Aconitase activity in rat liver

This paper presents data about the subcellular distribution of aconitases in rat liver and some properties of the aconitase activity in cytosol, mitochondria and soluble mitochondrial protein (SMP). The cytosolic and mitochondrial aconitase activity was 64.8% or 61.0% and 20.1% or 19.4% of the total rat liver aconitase activity when cis-aconitate or isocitrate was used as substrate. Aconitase activity of stored SMP and mitochondria with phosphate buffer (pH 7.4) and 0.25 M sucrose (pH 7.4) as isolation medium respectively, was reduced to an equal extent upon exposure to air. Fresh SMP preparations immediately and three hr after isolation had the same aconitase activity. It is concluded that phosphate has no role in the oxidative degradation of mitochondrial aconitase and does not inhibit it. Complete restoration of the decreased mitochondrial aconitase activity to the initial level was achieved with thiomalate and Fe2+ under anaerobic conditions or 60-70% was restored during the long period (60 min) of incubation with exogenous substrate. The aconitase activity of cytosol and mitochondria increased upon exposure to air for 7 1/2 hr. This finding is interpreted to suggest the existence of putative aconitase activity.