Fast-reacting thiols in rat hemoglobins can intercept damaging species in erythrocytes more efficiently than glutathione

The S-conjugation rates of the free-reacting thiols present on each component of rat hemoglobin with 5,5-dithio-bis(2,2-nitrobenzoic acid) (DTNB) have been studied under a variety of conditions. On the basis of their reactivity with DTNB (0.5 mM), three classes of thiols have been defined as follows: fast reacting (fHbSH), with t( 1/4 ) <100 ms; slow reacting (sHbSH), with t( 1/4 ) 30-50 s; and very slow reacting (vsHbSH), with t( 1/4 ) 180-270 s. Under paraphysiological conditions, fHbSH (identified with Cys.125β(H3)) conjugates with DTNB 100 times faster than glutathione and ~4000 times more rapidly than (v)sHbSH (Cys-13α(A11) and Cys-93β(F9)). Such characteristics of fHbSH reactivity that are independent of the quaternary state of hemoglobin are mainly due to the following: (i) its low pK (~6.9, the cysteinyl anion being stabilized by a hydrogen bond with Ser-123β(H1)) and (ii) the large exposure to the solvent (as measured by analysis of a model of the molecular surface) and make these thiols the kinetically preferred groups in rat erythrocytes for S-conjugation. In addition, because of the high cellular concentration (8 mM, i.e. four times that of glutathione), fHSHs are expected to intercept damaging species in erythrocytes more efficiently than glutathione, thus adding a new physiopathological role (direct involvement in cellular strategies of antioxidant defense) to cysteinyl residues in proteins.