Circulating IgG-LD complex, dissociable by addition of NAD+.

Abstract

Abstract Macromolecular LD (lactate dehydrogenase, EC 1.1.1.27) was present in the serum of a patient suffering from idiopathic fibrosis of the lung and presenting signs of autoimmune disease. By using gel filtration and affinity chromatography techniques, the vast majority of the patient's serum LD activity was shown to consist of LD-IgG complexes, which dissociated in the presence of added nicotinamide adenine dinucleotide (NAD+). Binding studies with tritiated NAD+ indicated that complex formation was not ascribable to a lack of circulating cofactor. The most likely explanation for the complex formation was the existence of LD binding sites on IgG molecules. The disruption of the complex by NAD+ might be explained by a competition between IgG molecules and NAD+ for the LD active site or by conformational changes induced in the LD molecules on binding of NAD+.