New role for leucyl aminopeptidase in glutathione turnover.

A manganese-dependent cysteinyl-glycine hydrolysing activityhas been purified to electrophoretic homogeneity from bovinelens. The characterization of the purified enzyme (molecularmass of the native protein, molecular mass of the subunit andextensive primary structure analysis) allowed the unequivocalattribution of the cysteinyl-glycine hydrolysing activity, whichis usually associated with alanyl aminopeptidase (EC 3.4.11.2)or membrane-bound dipeptidase (EC 3.4.13.19), to LAP (leucylaminopeptidase; EC 3.4.11.1). Analysis of the pH dependence ofCys-Gly hydrolysis catalysed by LAP, supported by a molecularmodelling approach to the enzyme-substrate conformation, gaveinsights into the ability of the enzyme to recognize Cys-Gly as asubstrate. Due to the effectiveness of LAP in hydrolysing Cys-Gly(Km = 0.57 mM, kcat = 6.0 × 103 min-1 at pH 7.4 and 25 oC)with respect to other dipeptide substrates, a new role for thisenzyme in glutathione turnover is proposed.