To examine the role of the distal His42 residue in the catalytic mechanism of pea cytosolic ascorbate peroxidase, two site-directed variants were prepared in which His42 was replaced with alanine (H42A) or glutamic acid (H42E). Electronic spectra of the ferric derivatives of H42A and H42E (pH 7.0, mu = 0.10 m, 25.0 degrees C) revealed wavelength maxima [lambda(max) (nm): 397, 509, approximately equal to 540(sh), 644 (H42A); 404, 516, approximately equal to 538(sh), 639 (H42E)] consistent with a predominantly five-co-ordinate high-spin iron. The specific activity of H42E for oxidation of L-ascorbate (8.2 +/- 0.3 U.mg(-1)) was approximately equal to 30-fold lower than that of the recombinant wild-type enzyme (rAPX); the H42A variant was essentially inactive but activity could be partially recovered by addition of exogenous imidazoles. The spectra of the Compound I intermediates of H42A [lambda(max) (nm) = 403, 534, 575(sh), 645] and H42E [lambda(max) (nm) = 404, 530, 573(sh), 654] were similar to those of rAPX. Pre-steady-state data for formation of Compound I for H42A and H42E were consistent with a mechanism involving accumulation of a transient enzyme intermediate (K(d)) followed by conversion of this intermediate into Compound I (k'(1)). Values for k'(1) and K(d) were, respectively, 4.3 +/- 0.2 s(-1) and 30 +/- 2.0 mM (H42A) and 28 +/- 1.0 s(-1) and 0.09 +/- 0.01 mM (H42E). Photodiode array experiments for H42A revealed wavelength maxima for this intermediate at 401 nm, 522 nm and 643 nm, consistent with the formation of a transient [H42A-H(2)O(2)] species. Rate constants for Compound I formation for H42A were independent of pH, but for rAPX and H42E were pH-dependent [pKa = 4.9 +/- 0.1 (rAPX) and pK(a) = 6.7 +/- 0.2 (H42E)]. The results provide: (a) evidence that His42 is critical for Compound I formation in APX; (b) confirmation that titration of His42 controls Compound I formation and an assignment of the pK(a) for this group; (c) mechanistic and spectroscopic evidence for an intermediate before Compound I formation; (d) evidence that a glutamic acid residue at position 42 can act as the acid-base catalyst in ascorbate peroxidase.