Brdička-type catalytic currents of hemoglobins: Part III. pH dependence of the catalytic current - an indication of quaternary structure changes?☆

Abstract Changes of the catalytic current of bovine, ovine, rabbit, porcine, human, dog and pigeon hemoglobins (Hb), differing in the number of SH groups from 2 to 10 per tetramer, were studied in ammonia buffer solutions covering the pH range between 8.6 and 11.0. Two Hb concentrations were used: 5.0 × 10 −7 and 1.54 × 10 −6 M . In all cases, the pH rise was accompanied by an increase in the catalytic current in the form of two steps, until a maximum value of I catal at about pH 10.6–10.8 was attained, after which a decrease followed. A similar course of this relationship was observed for both concentrations of all the hemoglobins. The shape of the I catal = f (pH) curves resembled titration curves, from which common characteristic points were evaluated for all the hemoglobins, corresponding to pH values of about 9.8 ± 0.1 and 10.5 ± 0.1, done in a similar way as for p K . In the discussion, ionization of hemoglobins, complex formation with Co(II), denaturation, adsorption phenomena, in connection with the charge of Hb, quaternary structure changes, the accessibility and reactivity of SH groups and possible changes of the preparations prior to catalytic wave registration were taken into account. The most probable reason for formation of the second step in the I catal = f (pH) curves was considered to be the dissociation of deoxy-Hb tetramers to dimers. In the catalytic process, depending on the conditions at least two types of hemoglobin-Co(II) complex of different character may participate. The drop in I catal at high pH values was assumed to be the result of the competing action of ammonia on complexation processes involving hemoglobin and Co(II).