Electrophoretic analysis of subcellular fractions prepared from rat and mouse liver indicate that the more acidic group of aconitase isozymes represents the soluble enzyme whilst the more basic group represents the mitochondrial form. The corresponding groups of isozymes in man can be similarly identified by analogy with the rodent patterns. Qualitatively similar isozyme patterns are found in a variety of human tissues during both adult and foetal life, but the tissues vary one from another both in the total levels of aconitase activity they contain and in the relative activities of the soluble and mitochondrial enzyme. During foetal development each isozyme undergoes an increase in activity in several tissues, although the relative activities in some tissues change during gestation. Investigation of the substrate specificities of the isozymes using activity stains designed to follow the enzymic conversion of citrate and cis aconitate to isocitrate, and of isocitrate and cis aconitate to citrate, reveal no evidence for the existence of separate α and β aconitases. Isoelectric focusing under standard conditions yields an isoelectric point value of 6.9 ± 0.2 for the human mitochondrial aconitase and a value of 5.1 ± 0.1 for the soluble aconitase. Gel filtration either in the presence or absence of substrate yields a molecular weight value of 65000 ± 4000 for the human mitochondrial aconitase and a value of 83000 ± 5000 for the soluble aconitase. A difference in molecular weights of this magnitude is unusual amongst systems of isozymes and is of interest with regard to the evolutionary history of aconitase. Investigation of the relative heat stabilities of the isozymes both by electrophoretic analysis of heated aqueous tissue extracts and by heating the isozymes in starch gels after electrophoretic separation at pH 7.4 indicates that the soluble aconitase is more stable than the mitochondrial. The isozymes of mitochondrial aconitase are found to give rise to secondary, more anodal components upon storage of aqueous tissue extracts at 4-6°C. The electrophoretic pattern given by the soluble aconitase, however, remains unchanged under similar conditions.
|Original language||English (US)|
|Number of pages||17|
|Journal||Annals of Human Genetics|
|Publication status||Published - Jan 1 1977|
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