114
C H A R I T Y W A Y M O U T H
in the medium. The relativity of the term "essential" is well illustrated
by the report of Katsuta and Takaoka (1960), that they could maintain
a substrain of L cells in a synthetic medium containing arginine,
cysteine, histidine, isoleucine, leucine, lysine, tryptophan and valine.
The addition of threonine and glutamine, and also alanine, appeared
to be needed for growth, and proliferation was further accelerated by
glutamic acid and methionine. Amino acids might be classified in
another sense as "essential" (because found as components of the
ultimate cell proteins), but it is found that they need not be supplied
as such, if they can be synthesized from other amino acids or from
keto acids. Kagawa, Kaneko, Takaoka and Katsuta (1960) have shown
that strain-L cells in their protein-free medium use leucine, isoleucine
and alanine in amounts exceeding those required for protein synthesis.
The proportions in which amino acids are combined in a given
medium are arrived at empirically. Fischer (1948) and others (White,
1946, 1949; Evans et al., 1956a,b) followed the principle of imitating
the composition of some biologically important protein. Later work
suggests that it by no means follows that the proportions in which the
amino acids are found by analysis in biological media, or even in the
protein of the cells themselves, are necessarily the optimal proportions
in which they should be included in nutrient media. A medium which
contains amino acids in proportions optimal for one cell type is not
necessarily optimal for another.
In an empirically compounded medium containing most of the known
amino acids, some are certainly interchangeable, or can be substituted
by keto acids. The minimum amino acids which will support growth of
strain-L (Eagle, 1955a; Sanford, McQuilkin, Fioramonti, Evans and
Earle, 1958), HeLa (Eagle, 1955b), and monkey kidney cells (Eagle,
Freeman and Levy, 1958b) appear to be twelve, namely: arginine,
cysteine, histidine, isoleucine, leucine, lysine, methionine, phenyl
alanine, threonine, tryptophan, tyrosine and valine. It is of interest
that methionine is not readily convertible to cysteine, as in the nutrition
of the whole mammal, though the results of Katsuta and Takaoka
(1960) quoted above, suggest that under proper conditions strain-L
cells can meet their requirements of methionine, phenyl alanine, and
tyrosine from other than direct sources. Rat connective tissue (Harris
and Jahnz, 1957) requires eleven of the above twelve amino acids, the
twelfth, threonine, being synthesized only slowly. Both HeLa and L
cells can, if supplied with the appropriate a-keto acids, form phenyl
alanine and tyrosine by transamination. Aspartic acid, glutamic acid,
leucine, isoleucine and alanine act as N H 2 donors (Barban and Schulze,
1959). The same eleven amino acids, plus glutamine, are adequate for
a strain of uterine fibroblasts (Swim and Parker, 1958b), and Eagle's
C H A R I T Y W A Y M O U T H
in the medium. The relativity of the term "essential" is well illustrated
by the report of Katsuta and Takaoka (1960), that they could maintain
a substrain of L cells in a synthetic medium containing arginine,
cysteine, histidine, isoleucine, leucine, lysine, tryptophan and valine.
The addition of threonine and glutamine, and also alanine, appeared
to be needed for growth, and proliferation was further accelerated by
glutamic acid and methionine. Amino acids might be classified in
another sense as "essential" (because found as components of the
ultimate cell proteins), but it is found that they need not be supplied
as such, if they can be synthesized from other amino acids or from
keto acids. Kagawa, Kaneko, Takaoka and Katsuta (1960) have shown
that strain-L cells in their protein-free medium use leucine, isoleucine
and alanine in amounts exceeding those required for protein synthesis.
The proportions in which amino acids are combined in a given
medium are arrived at empirically. Fischer (1948) and others (White,
1946, 1949; Evans et al., 1956a,b) followed the principle of imitating
the composition of some biologically important protein. Later work
suggests that it by no means follows that the proportions in which the
amino acids are found by analysis in biological media, or even in the
protein of the cells themselves, are necessarily the optimal proportions
in which they should be included in nutrient media. A medium which
contains amino acids in proportions optimal for one cell type is not
necessarily optimal for another.
In an empirically compounded medium containing most of the known
amino acids, some are certainly interchangeable, or can be substituted
by keto acids. The minimum amino acids which will support growth of
strain-L (Eagle, 1955a; Sanford, McQuilkin, Fioramonti, Evans and
Earle, 1958), HeLa (Eagle, 1955b), and monkey kidney cells (Eagle,
Freeman and Levy, 1958b) appear to be twelve, namely: arginine,
cysteine, histidine, isoleucine, leucine, lysine, methionine, phenyl
alanine, threonine, tryptophan, tyrosine and valine. It is of interest
that methionine is not readily convertible to cysteine, as in the nutrition
of the whole mammal, though the results of Katsuta and Takaoka
(1960) quoted above, suggest that under proper conditions strain-L
cells can meet their requirements of methionine, phenyl alanine, and
tyrosine from other than direct sources. Rat connective tissue (Harris
and Jahnz, 1957) requires eleven of the above twelve amino acids, the
twelfth, threonine, being synthesized only slowly. Both HeLa and L
cells can, if supplied with the appropriate a-keto acids, form phenyl
alanine and tyrosine by transamination. Aspartic acid, glutamic acid,
leucine, isoleucine and alanine act as N H 2 donors (Barban and Schulze,
1959). The same eleven amino acids, plus glutamine, are adequate for
a strain of uterine fibroblasts (Swim and Parker, 1958b), and Eagle's