Kunitz enzyme inhibitors

Some of the best investigated anti-nutrients are the enzyme inhibitors present in legumes and other plants. The Bowman-Birk and the Kunitz inhibitors of trypsin and other proteases are among the best characterized. In contrast to the non-specific and widespread influences of tannins and lectins (Carmona, 1996), the Bowman-Birk, Kunitz and other such inhibitors target specific enzymes. Corresponding with this, proteases and other digestive enzymes vary in sensitivity to the different inhibitors. 

Heibges, A., Salamini, F., Gebhardt, C. (2003). Funetional eomparison of homologous members of three groups of Kunitz-type enzyme inhibitors from potato tubers Solanum tuberosum L.). Mol Gen. Genomics, 269,535-541. 

Aprotinin is a polypeptide consisting of 58 amino acid residues derived from bovine lung tissues and shows inhibitory activity toward various proteolytic enzymes including chymo-trypsin, kallikrein, plasmin, and trypsin. It was also one of the first enzyme inhibitors used as an auxiliary agent for oral (poly)peptide administration. The co-administration of aprotinin led to an increased bioavailability of peptide and protein drugs [5,44,45], The Bowman-Birk inhibitor (71 amino acids, 8 kDa) and the Kunitz trypsin inhibitor (184 amino acids, 21 kDa) belong to the soybean trypsin inhibitors. Both are known to inhibit trypsin, chymotrypsin, and elastase, whereas carboxypeptidase A and B cannot be inhibited [7,46],... 

TIs are protease-inhibiting factors that bind the protease enzymes to decrease their catalytic power. The two predominant protease inhibitors are Kunitz trypsin inhibitor and Bowman-Birk inhibitor (BBI), and they are both protein in nature. 

Feeding rats raw soybeans or pure soybean inhibitor of the digestive enzymes chymotrypsin and trypsin, the so-called Bowman-Birk and Kunitz protease inhibitors, induce hypertrophy and hypersecretion of chymotrypsin and trypsin by the pancreas. Raw soy flour diets also potentiate the carcinogenicity of azaserine and nitrosamines in rats. A so-called biofeedback mechanism has been postulated to explain these consequences of soybean nutrition 4-17). 

Inhibitor activity is normally determined with commercial animal enzymes, e. g., bovine trypsin or bovine chymotrypsin. The evaluation of a potential effect of the inhibitors on human health assumes that the inhibition of human enzymes is known. Present data show that inhibitors from legumes generally inhibit human trypsin to the same extent or a little less than bovine trypsin. On the other hand, human chymotrypsin is inhibited to a much greater extent by most legumes. Ovomucoid and ovoinhibitor from egg white as well as the Kazal inhibitor from bovine pancreas do not inhibit the human enzymes. The Kunitz inhibitor from bovine pancreas inhibits human trypsin but not chymotrypsin. The data obtained greatly depend not only on the substrate used, but also on the enzyme preparation and the reaction conditions, e. g., on the ratio enzyme/inhibitor. The stability of an inhibitor as it passes through the stomach must also be taken into account in the evaluation of a potential effect (cf. Table 16.15). The Kunitz inhibitor of soybeans, for... 

Soybeans contain compounds that impair activity of enzymes trypsin, chymotrypsin, carboxypeptidase, elastase, and a-amylase. The presence of enzyme inhibitors, Bowman-Birk inhibitor (BBI) (an inhibitor of chymotrypsin and trypsin) and Kunitz trypsin inhibitor (KTI), in unheated soybeans hinders the activity of protease enzymes trypsin and... 

Dye-sensitized photooxidation of methionine occurs selectively in strong formic or acetic acid media (131). All four methionine residues in RNase-A appear to be converted to sulfoxides under these conditions. The product still showed 13% of the initial activity in the Kunitz RNA assay at pH 5 [see, however, Neumann et al. (ISO)]. Apparently an oxygen atom can be accommodated next to each Met sulfur atom in a structure closely resembling that of the native enzyme. The 3 buried tyrosine residues appeared to have been largely normalized. It would be interesting to know if the binding of inhibitors returned these to the buried condition. 

The most thoroughly studied mechanism of protein protease inhibitors is that of the standard mechanism (or Canonical or Laskowski mechanism) inhibitors of serine proteases (1) (Fig. 2). Standard mechanism inhibitors include the Kazal, Kunitz, and Bowman-Birk family of inhibitors and bind in a lock-and-key fashion. Ah standard mechanism inhibitors insert a reactive loop into the active site of the protease, which is complementary to the substrate specificity of the target protease and binds in an extended fi-sheet with the enzyme in a substrate-like manner. WhUe bound to the protease, the scissile bond of standard mechaiusm inhibitors is hydrolyzed very slowly, but products are not released and the amide bond is re-ligated. The standard mechanism is an efficient way to inhibit serine proteases, and it is thus used by many structurally... 

If thrombin and factor Xa, the major activated blood coagulation factors (Fig. 11.6), escape into healthy blood vessels, blood clots will develop and occlude capillaries throughout the body. Direct inhibition of these activated enzymes in the blood flow utilizes serine protease inhibitors, of which there are two common types a Kunitz inhibitor and a serpin. The former possess a Kunitz domain, a convex antiparallel (1-sheet that exactly fits into the concave active site of a serine protease, directly blocking it (lock and key mechanism). By contrast, serpins undergo complex interactions with other proteins to cause conformational changes that bait and block the catalytic action (Fig. 11.12 shows the bait). Table 11.3 fists the major coagulation inhibitors and cofactors, their targets and mechanisms of action. 

Figure 4, Kinin generating cascade and various vascular mediators in cancer tissues affecting the EPR effect. Mediators, enzymes, and inhibitors are shown. See text for details. M, macrophage PMN, polymorphoneuclear cells COXs, cyclooxygenases PCs, prostaglandins. SBTl, soybean trypsin inhibitor (Kunitz type) ONOO", peroxynitrite NO, nitric oxid MMP, matrix metalloproteinases.

Inhibitors of Bowman-Birk types are widely distributed in plants and significantly different from the Kunitz inhibitor in their amino acid composition and are able to interact not only with trypsin, but with chymotrypsin (Valueva Mosolov, 2002 Rawlings et al., 2004). Inhibitors of Bowman-Birk type characterized by the presence of two reactive centers on a single polypeptide chain rich in cysteine (7 or more residues in one polypeptide), and the lack of amino acid residues tryptophan and tyrosine. The molecular weight of such inhibitors can vary from 8 to 16 kDa. Sometimes there are inhibitors that contain two domains on one polypeptide chain and active only in relation to one type of enzyme (Valueva Mosolov, 2002 Yan et al., 2009 Mosolov Valueva, 2008). 

Inhibitors of proteolytic enzymes may play an important role in plant protection not only from harmful insects, but also other pests nematodes, the nematodes. Many nematodes are parasitic on plants and cause significant damage to agricultural production. In the gut of nematodes contains active cysteine and serine proteases, including chymotrypsin and kallikrein is similar. It is interesting that when infected with worms, among transcripts of tomato were found proteins related to the family of Kunitz inhibitor (SBH) (Valueva Mosolov, 2002 Schroeder et al., 1995). 

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