1. Oxidation of methionine residues of porcine and bovine pepsins
B Kassell, K Kido Biochemistry . 1975 Feb 11;14(3):631-5. doi: 10.1021/bi00674a026.
By treating porcine and bovine pepsins with H2O2 at pH 3.2, 3.5 of the 4 methionine residues of porcine pepsin and 1.6 of the 3 residues of bovine pepsin were oxidized to methionine sulfoxide. The effect of modification on activity varied with the substrate. There were no significant changes in catalytic constants in the hydrolysis of acetyl-L-phenylalanyl-L-tyrosine by both pepsins and in the hydrolysis of benzyloxycarbonyl-L-glutamyl-L-tyrosine by porcine pepsin. Hydrolysis of benzyloxycarbonyl-L-glutamyl-L-tyrosine by bovine pepsin was too slow to measure. With benzyloxycarbonyl-L-histidyl-L-phenylalanyl-L-tryptophan ethyl ester as substrate, the modification decreased the catalytic efficiency (kcat/Km) by two-thirds for porcine pepsin and by half for bovine pepsin. With hemoglobin substrate, digestion was significantly less than with native pepsin for modified porcine pepsin, and slightly less for modified bovine pepsin. The results are interpreted as indicating the presence of a methionine residue that participates in the binding of long substrates, but is not close enough to the active site to reach short substrates. Cleavage of the modified pepsins with cyanogen bromide identified the methionine nearest the carboxyl terminus of both pepsins as a resiude that remained partially unmodified.
2. L-Phenylalanyl-L-tryptophan 0.75-hydrate
Carl Henrik Görbitz Acta Crystallogr C . 2006 Jun;62(Pt 6):o328-30. doi: 10.1107/S0108270106014491.
The title compound, C20H21N3O3.0.75H2O, crystallizes as exceedingly thin fibers. The crystal packing arrangement is related to those of other hydrophobic dipeptides with phenylalanine residues, but the structure has pseudo-tetragonal symmetry in an orthorhombic space group with four peptide molecules and three water molecules in the asymmetric unit.
3. Determination of the pepsin activity in human gastric juice, using defined oligopeptides as substrates
E Schnaith Clin Biochem . 1989 Apr;22(2):91-8. doi: 10.1016/s0009-9120(89)80004-9.
Different methods for the determination of pepsin activity in human gastric juice, using defined oligopeptides as substrates, were investigated. N-Acetyl-L-phenylalanyl-L-3,5-diiodotyrosine and tripeptides like benzyloxycarbonyl-L-histidyl-L-phenylalanyl-L-tryptophan ethyl ester, benzyloxycarbonyl-L-histidyl-L-phenylalanyl-L-tyrosine ethyl ester, or benzyloxycarbonyl-L-histidyl-L-4-nitrophenylalanyl-L-phenylalanine methyl ester lead to only small absorption changes in the direct measurement of pepsin activity. Suitable substrates were shown to be the hexapeptide, L-leucyl-L-seryl-L-4-nitrophenylalanyl-L-norleucyl-L-alanyl-L- leucine-methyl ester and the octapeptide, L-prolyl-L-histidyl-L-leucyl-L-seryl-L-4-nitrophenylalanyl-L-norleucyl-L -alanyl-L-leucine methyl ester. Hydrolysis of these peptides can be measured continuously, using a spectral line photometer at 313 nm. Optimal test conditions and kinetic constants for substrate cleavage by pepsin in solution and by human gastric juice were determined.
