Food Lipids: Chemistry, Nutrition, and Biotechnology

on steroyl–ACP appear to be rare. When researchers looked at cDNA clones made
from mRNA from developing seed of mangosteen fruit (mangosteen seed oil typically
contains 40–50% stearic acid), a clone was identified that corresponds to an
enzyme with enhanced levels of stearoyl–ACP activity relative to other thioesterases.
Genetically engineered overexpression of that mangosteen enzyme in seed of transgenic
canola plants results in a seed oil enriched in stearic acid, up to 30% in some
seed.
Alternatively, one can consider engineering a seed to have less desaturase activity
so that fewer molecules of the saturated precursor stearoyl–ACP are converted
to oleoyl–ACP. This has been demonstrated in transgenic canola and soybean by
suppressing levels of the stearoyl–ACP desaturase enzyme. Suppression of an enzyme
can be achieved by genetic engineering methods of either antisense or cosuppression.
Each allows for tissue-specific suppression. In this particular application,
tissue-specific suppression is important because the stearoyl–ACP desaturase is an
essential enzyme in leaves and other tissues for plant viability.
3. Yield
As noted above, yield is a very important trait in reducing the cost of vegetable oils.
Theoretically, there are several arguments for the feasibility of raising seed oil content
in most crops. The cacao bean has 60% oil by weight, so the physiological limit
for crops like soybean and canola may be at least this high. Individual seed of
rapeseed can typically vary from 35% to 50%; so even within existing germ plasm,
the current average oil content from canola of 42% will likely be raised by straightforward
breeding selections in the coming years. Historically, soybean protein content
has been the most important component of the bean, with the 20% of the seed
that is oil a valuable by-product. Clearly, there is room to increase oil content in
soybean by some means; however, increased oil will be balanced against maintaining
value in the meal component for animal feed uses.
Despite the evident technical premise and motivation for increasing oil content,
exact scientific strategies are still developing. Acetyl–CoA carboxylase is an enzyme
activity often considered to be a rate-limiting step for fatty acid biosynthesis. The
data for this assumption seem clearest in animal cells and fairly convincing in the
bacterium E. coli, but somewhat less clear in higher plants. Transgenic modifications
of levels of this enzyme activity have been achieved with slight and possibly significant
increases in oil content; however, experiments continue. The development
of other strategies to increase fatty acid biosynthesis at the expense of nondigestible
fiber and other less desirable components of seed represent an area of increasing
research.
4. Removing Negatives
The availability of gene suppression technologies allows one to think of decreasing
certain constituents of vegetable oils. An early and far-reaching application is evident
in the use of cosuppression in canola and soybean to dramatically reduce the levels
of polyunsaturated fatty acids. Other potential project might be, for example, the
suppression of enzymes directly responsible for formation of cyclopropene fatty acids
such as malvalic acid in cottonseed oil or achieving a decrease in the high palmitic
acid content in palm oil by suppressing the level of palmitoyl–ACP thioesterase
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.