Further Confirmation of Carboxypeptidase Y as a Metal-Free Enzyme Having a Reactive Serine Residue1

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Abstract
The metal content of carboxypeptidase Y was analyzed by the atomic absorption method. After exhaustive dialysis against an EDTA solution, the enzyme showed no loss of activity nor any significant content of metals (Zn, Mg, Ca, Cu, Mn, Ni, Fe, and Co). The activity was, however, rather sensitive to preincubation with various metals. The reactivity of a serine residue of the enzyme was also reevaluated. Diiso-propyl fluorophosphate (DFP) and phenylmethanesulfonyl fluoride (PMSF) stoichio-metrically and irreversively inhibited the enzyme. The rate of inactivation with DFP was much faster than that for trypsin [EC 3.4.21.4] and chymotrypsin [EC 3.4.21.1], while the rate with PMSF was one-fifteenth of that for chymotrypsin. The pH-dependence of the inactivation by DFP was similar to that of the enzymatic hydrolysis of acetylphenylalanine ethyl ester. The present results indicate that carboxypeptidase Y is free of metals and has a serine residue with a vital role in the catalytic process, though the functional role of this SH group remains to be clarified.

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