MECHANISTIC STUDIES ON THE REACTIONS OF MOLYBDENUM(VI), TUNGSTEN(VI),VANADIUM(V), AND ARSENIC(V) TETRAOXO ANIONS WITH THE FEIIFEIII FORM OF PURPLE ACID-PHOSPHATASE FROM PORCINE UTERI (UTEROFERRIN)

The (FeFeIII)-Fe-II form of purple acid phosphatase (PAP(r)) from porc ine uteri (uteroferrin)catalyses the hydrolysis of phosphate esters. I n a previous paper kinetic studies on the reactions of PAP, with five different phosphate moieties, including [H2PO4](-) as a prototype, wer e reported. Here these studies have been extended to include the compl exing of tetraoxo XO(4) anions of molybdate(VI), tungstate(VI), vanada te(V, and arsenate(V) with PAP,. W-vis absorbance changes are small an d the range of concentrations is restricted by the need to maximise mo nomer XO(4) forms. Rate constants k(obs)(25 degrees C) were determined by stopped-flow monitoring of the reactions at similar to 520 nm. Abs orbance changes with arsenate(V) were too small for rate constants to be evaluated. At pH 5.6 the k(obs) values obtained are independent of [XO(4)] (X = Mo, W,V), in the range (0.4-10) x 10(-4) M. At concentrat ions of VO4 > 2.0 x 10(-4) M formation of [V3O9](3-) occurs, and a dec rease in experimentally determined rate constants is observed. In the pH range 3.5-6.3, as in the case of the PO4 reagents, rate constants d ecrease with increasing pH due to the slower reaction of Fe(III)-OH as compared to Fe(III)-OH2. A mechanism involving rapid XO(4) binding at the more labile Fe(II) is proposed. Slower [XO(4)]-independent bridgi ng to the Fe(III) then occurs with displacement of H2O at the lower pH 's. Nucleophilic attack of the conjugate-base Fe(III)-OH on the phosph ate-ester results in hydrolysis which peaks at pH 4.9. At this pH rate constants/s(-1) are for [MoO4](2-) (1.33), [WO4](2-) (0.77) and [H2VO 4](-) (1.44), as compared to values of similar to 0.5s(-1) for the fiv e different phosphates. There is therefore little dependence on the id entity of XO(4). With O-2 the PAP(r) . MoO4 adduct is less reactive th an PAP(r), while PAP(r) . AsO4 is more reactive. These results are exp lained by the effect of coordinated XO(4) on the PAP(0)/PAP(r) reducti on potential.