Vitamin precursors of NAD were first identified in 1938, when Conrad Elvehjem showed that liver has an "anti-black tongue" activity in the form of nicotinamide. Then, in 1939, he provided the first strong evidence that niacin is used to synthesize NAD. In the early 1940s, Arthur Kornberg was the first to detect an enzyme in the biosynthetic pathway. In 1949, the American biochemists Morris Friedkin and Albert L. Lehninger proved that NADH linked metabolic pathways such as the citric acid cycle with the synthesis of ATP in oxidative phosphorylation. In 1958, Jack Preiss and Philip Handler discovered the intermediates and enzymes involved in the biosynthesis of NAD; salvage synthesis from nicotinic acid is termed the Preiss-Handler pathway. In 2004, Charles Brenner and co-workers uncovered the nicotinamide riboside kinase pathway to NAD.