Proteases endogenous proteolytic enzymes

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. 

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. 

Second, of several proteases that are known to participate in peptide/protein hydrolysis, the cysteine and aspartic proteases, also known as cathepsins, constitute a majority of the endosomal/lysosomal proteolytic machinery. These enzymes are often synthesized in an inactive form requiring excision of the pro-domain facilitated by other proteases or autocatalytic mechanisms activated by acidic pH. Cathepsins seem to have originally evolved to catabolize both internalized and endogenous proteins crucial for cellular homeostasis, autophagy, apoptosis, and antigen presentation. The optimal pH range for enzymatic activity of cathepsins ranges from 5.0 to 6.5, as is the case with endosomes/lysosomes. 

The only other plant enzyme which has been reported to be stimulated by the same three treatments is isofloridoside-phosphate synthase (4) as previously described. Those authors presented evidence for the presence of an endogenous acid protease which could stimulate the enzyme. They also proposed (4) that calmodulin and protein kinase somehow stimulated this protease which in turn proteolytically-activated the isofloridoside-phosphate synthase. Such a mechanism is also conceivable for the potato leaf phospholipase. The only other plant enzyme which has been reported to be stimulated by proteolytic activation is a glucan synthase in soybean cells (6). It is also interesting to note that an animal phospholipase, pancreatic phospholipase A, is activated by the proteolytic removal of a heptapeptide from the ammo terminus (7). 

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