The history of riboflavin, or vitamin B₂, is an especially colorful one. It begins in 1879, when a water-soluble pigment was first discovered in milk. The pigment, which possessed a distinctive yellow-green fluorescence, was generally called lactoflavin-- lacto, for the milk, flavin, for the yellow coloring. But for several decades, no one really had a clue to the purpose of this oddly glowing substance.

Then, in the 1930s, a chemist named Otto Warburg —who was doing research on how damaged cells manage to repair themselves--isolated a substance from yeast that also had a yellowish glow. Warburg termed it the "yellow enzyme" and--because it appeared to play a role in cellular repair--subjected it to intense study. Before long, Warburg and his coworkers learned that the so-called yellow enzyme actually consisted of two factors: a protein factor and a nonprotein one, in this case a pigment. Somehow, both factors appeared...