Properties of pig heart aconitase

Comparison of pig heart aconitase (Kennedy et al., 1972) with yeast (Candida lipolytica) aconitase (Suzuki et al., 1973) reveals similarities in molecular weight and iron content but not in sulphide content. Comparison with the Mildvan & Villafranca (1971) pig heart aconitase preparation reveals differences in iron ligands, specific activity and other properties; these differences possibly arise from protein association as pig heart protein associates under a variety of conditions. The electron spin resonance spectrum, g 4.25, and the low molar relaxivity, 473m−1·s−1, of water H+ suggest the presence of high-spin Fe(III) unco-ordinated to water in the enzyme. The iron chromophore on acid titration at 320nm gives a curve with an inflexion at pH4.2. Ten of 16 expected thiol equivalents are titrated with p-hydroxymercuribenzoate suggesting the presence of cystine as well as cysteine residues. Inhibition of the activation of inactive (activatable) enzyme is sigmoidally related to the molar ratio, p-hydroxymercuribenzoate/enzyme with 10–11mol of mercurial compound causing complete inhibition. Active enzyme, free from activating reagents, requires high molar ratios of mercurial compound for rapid inhibition. In terms of p-hydroxymercuribenzoate the enzyme then lacks an essential thiol group.