Farm Animal Metabolism and Nutrition

denote lectin activity. In contrast to most
dietary proteins, lectins resist proteolytic
degradation in vivo, and thus substantial
quantities of ingested lectins may be
recovered intact from the digestive tract
and faeces of animals fed diets containing
one of a number of legume seeds (D’Mello,
1995). The prime example of a lectin with
potent anti-nutritional and toxic properties
is concanavalin A, a component of the jack
bean (Canavalia ensiformis). This lectin is
composed of four identical subunits each
containing two metal-binding sites and an
additional site for a sugar residue. Due to
its particular sugar specificity, concanavalin
A binds mainly to the lower regions of the
intestinal villi where membrane proteins of
the less differentiated crypt cells have
predominantly polymannose-type side
chains. The lectin of the winged bean
(Psophocarpus tetragonolobus), on the other
hand, with its requirement for more
complex sugar structures, binds largely to
the upper part of the villi. Despite these
differences, both lectin types confer
profound anti-nutritional properties to diets
containing raw jack beans and winged
beans (D’Mello, 1995).

Proteinase inhibitors

The proteinase inhibitors are typical
examples of heat-labile ANFs. They consti-
tute a unique class of proteins with the
ability to react in a highly specific manner
with a number of proteolytic enzymes in
the digestive secretions of animals. The
proteinase inhibitors of soybeans are now
well characterized, and the complete
amino acid sequence of two of these
(Kunitz and Bowman–Birk inhibitors) have
been elucidated (Norton, 1991). They exert
their effects in a strictly competitive
manner and the complex formed between
inhibitor and enzyme is devoid of catalytic
activity. Proteinase inhibitors are widely
distributed in the plant kingdom, parti-
cularly in leguminous seeds such as soy-
bean, field beans (Vicia faba), winged
beans, pigeon pea (Cajanus cajan) and cow
pea (Vigna unguiculata).

Antigenic proteins

There is now unequivocal evidence that

certain storage proteins of legume seeds are

capable of crossing the epithelial barrier of

the intestinal mucosa to elicit adverse effects

on immune function in farm animals. In the

case of the soybean, the antigenic proteins

have been identified as glycinin and

conglycinin. The latter is subdivided further

into 
, and isotypes, with the com-

ponent representing the predominating

protein. The antigenic proteins are charac-

terized by their resistance to denaturation by

conventional thermal processing procedures

and to digestive attack in the alimentary

canal of mammals.

Condensed tannins

Tannins belong to a class of compounds

with a molecular weight in excess of

500 Da with a sufficiently large number of

phenolic hydroxyl groups to form cross-

linkages with proteins and other macro-

molecules. Condensed tannins (CTs) are a

subset of this group, being dimers or higher

oligomers of variously substituted flavan-3-

ols (Griffiths, 1991). It is widely accepted

that CTs are the most abundant type of

tannins in economically important plants,

occurring in leguminous seeds and forages

and certain cereal grains, particularly

sorghum (Sorghum bicolor). On heating

with strong acids, CTs polymerize further

to yield small quantities of anthocyanidins,

giving rise to the term ‘proanthocyanidins’

as an alternative generic name for CTs.

Quinolizidine alkaloids

The alkaloids of lupins (Lupinus spp.) are

commonly bicyclic, tricyclic or tetracyclic

derivatives of quinolizidine (Petterson et

al., 1991). The bicyclic alkaloids are

exemplified by lupinine, the tricyclics by

angustifoline and the tetracyclics by

sparteine. In addition, lupanine, a deriva-

tive of sparteine, as well as hydroxylated

compounds of lupanine occur regularly in

most lupin species.

Anti-nutritional Factors and Mycotoxins 385