Pepsin occurrence

A better understanding of the structure of collagen IV molecules explains some previous observations. The occurrence of the 7 S domain and nonhelical interruptions in the molecule can explain in part the heterogeneous mixture of collagenous components isolated from basement membrane, particularly when pepsin or other proteases are used to solubilize the protein. The molecules isolated from lathyritic basement membrane by acid extraction are truncated and lack the 7 S domain presumably due to cleavage by endogenous proteases. 

Acid proteases have been found in other organisms and have been the subject of recent reviews 21, 29), The widespread occurrence of these enzymes is surprising since they function optimally at pH values which may be 5 to 6 units below a physiological pH. The necessity for a low pH optimum for pepsin and chymosin is understandable, but why do cathepsins require a low pH for optimal activity Their role is at present unclear, but at least one enzyme—an acidic cathepsin from polymorphonuclear leucocytes—has been implicated in responses to inflammatory processes (30). It is not clear why an acidic enzyme would be required in such a situation unless local pockets of low pH are created intracellularly. 

Further evidence for the occurrence of a diester linkage in this protein is reflected in the electrophoretic behavior. Pepsin still moves anodically in 0.1 N hydrochloric acid, pH 1.08. On the other hand, the phosphorus-free pepsin is positively charged at this pH. It has an isoelectric pH of 1.7. As shown in Fig. 6, the mobilities of these two proteins differ by a... 

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