The Biogenesis of Vitamin

Vitamin D is not widely distributed in nature being quite low in the plant world except special circumstances It is found in sizable amounts in livers of cartilaginous aquatic animals such as sharks and in fish The source of vitamin D in these species is still unknown. There is evidence that it arises by means of a non- 

Vitamin D is produced in skin as a result of ultraviolet irradiation, a phenom enon realized in 1919—1924 It is known that ultraviolet light brings about a photolyas of 5,7-diene sterols giving rise to previtamin D and other photoisomers, lumisterol and tachysterol (Fig. 2). The previtamin D spontaneously isomerizes to vitamin D to yield an equilibrium mixture favoring vitamin D. This photolysis known to occur in organic solvents also occurs in epidermis of skin. Ultraviolet light is known to penetrate to the site of 7-dehydrocholesterol in epidermis Recently, the antirachitic substance produced in rat skin by ultraviolet irradiation has been isolated in pure form and unequivocally identified as vitamin D3 The 

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Microorganisms have invariably been used to study the biogenesis of vitamin Bj since many organisms are capable of producing the substance under well-defined conditions (I), and since a number of these produce more than appears necessary for growth and function. A thorough survey of the literature on microorganisms which have been studied for their unusual ability to synthesize flavins was made by Pridham 2). 

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