Comparison of substrates for measuring cystyl-amino peptidase activity in serum of pregnancy and hepatic disease and in various tissues.

The specificity of substrates for measuring cystyl-amino peptidase activity in pregnant serum in the assessment of placental function was studied, using £-cystine-di-j3-naphthylamide, L-cystine-bis-/?-nitroanilide,/,-cysteine-j3naphthylamide, and S-benzyl-i-cysteine-p-nitroanilide. The enzyme activity with S-benzyl-Z,-cysteine-/?-nitroanilide increased more rapidly and markedly than with other substrates with advancing gestation. At term, the highest value was obtained with S-benzyl-i-cysteine-p-nitroanilide, followed by £-cysteine-/J-naphthylamide and cystineamides. The cystyl-amino peptidase activity with these substrates in pregnant serum was well correlated with the plasma oestriol level.