Substrate Specificity of the Milk-clotting Enzyme from Irpex lacteus on Peptide Hormones

概要

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To clarify the substrate specificity of the milk-clotting enzyme excreted by Irpex lacteus (IR), various peptide hormones were used as substrates to find the cleavage sites and to compare the specificity with those of other milk-clotting enzymes such as calf chymosin (CR), Mucor miehei enzyme (MR), and Endothia parasitica enzyme (ER). IR hydrolyzed substance P, physalaemin, β-neo-endorphin, neurotensin, angiotensin I, LHRH, and DSIP, but not bradykinin. Although IR generally require hydrophobic amino acids in the P1 and P1 sites, it also hydrolyzed peptide bonds which have only one hydrophobic amino acid either in the P1 or P1 site. CR did not hydrolyze either angiotensin I or DSIP, and MR did not hydrolyze DSIP, but both enzymes hydrolyzed the Tyr(5)-Glu(6) bond of LHRH slightly. On the other hand, ER hydrolyzed these substrates at several bonds. Besides the common cleavage sites with IR, ER hydrolyzed the His(6)-Pro(7) bond of angiotensin I, the Leu(7)-Arg(8) bond of LHRH, and the Ala(2)-Gly(3) and Gly(3)-Gly(4) bonds of DSIP. Therefore, these milk-clotting enzymes can be classified into two groups according to their substrate specificities on peptide hormones. CR and MR belong to the first group which has restricted specificity, while IR and ER belong to the second group which shows relatively broad specificity.
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