346 CELLS, TISSUES, AND ORGANISMS 



human cells in tissue culture ( in addition to any vitamins that may be 

 bound to the serum protein ) : thiamine, riboflavin, nicotinic acid, vita- 

 min Be, pantothenic acid, folic acid, inositol, and choline. In experi- 

 ments with mouse fibroblasts, thiamine pyrophosphate and thiamine 

 were equivalent in activity; riboflavin and riboflavin phosphate were 

 superior to flavin adenine dinucleotide. Nicotinamide, nicotinic acid, 

 and di- and tri-phosphopyridine nucleotides were approximately equal 

 in molar activity, and so were pyridoxine and pyridoxal, but pyridoxa- 

 mine and pyridoxal phosphate were somev/hat less active. Pantothenic 

 acid was about ten times as active as coenzyme A, which other in- 

 vestigators have reported does not readily penetrate mammalian cells. 

 Folic acid was less active than the natural citrovorum factor, L ( I ) 

 5-formyltetrahydro-pteroylglutamic acid. These findings are illuminat- 

 ing in that they demonstrate the ability of human cells to convert these 

 vitamins to the coenzymatic forms necessary for cellular growth. 



The transfer of chemical units by vitamins 



A subject related to the growth of animals is the study of biochem- 

 ical synthetic mechanisms. It has long been evident that living organ- 

 isms are able to synthesize large and complex molecules from small 

 units. The outstanding example of this is the synthesis of proteins from 

 amino acids. More recently it has become apparent that even these 

 small molecules are put together from a handful of smaller units, such 

 as acetate, amonia, carbon dioxide, water, hydrogen, and the "single- 

 carbon" unit. Some of the vitamins serve as transfer agents to shuttle 

 these small units into position. Although it was apparent soon after 

 this discovery that the flavin and pyridine coenzymes transferred hy- 

 drogen, it was not until later that it was demonstrated that other 

 coenzymes actually participated in reactions in which carbon-contain- 

 ing units were transferred. 



One of the earlier observations in this field was that an "incom- 

 plete" purine molecule, 4-amino-5-imidazole carboxamide, was pro- 

 duced by E. coli grown on a medium containing sulfanilamide. This 

 led Gordon and co-workers (1948) to predict that the formyl group 

 necessary to close the ring was carried by a folic-acid coenzyme which 

 could then be "reformylated" to repeat the procedure. This prediction 

 has been amply fulfilled, and a number of other "single-carbon-trans- 

 fer" reactions of folic-acid coenzymes have been demonstrated by vari- 

 ous pieces of experimental work. Formyl and hydroxymethyl groups 

 are tranf erred by folic-acid coenzymes in the synthesis of metabolites. 

 In addition, a tetrahydrofolic coenzyme, together with thymidylate syn- 

 thetase, transfers hydrogen and a single-carbon unit to deoxyuridylic 

 acid to form the methyl group of thymidylic acid, and the tetrahydro- 



