Legume seed protease inhibitors: their functions, actions and characteristics

Protease inhibitors are found in many legumes, especially their seeds. They are classified into families based on the amino acid sequence, the position and nature of reactive site, and the number of disulfide bridges. These inhibitors inhibit the pancreas proteases of higher animals and impair the nutritional quality of seeds by reducing protein digestibility and absorption by inducing pancreatic hypertrophy and by depressing growth. Several functions of these proteins have been proposed, including the regulation of endogenous proteinases and the protection of plants from insects and microorganisms. Clarification of the antinutritional status of legumes, especially of protease inhibitors, is important, not only because these proteins may be included at higher levels in diets than hitherto assumed, but also because, in general, protease inhibitors are desirable proteins from the view point that their content of sulphur containing amino acids as well as for their potential rules in protecting seeds and plants from insect attack. Protease inhibitors are very effective in their ability to suppress carcinogenesis in many different in vivo and in vitro assay systems. Although the mechanisms for the anticarcinogenic activity of protease inhibitors are unknown, many hypotheses have been proposed. INTRODUCTION Legume seeds are important staple foods, particularly in developing countries, due to their relatively low cost, ease of preservation, and high nutritional value. A lot of legume seeds are consumed by various segments of Egyptians and it is known that these seeds are an important source of energy and protein. However, their wider use is some how limited by the presence of undesirable components known as antinutritional factors (ANFs) in the seeds which may have adverse effects for human or animal nutrition. Some examples of these compounds include protease inhibitors (PIs), lectins (heamagglutinnis), αamylase inhibitors, phenolics, phytates and α-galactosides (raffinose family sugars) (El-Morsi 1996, 1997, 1998; Zaki et al. 1999; Trugo et al. 2000; Ismail & Fawzi 2001). This article deals with PIs in the seeds of some Egyptian legumes. PIs content in the seeds of some Egyptian legumes: The nutritional properties of various PIs have been well documented. The results of determining PIs in the seeds of four legume crops studied by Aboul-Fotouh et al. (1998) are shown in table 1. TI activities were detected in both albumin and globulin fractions, but their levels were higher in albumin fractions than in globulin fractions in all samples. The highest level of total TIA was recorded in kidney bean Nomad variety (40.4 mg/g) and the lowest amount was found in pea variety Master B (2.5 mg/g). Of the two kidney bean varieties, Belfin contained about one third of the TIA recorded for the Nomad variety. The levels of TIA in cowpea and mungbean were lower than those recorded for kidney beans but higher than peas (Table 1). With the exception of kidney bean Nomad variety all other legumes demonstrated TIA (Table 1) lower than that recorded for soybean (26.5 mg/g) (EL-Morsi et al. 1990). Cowpea variety 331 (Table 1), produced higher levels of TIA than that reported by Ismail, et al. (1995a) for black-eyed cultivar. The levels of TIA in Kawmy 1 mungbean seeds (Table I) were much lower than the values recorded for two mungbean genotypes studied by Zaki (1996). The value of TIA in pea seeds was within the range reported by other workers (Abdel-Galil 1998; Van der Poel et al. 1992). El-Morsi: Legume seed protease inhibitors Table 1: Trypsin inhibitory activity (TIA) in dehulled legume seeds (Mean ±SD)*