythrocytes. The activity of catalase and superoxide dismutase was not significantly influenced by any of the hydroxylamine s tested. However, the activity of glutathione peroxidase (GPX) and gl utathione S-transferase (GST) was strongly inhibited by hydroxylamine and its O-derivatives (O-methyl and O-ethyl hydroxylamine). GPX was al so inhibited by two N-derivatives of hydroxylamine (i.e. N-dimethyl an d N,O-dimethyl hydroxylamine). This indicates that exposure to hydroxy lamines not only changes the cellular oxidation-reduction status but a lso leads to inhibition of the glutathione dependent antioxidant enzym es. GST as well as GPX have cysteine residues at the active site of th e enzymes. Such an accessible thiol group is generally susceptible to formation of protein-mixed disulphides or intramolecular disulphides. If these thiol groups are essential for activity this would be accompa nied by an increase or decrease in the enzyme activity. In principle t his is also true for glutathione reductase (GR), which in this study w as only inhibited by N,O-dimethyl and N-methyl hydroxylamines. However , GR is capable to reduce these disulphides by taking up two electrons , either from its substrate NAPDH or from another reductant. Oxidation of these thiol groups in GR would thus not lead to impairment of GR a ctivity. The fact that NODMH and NMH do decrease the GR activity can t herefore only be explained by other modifications. The activity loss o f GST and GPX on the other hand, is likely to involve oxidation of cri tical cysteine residues. The practical consequence of these findings i s that the cellular prooxidant state that may arise in erythrocytes ex posed to hydroxylamines can be further increased by activity loss of p rotective enzymes, which may decrease the average life span of the red blood cell.