Nutritional Characteristics of Potatoes - The Potato: Botany, Production and Uses - page 325

decreased when stored frozen or under modified
atmosphere (Tudela et al ., 2002b).

anticancer drugs (Liang et al ., 2008). In addition
to chemopreventive effects, GAs can boost the
immune response. Mice fed GAs were more re-
sistant to Salmonella infection (Gubarev et al .,
1998), while tomatine potentiated the mice im-
mune response to vaccines (Rajananthanan
et al ., 1999). GAs are reported to inactivate her-
pes viruses (Chataing et al ., 1997).

18.6 Glycoalkaloids

Potatoes and many other Solanaceae contain gly-
coalkaloids (GAs), compounds that can be toxic
when present in too high a concentration, caus-
ing vomiting, nausea, and other ill effects (Mc-
Millan and Thompson, 1979; Hopkins, 1995).
Most of the effects of GAs are due to their disrup-
tion of cell membranes and cholinesterase in-
hibition. They occur in higher concentrations in
fruits, leaves, and sprouts than in tubers, with
GA concentrations of 18 g kg - 1 FW reported in
sprouts (Valkonen et al ., 1996). In the USA, po-
tatoes must contain less than 20 mg 100 g - 1 FW
of GAs, but the guidelines established in other
countries vary (Wilson, 1959).
GAs are steroidal alkaloids with a hetero-
cyclic nitrogen, and a C27 steroid conjugated to
a sugar, typically a tri- or tetrasaccharide. GAs
are derived from the mevalonate pathway via
cholesterol (Johnson et al ., 1963; Heftmann,
1983), but the full pathway has not yet been
worked out (Ginzberg et al ., 2012). The hetero-
cyclic nitrogen is derived from arginine (Kaneko
et al ., 1976). Glycosylation steps are involved in
GA biosynthesis, and some of the glycosyltrans-
ferases have been identified (Stapleton et al .,
1991; Zimowski, 1991; Moehs et al ., 1997;
McCue et al ., 2007, 2011). Recently, several
genes in the pathway were identified, and it was
shown that silencing of GAME4 greatly reduced
GA accumulation in tubers (Itkin et al ., 2013).
Interestingly, recent studies show that some
GAs can have health-promoting effects, such as
inhibition of mice sarcoma tumors by a solama-
rine (Kupchan et al ., 1965), whereas solasodine
may protect against skin cancer (Cham, 1994).
Tomatine, solanine, and chaconine inhibited
growth of human colon and liver cancer cells
in cell culture assays (Lee et al ., 2004; Friedman
et al ., 2005). Tomatine was effective against can-
cer when provided in a feeding study using rain-
bow trout (Friedman et al ., 2007). Tomatidine is
a chemosensitizing agent, increasing the effect-
iveness of cancer chemotherapy (Lavie et al .,
2001). Likewise, solamargine increased the
susceptibility of human lung cancer cell lines to

Future directions for
glycoalkaloid research

These findings demonstrate that assessing the
value of potato GAs is complex. GAs contribute
to pest and pathogen resistance, so reducing
the amount in potatoes can lead to plants that
are more susceptible to pests and require more
pesticide use. Although not easily accomplished
with the current state of the art, it may be desir-
able to have high concentrations of GAs in foli-
age but low amounts in tubers to provide at least
foliar pest resistance without impacting food
safety. However, GAs are also involved in tuber
resistance to various pests, so even plants with
normal levels in leaves but lower amounts in
tubers may still face increased pest pressure.
Solanine and chaconine typically make up over
90% of the total GA complement of commercial
cultivars, with chaconine often more abundant
than solanine (Griffiths et al ., 1997; Sotelo and
Serrano, 2000). However, potatoes can contain
a much greater diversity of GAs because wild
species and primitive germplasm likely contain
over 100 different GAs (Friedman and McDon-
ald, 1997; Shakya and Navarre, 2008; Mweet-
wa et al ., 2012). Such diversity offers opportun-
ities to develop new cultivars with desired types
and quantities of GAs. Some GAs have markedly
less toxic effects than others, and if they have
health-promoting effects or contribute to pest
resistance, then they might be more desirable to
have in tubers than chaconine. In general, sola-
nidanes are more toxic than spirosolanes. Chaco-
nine is more toxic than solanine, and both
become less toxic with the loss of sugars, with
the aglycone the least toxic (Friedman and
McDonald, 1997). Friedman suggests replacing
solanidine and chaconine with tomatine (Fried-
man, 2002), which also has health-promoting
properties and is less toxic. The predominance of