ANTI-NUTRITIONAL CONSTITUENT OF COLOCASIA ESCULENTA ...

essential amino acids not available for the synthesis of other proteins (Balter and
Jongsrna, 1997; Pompermayer etaI., 2001).
Kowalska et al (2007) commented that an abundant source of protein proteinase
inhibitors may be found in plant seeds and storage. Other substances, such as tannins
(Price and Butler, 1980; Quesada et al., 1996) and several indigestible polysaccharides
(Price et al. 1988), may also inhibit digestive enzymes. However, the contribution of
these compounds to the total trypsin inhibitor activity (TIA) is minimal in most foods
(lkeda and Kusano, 1983).
Trypsin is one of the serine-protease inhibitors identified and characterized in plants
(Tremacoldi and Pascholati, 2002; Richardson, 1991). In general, trypsin inhibitors are
low molecular weight proteins formed by association of identical peptide chains of
smaller size. Normally these chains are devoid of carbohydrates, but have between one
and eight disulphide bonds (Mueller and Weder, 1990). A common characteristic of
plant protease inhibitors is their resistence to pH extremes, heat and hydrolysis by
proteases as a result ofthe level ofS-S bridges (Proteinase inhibitors, 1999).
Based on the molecular masses, cysteine content and number of reactive sites, these
inhibitors have been categorized into two families, the Kunitz Type and the Bowman
Birk Type. Bhattacharyya (2007) identified Bowman Birk Type iubibitors as " ... usually
8-kDa proteins with seven disulfide bridges... " and Kunitz Type as " ... 20-kDa proteins
with just two disulfide linkages". Although active sites are much the same, it would
appear that protease inhibitors from different sources have varied structores, molecular
weights and chemical compositious (Richardson, 1977; Liener, 1994). These inhibitors
inhibit trypsin and chymotrypsin at independent binding sites by reacting with some
functional groups in the active site of the enzyme. This inlnbition prevents the substrate
from entering the active site or leaves the site catalytically inactive (Bender and Hezdy,
1965; Rittschof et aI., 1990).
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