Protein endogenous proteolytic enzymes

Autolytic activities in tissues can have a significant effect on the yields of proteins in the preparation of concentrates from animal and plant tissues. Koury et al. (29) observed that fish flesh and viscera contain highly active proteolytic enzymes. In preparing solvent-extracted FPC from hake, these workers observed that if the fish was held for 20 min at 50°C at a pH of 5.5, the yield of FPC decreased ca. 9%. Apparently the endogenous proteolytic enzymes in fish hydrolyze the... 

For example, intense proteolyses of muscle proteins, due to the action of endogenous proteolytic enzymes, have been reported to occur during the processing of dry-cured ham. This gives rise to the formation of free amino acids and short peptides (especially from actin through the action of cathepsin D in meat and from caseines as a result of plasmin and other proteases in cheeses) that contribute directly or indirectly to the flavour characteristics of the final product. In the case of octapeptide Lys-Gly-Asp-Glu-Glu-Ser-Leu-Ala, isolated from beef broth, this reportedly showed umami taste with a threshold value of about 500 mg/1. 

A number of endogenous chemicals provide effective barriers in innate immunity. They include hydrolytic enzymes of saliva, low pH of stomach and vagina and proteolytic enzymes in the small intestine. Additional examples include cryp-tidins, a-defensins, (3-defensins, interferons (IFNs), and surfactant proteins A and D. Cryptidins and a-defensins are produced in the small intestine and p-defensins are produced by the skin and respiratory tract. 

The mechanism by which proteins are catabolized in liver is not elucidated. Like most other organs, the liver contains an impressive arsenal of proteolytic enzymes. Some act optimally at alkaline pH, others at acid pH. The activity of the alkaline proteases is usually recovered in the supernatant fraction after differential centrifugation of the liver homogenate. The activity of the acid protease is associated with all cell fractions, but the highest specific activity is found in the lysosomal pellet. Whether the alkaline or the acid proteases attack the endogenous liver proteins is not known. 

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