IntroductionProteases are amongst t

Introduction
Proteases are amongst the most studied enzymes. They catalyse the cleavage of peptide bonds in peptides and proteins, and represent a class of enzymes having diverse applications in both physiological and commercial fields. They are classified into four groups: serine proteases, cystein proteases, aspartic proteases and metalloproteases
(Rao, Tanksale, Ghatge, & Deshpande, 1998). Alkaline proteases are particularly suitable for industrial applications, due
to their high stability and activity under harsh conditions, such as high temperature, alkaline pH and in the presence of surfactants or oxidising agents (El Hadj-Ali, Hmidet, Bougatef, Nasri, & Nasri,2009). Proteases have been widely used for basic research and they have extensive applications in a range of industrial products and processes, including detergents, food, pharmaceuticals, leather, silk and for silver extraction from used X-ray films (Gupta, Beg, & Lorenz, 2002). Proteases are derived from animals, plants and microorganisms. To obtain a variety of proteases, particularly those
with unique properties, new sources of proteolytic enzymes have been studied, including proteases from fish, especially fish viscera (Balti, Barkia, Bougatef, Ktari, & Nasri, 2009; Ben Khaled, Nasri, Bougatef, Ghorbel, & Nasri, 2011).
The most important proteolytic enzymes from viscera of fish and aquatic invertebrates are the aspartic protease pepsin, and the serine proteases, trypsin, chymotrypsin, collagenase and elastase. Acidic proteases from fish stomach display high activity between pH 2.0 and 4.0, while digestive serine proteases, such as trypsins, are most active between pH 8.0 and 10.0, and inactive or unstable at acid pH (Simpson, 2000).