PubMed · 5960517
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E Toro-Goyco, M Matos. 1966-04-30. Pinguinain: a simple method for its crystallization.. https://doi.org/10.1038/210527b0
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The thermal denaturation of bromelain, a cysteine proteinase from the papain family, was studied by means of circular dichroism (CD) and differential scanning calorimetry (DSC). It was found that this process is completely irreversible and apparently follows a simple two-state mechanism of the type N-->D. The activation energy, E, that characterizes this reaction was calculated by the use of different approaches: (i) the effect of heating rate on the temperature at which the transition is half completed; (ii) analysis of individual transition curves; (iii) kinetic studies at fixed temperatures; and (iv) single DSC tracings. The obtained values for E were rather similar to one another, varying from 164 to 226 kJ/mol. In comparison, the total calorimetric enthalpy change was 334 kJ/mol. When a more complex mechanism is considered (N<-->U-->D), which takes into account the presence of a reversibly unfolded state (U), our results suggest that the rate-limiting step is precisely the formation of U. Calculation of the corresponding activation enthalpy and entropy also seems to support this proposal.
Protein inhibitors of cysteine proteinases possessing unusual properties have been found in soya (Glycine max) seeds. One of the inhibitor forms has also been detected in Bowman-Birk inhibitor preparations (both commercial and purified by affinity chromatography on chymotrypsin-Sepharose ones). A peculiarity of the inhibitors is that they irreversibly lose their activity in the presence of reducing agents; therefore their effects are normally unobserved under standard conditions of cysteine proteinase inhibitor assays. Soybean inhibitors are represented by two forms with pI of 5.9 and 3.2. The molecular mass of the inhibitor whose pI is equal to 5.8 is about 14 kDa. Both inhibitors suppress the activity of papain, ficin and bromelain.
Two forms of stem bromelain (EC 3.4.22.4) were isolated from commercial, crude and chromatographically purified preparations of the enzyme by means of gel-filtration and cation-exchange liquid chromatography. These forms possess nearly identical secondary and tertiary structures, as judged from their circular dichroism (c.d.) spectra. The spectral characteristics of stem bromelain suggest that this enzyme belongs to the alpha + beta protein class, as other cysteine proteinases do. In agreement with these results, quantitative estimation of secondary structures yielded amounts similar to those for papain and proteinase omega. However, the bromelain c.d. curve is clearly distinguishable from those reported for papain and proteinase omega, on one hand, and that of chymopapain, on the other. Thus, it is apparent that there are at least three types of c.d. spectra associated with the family of cysteine proteinases.