Simple, refined fluorometric method for measuring cystyl-amino peptidase activity

Cystyl-amino peptidase (EC 3.4.11.3) activity in serum or plasma was measured fluorometrically using L-cystine-di- β-naphthylamide in the absence and presence of thiol such as mercaptoethanol. In the presence of thiol, L-cystine-di- β-naphthylamide is converted to L-cysteine- β-naphthylamide, and the enzyme activity to hydrolyze L-cysteine- β-naphthylamide can be measured, while in the absence of thiol, the enzyme activity to hydrolyze L-cystine-di- β-naphthylamide is determined. Thiol added did not affect various aminopeptidase activities. The present method is able to measure the enzyme activity hydrolyzing L-cystine-di- β-naphthylamide and L-cysteine- β-naphthylamide simultaneously and separately using only L-cystine-di- β-naphthylamide. This method is simple, sensitive and useful in clinical routine work, assessing placental function for the evaluation of the pregnant status.