Catalytic activity of hexokinase in reversed micelles

Reversed micelles can control the size of water pools and the physical prop erty of water by changing W-0 (= [water]/[surfactant]). Hexokinase (HK) act ivity seems to be easily affected by the microenvironment in the neighborho od of the enzyme because it is assumed that HK binds to the outer mitochond rial membrane by insertion of its hydrophobic NH2 tail. The catalytic activ ity of HK was examined in reversed micelles in order to study the effect of the microenvironment in the neighborhood of HK on the activity. Sodium bis (2-ethylhexyl)sulfosuccinate (AOT), hexadecyltrimethyl ammonium chloride (H TAC), and octaoxyethylene dodecyl ether (C12E8) were used as anionic, catio nic, and nonionic surfactants, respectively. HK activity was obtained by me asuring ATP and ADP amounts with HPLC, The high electrostatic inner surface s of AOT and HTAC reversed micelles were not favorable for HK to exhibit th e catalytic activity, but the activity in HTAC reversed micelles was 2-3 ti mes higher than that in AOT reversed micelles and the activities in both re versed micelles revealed an optimum at W-0 = 10. The phenomenon was discuss ed in connection with the location of HK, nonuniform distribution of substr ates, and the size and physical properties of the water pools. On the other hand, HK activity was much higher in C12E8 reversed micelles than in AOT a nd HTAC reversed micelles and increased with the concentration of C12E8 Thi s suggests that HK activity is easily revealed in hydrated ethylene oxide c hains. In conclusion, it was demonstrated that HK activity depends on the m icroenvironment such as the electrostatic field, the physical properties of water, and the hydrophobicity. (C) 2001 Academic Press.