Effect of Surfactant Structure on Urease Stability in Reversed Micellar Systems

A. V. Puchkaev,¹ N. V. Girina,¹ A. P. Vlasov,¹ and D. I. Metelitza^1,2

¹Institute of Bioorganic Chemistry, Academy of Sciences of Belarus, ul. Zhodinskaya 5/2, Minsk, 220141 Belarus; fax: (0172) 63-7274; E-mail: ibochbel@igs.ac.by

²To whom correspondence should be addressed.

Submitted March 25, 1997; revision submitted April 8, 1997.

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The effective inactivation constants (k_in) were calculated from semilogarithmic plots of urease inactivation at 55°C in reversed micelles of anion aerosol OT (AOT) in octane, cationic cetyl-trimethylammonium bromide (CTAB) in octane with 15% hexanol, and in mixed micelles of AOT--Triton X-100 (1:1) and CTAB--Triton X-100 (1:1) in octane with 5 or 15% hexanol. The effect of the initial urease concentration (34-136 nM) and hydration degree w₀ of all four micellar systems on the k_in values was studied at 55°C. In all micellar systems increase of concentration of solubilized urease caused significant decrease in k_in. In AOT micelles in octane the dependence of k_in on w₀ has a maximum at hydration degree 20; this corresponds to the maximal catalytic activity of urease. In three other micellar systems k_in values gradually decreased with increasing w₀, i.e., micelle moistening stabilizes urease. The enzyme stability in the system AOT--Triton X-100 was twofold higher than in aqueous medium. The temperature dependences of excess heat capacity of urease were determined by differential scanning calorimetry and compared with curves of heat consumption of the enzyme in aqueous medium. Urease structure in less stable and less cooperative in reversed AOT micelles (w₀ = 20) than in aqueous solution. The dissociation-association scheme of urease inactivation is discussed.

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KEY WORDS: urease, reversed micelles, aerosol OT, Triton X-100, CTAB, mixes micelles, urease inactivation, enzymes in non-aqueous media.

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