3. C O N S T R U C T I O N A N D USE OF S Y N T H E T I C M E D I A
113
enzymes in the cells themselves. Some peptides can certainly be utilized
but complete hydrolysis to amino acids may not be necessary. There
is some flexibility in the number and proportions of amino acids in
any medium which will produce equivalent effects, on account of
varying degrees of interconvertibility. The terms "essential" and
"non-essential" amino acid, rather commonly used, are not, therefore,
strict designations and should be accepted with scepticism unless
carefully defined in a particular context, as, for example by Evans
et al. (1958).
Biggers et al. (1957) made an important contribution to the clarification of the concept of "essentiality" in their treatment of the question
of dose-response relationships. As they demonstrated, when two components must both be present for a response to take place, there may
be more than one specific combination which gives the maximum
effect. Various combinations may be effective, and a dose-response
surface may be plotted, the consequence of which is that "an infinite
number of chemically defined media are associated with any one
response surface". The earlier work of Eagle (1955a,b,c,d), though
valuable in helping to define standard states of nutrition which permit
the continuous growth of certain cell lines, erred on the side o f dogmatism in stating categorically that certain nutrients are "essential",
and in laying down ex cathedra the concentrations of these nutrients
required by the cells. Later work by this and other authors has modified
this attitude, and we are coming back (Levintow and Eagle, 1961) to
the more moderate view (Waymouth, 1954a) that the term "essential"
is a dangerous one to use in systems where complex interrelations may
render a component essential in one context and inessential in another,
e.g. galactose or some other sugar, in the absence of glucose for MB III
cells (Rueckert and Mueller, 1958); many sugars for several human
cell lines (Eagle et al., 1958a); asparagine for a dependent line, but not
for the nutritional variant of Jensen sarcoma which is independent of
the need for it (McCoy, Maxwell, Irvine and Sartorelli, 1959c).
Criteria for essentiality differ according to the culture conditions. The
functions in which amino acids have an "essential" character are
usually survival or proliferation, with or without differentiation.
A minimum mixture of amino acids may support virtually unlimited
survival, with little proliferation, of a cell population. The addition
of another amino acid may greatly stimulate the growth rate in an
otherwise adequate medium, and so qualify as an "essential" amino
acid for rapid growth. Thus, Lucy and Rinaldini (1959) found a
large requirement for leucine for chick skeletal myoblasts, though it
is of interest that, provided sufficient of the amino acid is available for
the needs of the cells, its utilization is independent of the concentration
113
enzymes in the cells themselves. Some peptides can certainly be utilized
but complete hydrolysis to amino acids may not be necessary. There
is some flexibility in the number and proportions of amino acids in
any medium which will produce equivalent effects, on account of
varying degrees of interconvertibility. The terms "essential" and
"non-essential" amino acid, rather commonly used, are not, therefore,
strict designations and should be accepted with scepticism unless
carefully defined in a particular context, as, for example by Evans
et al. (1958).
Biggers et al. (1957) made an important contribution to the clarification of the concept of "essentiality" in their treatment of the question
of dose-response relationships. As they demonstrated, when two components must both be present for a response to take place, there may
be more than one specific combination which gives the maximum
effect. Various combinations may be effective, and a dose-response
surface may be plotted, the consequence of which is that "an infinite
number of chemically defined media are associated with any one
response surface". The earlier work of Eagle (1955a,b,c,d), though
valuable in helping to define standard states of nutrition which permit
the continuous growth of certain cell lines, erred on the side o f dogmatism in stating categorically that certain nutrients are "essential",
and in laying down ex cathedra the concentrations of these nutrients
required by the cells. Later work by this and other authors has modified
this attitude, and we are coming back (Levintow and Eagle, 1961) to
the more moderate view (Waymouth, 1954a) that the term "essential"
is a dangerous one to use in systems where complex interrelations may
render a component essential in one context and inessential in another,
e.g. galactose or some other sugar, in the absence of glucose for MB III
cells (Rueckert and Mueller, 1958); many sugars for several human
cell lines (Eagle et al., 1958a); asparagine for a dependent line, but not
for the nutritional variant of Jensen sarcoma which is independent of
the need for it (McCoy, Maxwell, Irvine and Sartorelli, 1959c).
Criteria for essentiality differ according to the culture conditions. The
functions in which amino acids have an "essential" character are
usually survival or proliferation, with or without differentiation.
A minimum mixture of amino acids may support virtually unlimited
survival, with little proliferation, of a cell population. The addition
of another amino acid may greatly stimulate the growth rate in an
otherwise adequate medium, and so qualify as an "essential" amino
acid for rapid growth. Thus, Lucy and Rinaldini (1959) found a
large requirement for leucine for chick skeletal myoblasts, though it
is of interest that, provided sufficient of the amino acid is available for
the needs of the cells, its utilization is independent of the concentration