Porcine Muscle Prolyl Endopeptidase and Its Endogenous Substrates

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Abstract
Prolyl endopeptidase [EC 3.4.21.26] was purified 4,675-fold with a yield of 26.3% from porcine muscle.The purified enzyme was shown to be very similar to the liver enzyme with respect to its molecular weight (72,000–74,000), antigenicity, substrate specificity, and susceptibility to protease inhibitors. Among several bioactive peptides, angiotensins I, II, and III had the lowest Km of 0.6 to 3 μM with the lowest kcat of 0.19 to 0.85 s while thyrotropin-releasing hormone had the highest kcat of 98 μM with the highest kcat of 14.4 s−1 Interestingly, mastoparan was hydrolyzed at alanyl bonds, but insulin was only slightly hydrolyzed and glucagon was not hydrolyzed although the latter two peptides contain prolyl and/or alanyl bonds. Muscle prolyl endopeptidase failed to hydrolyze proteins with high molecular weight such as albumin, immunoglobulin G, elastin, collagen, and muscle soluble and insoluble proteins. However, 8 of 14 peptides with molecular weights lower than 3,000, which were isolated from muscle extract, were digested by this enzyme, and they were proved to contain prolyl and/or alanyl residues in their molecules. The data suggest that they are probable endogenous substrates for prolyl endopeptidase.