Subcellular localization, substrate specificity and crystallization of duodenase, a potential activator of enteropeptidase

Duodenase, a serine protease from bovine duodenum mucosa, was located in endoplasmic reticulum, the Golgi secretory granules of epithelial cells and ducts of Brunner's glands by the A-gold immunocytochemical method. Duodenase exhibits trypsin-like and chymotrypsin-like specificities with a preference for substrates having lysine at the P1 and proline at the P2 positions. The kinetic constants for the hydrolysis of 21 potential duodenase substrates are reported. The best substrates were found to be alpha-N-tosylglycylprolyllysine 4-nitroanilide (k[cat]/Km of 35000 M[-1] s[-1]), alpha-N-succinylthreonylprolyllysine 4-nitroanilide (k[cat]/Km of 18000 M[-1] s[-1]) and alpha-N-serylprolyllysine 4-nitroanilide (k[cat]/Km of 2600 m[-1] s[-1]), all of which contain the P1-P3 sequence of the enteropeptidase zymogen/activation site. On the basis of its catalytic properties and sites of localization, duodenase has been postulated to be an activator of the enteropeptidase precursor. A tetradecapeptide (LVTQEVSPKIVGGS) having the P9-P5'sequence of the cleavage site of zymogen activation of bovine proenteropeptidase was synthesized, and kinetic parameters of its hydrolysis by duodenase were determined (Km of 87 microM; k[cat] of 1.4 s[-1]; k[cat]/Km of 16000 M[-1] s[-1]). Crystals of duodenase frozen in a stream of liquid nitrogen diffracted synchrotron X-rays to 0.2-nm resolution.     

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Sorption of Cs,Sr, U,and Pu radionuclides on natural and modified clays

The ability of natural and modified montmorillonite clays from Belgorod oblast to sorb Cs, Sr, U, and Pu radionuclides was studied. The clays were modified by treatment with metal (Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Fe²⁺, Zn²⁺) chloride solutions or aqueous HCl. The natural and modified clays studied show high performance in sorption treatment of solutions to remove Cs radionuclides. The natural clay and the Na and Mg forms of clays show the best sorption characteristics with respect to Cs. The distribution coefficient K _d of ¹³⁷Cs in sorption on the above samples from a 0.1 M NaNO₃ solution is (1.1–1.4) × 10⁴ cm³ g⁻¹, which is 4–5 times higher compared to natural clinoptilolite. The Sr, U, and Pu radionuclides are sorbed on the examined clay samples to a considerably lesser extent. The K _d values in sorption of these radionuclides from tap water are lower by 2–3 orders of magnitude than in sorption of Cs. Addition of clay materials in the course of cementation of liquid radioactive wastes, including NPP bottom residues, allows the rate of radiocesium leaching from the hardened cement compounds to be decreased by a factor of 5–16. The most efficient sorption additive in cementation of NPP bottom residues is natural montmorillonite clay.