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.