Characterization of a low-activity allele of NADP+-dependent isocitrate dehydrogenase from Drosophila melanogaster

We have characterized biochemical effects of IdhGB1 in Drosophila melanogaster. This is a "null"-activity allele for NADP+-dependent isocitrate dehydrogenase (NADP-IDH) isolated from a natural population. The homozygous mutant strain has 5% of the NADP-IDH specific activity found in controls and less than 24% of the immunologically cross-reacting material (CRM). This mutation maps to 27.2 on the third chromosome, to the right of h. The biochemical phenotype of this mutant strain includes a coordinate reduction in malic enzyme (ME) specific activity and CRM and an increase in specific activity for the pentose-phosphate shunt enzymes, 6-phosphogluconate dehydrogenase and glucose-6-phosphate dehydrogenase. The Km values for purified NADP-IDH are not different from those found for the purified control enzyme for NADP+ or isocitrate. It is suggested that this allele may represent a cis-acting control mutation for one of at least two loci involved in the production of NADP-IDH in D. melanogaster.