CATALYTIC PROPERTIES OF RABBIT SERUM ESTERASES HYDROLYZING ESTERIFIEDMONOSACCHARIDES

Rabbit serum and one enzyme fraction isolated from rabbit serum by col umn chromatography (Fraction II) were used as catalysts in regioselect ive hydrolysis of radiolabelled pivaloylated monosaccharides (Piv=Me(3 )CCO). The hydrolysis of C-14-labelled methyl 2-O-pivaloyl-(2-MP)-, 6- O-pivaloyl (6-MP)-, 2,6-di-O-pivaloyl-(2,6-DP) alpha-D-glucopyranoside s and methyl 2-acetamido-2-deoxy-3,6-di-O-pivaloyl-(3,6-DPNAc) alpha-D -glucopyranosides, was studied, as well as that of the non-sugar subst rates butyrylthiocholine, thiophenylbutyrate, phenylacetate and paraox on, The specific activities of 2,6-DP, 3,6-DPNAc, butyrylthiocholine a nd thiophenylbutyrate were higher in Fraction II than in native sera, while those of phenylacetate and paraoxon were lower, Inhibition studi es were done using the substrates mentioned and five different inhibit ors, namely bis(p-nitrophenyl phosphate) (BNPP), eserine, paraoxon, Hg Cl2 and EDTA. The hydrolysis of 2,6-DP and 3,6-DPNAc was not inhibited by HgCl2 and only slightly by EDTA. Paraoxon, eserine and BNPP were p rogressive inhibitors of the hydrolysis of the two sugar substrates, a nd the pattern of inhibition resembled closely the inhibition of butyr ylthiocholine and thiophenylbutyrate hydrolysis. This result applied t o both, native serum and Fraction II. It was concluded that esterases in rabbit serum which hydrolyze pivaloylated sugar substrates belong t o the category of serine esterases. Kinetic parameters (K-M and V-max) , effects of temperature and pH on activity of esterases from Fraction II were also determined for the hydrolysis of sugar substrates.