NAD1 use is an ancestral trait of isocitrate dehydrogenase (IDH), and the NADP1 phenotype arose through
evolution as an ancient adaptation event. However, no NAD1-specific IDHs have been found among type II
IDHs and monomeric IDHs. In this study, novel type II homodimeric NAD-IDHs from Ostreococcus
lucimarinus CCE9901 IDH (OlIDH) and Micromonas sp. RCC299 (MiIDH), and novel monomeric
NAD-IDHs from Campylobacter sp. FOBRC14 IDH (CaIDH) and Campylobacter curvus (CcIDH) were
reported for the first time. The homodimeric OlIDH and monomeric CaIDH were determined by size
exclusion chromatography and MALDI-TOF/TOF mass spectrometry. All the four IDHs were
demonstrated to be NAD1-specific, since OlIDH, MiIDH, CaIDH and CcIDH displayed 99-fold, 224-fold,
61-fold and 37-fold preferences forNAD1 overNADP1, respectively. The putative coenzyme discriminating
amino acids (Asp326/Met327 in OlIDH, Leu584/Asp595 in CaIDH) were evaluated, and the coenzyme
specificities of the two mutants, OlIDH R326H327 and CaIDHH584R595, were completely reversed from NAD1
to NADP1. The detailed biochemical properties, including optimal reaction pH and temperature,
thermostability, and metal ion effects, of OlIDH and CaIDH were further investigated. The evolutionary
connections among OlIDH, CaIDH, and all the other forms of IDHs were described and discussed
thoroughly.