The reversible binding of half-cystine residues to serum protein, and its bearing on the cystine requirement of cultured mammalian cells.

All the serially propagated mammalian cell cultures SO far examined, as well as monkey kidney cells in primary passage, have been found to require a minimum of 13 amino acids (l-4). One of the 5 amino acids regularly required in culture, but not necessary for nitrogen balance in man, was L-cystine. In many of the experiments, however, there was definite if limited initial cellular growth in a cystine-free medium, indicating the presence of a mechanism for its provision. It was subsequently found that a number of inorganic reducing agents containing sulfur (S’, S~0’3, S30;, SO;) would greatly promote the growth of human, monkey, and mouse cells in a presumably cystine-free medium, and that D-cystine, D-homocystine, L-cysteamine or L-cystamine, and thioglycolate were also effective. As will be here shown, these compounds are not used for the biosynthesis of L-cystine, but act by mobilizing half-cystine residues bound through disulfide bonds to the serum protein of the medium.