MAGNETIC-SUSCEPTIBILITY STUDIES ON THE DIIRON FORMS OF THE METALLOPROTEIN PURPLE ACID-PHOSPHATASE FROM BOVINE SPLEEN AND KIDNEY BEAN

Temperature dependent magnetic susceptibility measurements of the redu ced and of the oxidized diiron forms of purple acid phosphatase from k idney bean (KBP) and from bovine spleen (BSP) have been carried out in the temperature range 4.2-220 K in order to detect the strength of th e exchange coupling between the two metal centers in the active site. Investigations of BSP were additionally carried out at different pH va lues. For the reduced forms with the Fe(III)-Fe(II) center, values for the exchange coupling constant in the range of J = -5 to -15 cm(-1) w ere obtained for KBP (pH = 6) and BSP (pH = 3.9, 4.9 and 5.6), based o n the Hamiltonian (H) over cap = -2JS(1)S(2). In the presence of phosp hate at pH = 3.6 and 5.6 a significant reduction of the strength of th e exchange coupling was observed. The analysis of the magnetic data on the oxidized, diferric metalloproteins yielded coupling constants wit h similar values as found for the reduced forms. After addition of pho sphate to the oxidized BSP. a reduction of the exchange coupling was a lso observed, indicating a similar change in the magnetic properties a s found for the reduced form.