8. P R O T E I N M E T A B O L I S M IN C E L L LINES
279
II. N U T R I T I O N A L R E Q U I R E M E N T S OF S E R I A L L Y P R O P A G A T E D
C E L L S
A. A M I N O A C I D R E Q U I R E M E N T S
Evidence from many laboratories indicates that there is a striking
similarity, both qualitative and quantitative, in the amino acid
requirements of a variety of cell lines of widely diverse origin, and that
cells growing under usual conditions in vitro must be supplied with
certain amino acids not required for the nutrition of the whole animal.
Subject to the qualifications discussed below, this underlying similarity
in nutritional requirements permits the use of a standard "minimum"
medium, consisting solely of demonstrable essential nutrients which are,
with the exception of serum protein, chemically denned (Table I ) .
No cultured cell has yet been propagated in the absence of any of
the 8 amino acids required by the whole animal, and it seems clear
that adaptation to serial growth in vitro does not involve the acquisition
of new synthetic capacities of this sort. Certain of these "essential"
amino acids can be replaced by their corresponding keto acids, and in
most cases this amination appears to be the sole step in the biosynthetic
sequence that the cell is able to perform. A different situation prevails
with respect to the 5 amino acids not essential for N balance in man,
but typically required by serially cultured cells (arginine, cyst(e)ine,
glutamine, histidine, and tyrosine). Citrulline, but not ornithine, can
fulfil the arginine requirement, indicating that the cell is unable to
carry out the transcarbamylation step in this biosynthetic pathway.
This is in accord with the finding that, of a variety of mammalian
tissues examined, only liver parenchyma exhibited significant transcarbamylase activity (Jones, Anderson, Anderson and Hodes, 1961).
In the intact animal, on an arginine-free diet, the liver presumably
provides sufficient arginine for sustained N balance; in cell culture,
however, preformed arginine must be supplied. The hydroxylation
of phenylalanine to tyrosine also appears to be an exclusive function
of the liver (Udenfriend and Cooper, 1952; Udenfriend, personal
communication), and the requirement of serially cultured cells for
this amino acid follows naturally.
function, rather than a reversion to the undifferentiated but totipotential embryonic cell. The basic question remains, however, unanswered as to why the capacity for serial growth in vitro is almost
invariably restricted to precisely such dedifferentiated cells, with no
functional or structural identification.
279
II. N U T R I T I O N A L R E Q U I R E M E N T S OF S E R I A L L Y P R O P A G A T E D
C E L L S
A. A M I N O A C I D R E Q U I R E M E N T S
Evidence from many laboratories indicates that there is a striking
similarity, both qualitative and quantitative, in the amino acid
requirements of a variety of cell lines of widely diverse origin, and that
cells growing under usual conditions in vitro must be supplied with
certain amino acids not required for the nutrition of the whole animal.
Subject to the qualifications discussed below, this underlying similarity
in nutritional requirements permits the use of a standard "minimum"
medium, consisting solely of demonstrable essential nutrients which are,
with the exception of serum protein, chemically denned (Table I ) .
No cultured cell has yet been propagated in the absence of any of
the 8 amino acids required by the whole animal, and it seems clear
that adaptation to serial growth in vitro does not involve the acquisition
of new synthetic capacities of this sort. Certain of these "essential"
amino acids can be replaced by their corresponding keto acids, and in
most cases this amination appears to be the sole step in the biosynthetic
sequence that the cell is able to perform. A different situation prevails
with respect to the 5 amino acids not essential for N balance in man,
but typically required by serially cultured cells (arginine, cyst(e)ine,
glutamine, histidine, and tyrosine). Citrulline, but not ornithine, can
fulfil the arginine requirement, indicating that the cell is unable to
carry out the transcarbamylation step in this biosynthetic pathway.
This is in accord with the finding that, of a variety of mammalian
tissues examined, only liver parenchyma exhibited significant transcarbamylase activity (Jones, Anderson, Anderson and Hodes, 1961).
In the intact animal, on an arginine-free diet, the liver presumably
provides sufficient arginine for sustained N balance; in cell culture,
however, preformed arginine must be supplied. The hydroxylation
of phenylalanine to tyrosine also appears to be an exclusive function
of the liver (Udenfriend and Cooper, 1952; Udenfriend, personal
communication), and the requirement of serially cultured cells for
this amino acid follows naturally.
function, rather than a reversion to the undifferentiated but totipotential embryonic cell. The basic question remains, however, unanswered as to why the capacity for serial growth in vitro is almost
invariably restricted to precisely such dedifferentiated cells, with no
functional or structural identification.