Held et al. (1984a,b) demonstrated previously that
glutathione (GSH), a negatively charged
thiol, is significantly less efficient in the
hydrogen atom donation repair reaction with radicals induced by radiation in transforming
DNA (
t-DNA) than are other
thiol compounds. Fahey et al. (1991a,b) postulated that the charge on
thiols can influence their ability to radioprotect
DNA. GSH, which is excluded from the vicinity of
DNA due to its negative charge, is less protective than neutral or positively charged
thiols. We have investigated this phenomenon further with
trypanothione, the conjugate of
glutathione and
spermidine, N1,N8-bis (L-gamma-glutamyl-L-hemicystinyl-glycyl)-
spermidine.
Trypanothione exists in aerobic
solution largely as the disulphide (T(S)2) but is maintained in the cell in the reduced form (T(SH)2) by means of an
NADPH-dependent flavo-
enzyme,
trypanothione reductase (TR). Experimental data show that T(S)2 in the presence of TR radioprotects
t-DNA in the absence of
oxygen much better than GSH or
spermidine alone or in combination. Little radioprotection by T(S)2 is seen when TR is not present. The results obtained with reduced
trypanothione at low concentrations suggest that radioprotection of
t-DNA in
hypoxia occurs predominantly by H atom donation and slightly by .
OH radical scavenging, and the protection is greater than that by GSH or
spermidine because the
polyamine moiety in
trypanothione allows a greater concentration of GSH near the
DNA molecule.