The hexameric form of LAPTc was confirmed by ana lytical ultracentrifugation, a versatile and power ful tool for the identification of oligomeric states and the determination of protein molecular masses. Fig ure 3B shows the experimental and fitted sedimentation velocity profiles obtained at 56 uM by monitoring the absorbance at 295 nm. The derived sedimentation coef ficient former distribution exhibits four main spe cies sedimenting at 5. 1, 10. 2, 15. 3 and 19. 5 S. The s value depends on the molar mass, M, and Stokes radius, RS, of the particle, according to the Svedberg equation, s M To calculate the correspond ing molecular masses, calibrated size exclusion chroma tography was performed with the same samples, giving Stokes radii for the two main species eluting at 9 and 10 ml of 6. 8 and 5.

7 nm, respectively. The combina tion of the s values of 15. 3 and 10. 2 S with RS 6. 8 and 5. 7 nm gives the estimates for the species of M 593 and 330 kDa, respectively, confirming the results obtained by SEC MALLS. Con sidering the monomer molecular mass deduced from the sequence, 58. 7 kDa, the calculated number of subu nits present in the main species eluting at 10 ml is 5. 6, suggesting a pentamer or, more likely, a hexamer. Tak ing into account 5 or 6 as the number of subunits, the inferred RS values from the Svedberg equation are 5. 1 and 6. 1 nm, which correspond to frictional ratios of 1. 16 and 1. 31, respectively. These are within the values expected for globular proteins. However, the frictional ratio obtained for the pentamer hypothesis is somewhat low for a 330 kDa protein.

Thus, these data indicate that the main rLAPTc species is a hexamer. The sedi mentation distributions of rLAPTc at 170, 56 and 10 uM present the same main features. However, the ratio of hexamer to trimer decreases when the concentration of the enzyme goes from 56 to 10 uM. In addition, at concentrations as high as 170 uM the amount of large aggregates increases significantly. Our data thus show a complex equilibrium among different multimers depending on enzyme concentration. Recombinant and native forms of LAPTc display distinct activity features The influence of pH on the activity of purified LAPTc and rLAPTc was determined. Maximal specific activity for the native enzyme was measured at pH 7. 0. At pHs 6. 0 and 8. 0 the recorded specific activ ities were 45% of that measured at pH 7. 0, whereas at pHs 5. 0 and 9. 0 the enzyme was shown to be inactive. Conversely, for rLAPTc the optimal pH is 8. 0, at pH 7. 5 and 9. 0 the enzyme loses 60 and 75%, respectively, of its activity recorded at pH 8. 0. These data demonstrate that LAPTc has a strong dependence on neutral pH, whereas its recombinant form Batimastat displays maximal activity at pH 8. 0.