Gastric proteases

FIGURE 14.11 The pH activity profiles of four different enzymes. Trypsin, an intestinal protease, has a slightly alkaline pH optimnm, whereas pepsin, a gastric protease, acts in the acidic confines of the stomach and has a pH optimmn near 2. Papain, a protease found in papaya, is relatively insensitive to pHs between 4 and 8. Cholinesterase activity is pH-sensitive below pH 7 but not between pH 7 and 10. The cholinesterase pH activity profile suggests that an ionizable group with a pK near 6 is essential to its activity. Might it be a histidine residue within the active site ... 

The greatest variety of industrial enzymes are presently derived from microbial sources, with a lesser diversity coming from plant and animal sources 34), Enzymes derived from plant sources and which are used extensively in the food industry include papain, bromelain, ficin, and amylases. Animal enzymes of economic importance include trypsins, lipases, and gastric proteases. 

Aspartic proteases include the gastric proteases pepsin A, pepsin B, gastricsin and chymosin that are secreted as inactive zymogens and are activated through removal of autoinhibitory domains in the low pH conditions of the stomach [7-9]. Oesophagitis is caused by undue... 

Fig. 1. pH Activity curve of a human gastric juice and structure of gastric protease. From Buchs (B48). 

A third proteinase appears to exist in the gastric juice, which exerts proteolytic activity at a neutral pH of about 7.0, i.e., in the range where pepsin and cathepsin no longer are active. The existence of a protease called parachymosin, active at pH 5.0-7.0, has been described in the past (Fig. 1) (see B46). Buchs (JM6) considered it to be an integral part of the gastric protease, the two other components of which were pepsin and cathepsin. In the early thirties, Castle s group found some paral-... 

Application of Electrophoresis to Special Problems 1.9.1. Localization, Fractionation, and Qtumtitation of Gastric Proteases... 

The 130-140,OOOd 6-endotoxin is not toxic in its native form, but must be cleaved to a 60-65,000 fragment to be active. This cleavage takes place in the insect gut. After ingestion by a caterpillar, the proteinaceous crystal dissolves in the alkaline gut juices. Digestion by gastric proteases then cleave the... 

Like the gastric proteases, digestive cysteine or thiol proteases are active at acidic pH and inactive at basic pH. They are important components of the hepatopancreas of many marine crustaceans, and are responsible for over 90% of the protease activity in the hepatopancreas in short-finned squid (Jllex illecebrosus) (Raksakulthai and Haard, 2001). Cathepsin B is one example of a marine-derived digestive thiol protease. Only a few marine sources have been identified for cathepsin B, including surf clam (Spisula solidissimd), horse clam (Tresus capax), and mussel (Perna perna L.) (Simpson, 2000). 

The names of enzymes are based on what they do and where they occur. For example, gastric protease, commonly called pepsin, is an enzyme released by the stomach (gastric), which splits protein molecules as part of the digestion process (protease). Similarly, the enzyme produced by the pancreas that breaks down fats (lipids) is called pancreatic lipase. Its common name is steapsin. In general, lipase enzymes cause lipid triglycerides to dissociate and form glycerol and fatty acids. 

The results shown in Table I and this discussion indicate that the gastric proteases form a family of enzymes, closely related structurally. It also appears that pepsin A from different species (pig, ox and humans) are more closely related to each other than pepsin A is to other proteinases of the same species (e.g. bovine pepsin A, B, and chymosin) but with our present knowledge it is difficult to determine the interspecies relationship between bovine pepsin B and human pepsin C. 

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