Riboflavin
The suggestion that water-soluble B contained more than one active principle was given impetus by the work of Emmett and McKim. These workers indicated the presence of two water-soluble vitamins in rice polishings, one that cured rat polyneuritis and another that produced weight gain in rats on specific rations. We now know that the second factor contained riboflavin, and at that time this principle was called vitamin B2 or vitamin G. Various investigators had studied naturally occurring pigments in foods. Bleyer and Kallmann isolated a crude yellow pigment from whey, and Blooher presented evidence pointing to the probable identity of vitamin G (B2) and the water-soluble yellow, green fluorescent pigment of whey.
In 1932 Warburg and Christian isolated a yellow respiratory ferment (Warburg's yellow enzyme) from bottom yeast. This was later separated into a protein fraction and a small pigment molecule (riboflavin), neither of which alone possessed enzyme activity. Pure riboflavin was isolated from milk and other foods in 1933 by Kuhn and co-workers. In the early phases of this work 100 mg of flavin were obtained from a quantity of egg white corresponding to 33,000 eggs. This represents a yield of about 7 per cent based on the content of this pigment now known to be present. Lactoflavin (milk), hepatoflavin (liver), ovoflavin (eggs), and verdoflavin (grass) proved to be chemically identical with riboflavin, and it was recognized that riboflavin is actually what had been called vitamin B2. The synthesis of riboflavin was accomplished by two groups of German workers in 1935. In nature the vitamin is synthesized by all green plants and by most microorganisms, although some bacteria are without this capacity.
