Biochemical selection systems for mammalian cells: the essential amino acids

The essential amino acid requirement of cultured mammalian cells can be satisfied by 19 amino acid derivatives. This finding (a) confirms the results of animal nutritional studies and (b) identifies 19 essential amino acid derivatives and should permit the isolation of a new class of auxotrophic mutants. Five naturally occurring auxotrophic markers have been detected in this survey; namely, inability to utilize cystathionine and citrulline in Chinese hamster ovary (CHO) cells, inability to metabolize citrulline by HTC+hepatoma cells, and confirmation of Eagle's observation that KB cells can utilize homocystine in place of methionine or cystine and D-cystine in place of L-cystine.