Identification and purification of a distinct dihydrolipoamide dehydrogenase from pea chloroplasts

Two distinct dihydrolipoamide dehydrogenases (E3s, EC 1.8.1.4) have been detected in pea (Pisum sativum L. cv. Little Marvel) leaf extracts and purified to at or near homogeneity. The major enzyme, a homodimer with an apparent subunit M(r) value 56,000 (80-90% of overall activity), corresponded to the mitochondrial isoform studied previously, as confirmed by electrospray mass spectrometry and N-terminal sequence analysis. The minor activity (10-20%), which also behaved as a homodimer, copurified with chloroplasts, and displayed a lower subunit M(r) value of 52,000 which was close to the M(r) value of 52,614 +/- 9.89 Da determined by electrospray mass spectrometry. The plastidic enzyme was also present at low levels in root extracts where it represented only 1-2% of total E3 activity. The specific activity of the chloroplast enzyme was three- to fourfold lower than its mitochondrial counterpart. In addition, it displayed a markedly higher affinity for NAD+ and was more sensitive to product inhibition by NADH. It exhibited no activity with NADP+ as cofactor nor was it inhibited by the presence of high concentrations of NADP+ or NADPH. Antibodies to the mitochondrial enzyme displayed little or no cross-reactivity with its plastidic counterpart and available amino acid sequence data were also suggestive of only limited sequence similarity between the two enzymes. In view of the dual location of the pyruvate dehydrogenase multienzyme complex (PDC) in plant mitochondria and chloroplasts, it is likely that the distinct chloroplastic E3 is an integral component of plastidic PDC, thus representing the first component of this complex to be isolated and characterised to date.