Isozymes of isocitrate dehydrogenase: Subunit structure and intracellular location

Abstract

Isocitrate dehydrogenase (IDH) exists in mammalian tissues as three enzymes, one specific for NAD and two specific for NADP. The two NADP isozymes differ in electrophoretic mobility, immunological characteristics, and tissue and subcellular location. One is found mainly in the mitochondrial fraction and one in the supernatant fraction of tissue homogenates. Two allelic forms of the supernatant NADP isocitrate dehydrogenase have been described in inbred strains of mice. In a heterozygote containing both alleles, three forms of the supernatant NADP-IDH are generated in a ratio of 1:2:1. This implies that the supernatant enzyme is a dimer. The mitochondrial form is not affected by the gene, indicating separate genetic control. Liver mitochondria contain both the supernatant and the mitochondrial isozymes. Thus at least one protein found in the mitochondria is encoded in a nuclear gene.Research in biochemical genetics led to the formulation of the one-gene-one enzyme hypothesis with the consequent expectation that one enzyme would exist as a single molecular type within an organism. Exceptions would be diploid organisms carrying allelic genes for the enzyme, in which case two forms of the enzyme would be found. However, in recent years an increasing amount of evidence has shown that many enzymes exist in multiple molecular forms, or as isozymes, within the cells of a single organism, even when that organism is homozygous (Markert and Mller, '59; Wieland and Pfleiderer, '57; Vessel1 and Bearn, '58). Such molecular heterogeneity has been detected by many different techniques, including electrophoresis, column chromatography, centrifugation, salt fractionation, and by determination of tissue specific differences in enzyme kinetics, coenzyme specificity, and immunochemical behavior. The moleculuar basis for this heterogeneity may be different for different enzymes. The binding of small molecules to the enzyme can give rise to multiple forms, as in the case of the two active forms of phosphoglucomutase, which differ in the presence of a phosphate group bound to one form during the enzymatic reaction (Koshland, '64 be polymers of different numbers of the same polypeptide subunit. The two forms of phosphorylase, a and b, differ in that one is a dimer of the other as well as in the presence of two phosphate groups bound to serine residues in the dimer (Brown and Cori, '61).On the other hand, the several forms of an enzyme may differ significantly in primary protein structure, and thus be under multiple gene control, as has been clearly shown for the five principal isozymes of lactate dehydrogenase (LDH) . These isozymes are found in characteristic relative amounts in the different tissues of an organism (Markert and MBller, '59) and have the same molecular weight but differ in electrophoretic mobility. The two extreme forms, LDH-1 and LDH-5, differ in amino acid content, immunological specificity, kinetic properties, coenzyme specificity, heat stability, and other characteristics. The intermediate forms, , are also...

Glucose-6-Phosphate Dehydrogenase Modulates Cytosolic Redox Status and Contractile Phenotype in Adult Cardiomyocytes

“…Furthermore, expression of the cytosolic isoform of ICD (ICD1) is minimal in the heart, 38 and is several orders of magnitude lower relative to ICD1 expression in other tissues, including corneal epithelium, liver, kidney, and spleen. 39 Cytosolic ICD1 is therefore also unlikely to contribute significantly to the maintenance of cytosolic thiols. Furthermore, we found that in cardiomyocytes inhibition of G6PD caused a near complete depletion of total thiols and reduced glutathione.…”

The third allele of supernatant isocitrate dehydrogenase of house mouse, iId-1c, originates from asian continent.

“…Two alleles, Id-la and Id-l", were reported in inbred strains of mice (Henderson 1965;Roderick et al 1971;Staats 1976), and were also observed in the Western Hemisphere wild house mice, though the frequencies of the two alleles were variable among the populations investigated (Ruddle et al 1969;Hunt and Selander 1973;Berry and Peters 1977;Britton and Thaler 1978). One female carrying an Id-P allele has been found in the Hawaiian Islands (Wheelar and Selander 1972).…”

The Distribution of Blood, Serum Protein and Enzyme Groups in a Series of Lebanese in Australia

“…Haemoglobin was examined on the same gels used for the G6PD tests. The other enzyme systems were studied, using the procedures outlined by the original authors: phosphoglycerate kinase (Chen, Malcolm, Yoshida and Giblett, 1971), phosphohexose isomerase (Detter, Ways, Giblett, Baughan, Hopkinson, Povey and Harris, 1968), NADH diaphorase (Hopkinson, Gomey, Gook, Robson and Harris, 1970), isocitrate dehydrogenase (Henderson, 1965) and glutamic oxaloacetic transaminase (Davidson, Gortner, Rattazzi, Ruddle and Lubs, 1970).…”