Comparison of Substrates for Measuring Cystyl-Amino Peptidase. Activity in Serum of Pregnancy and Hepatic Disease and in Various Tissues

Summary: 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. In the serum of normal individuals and in patients with hepatic disease, the highest catalytic activity was obtained with Z,-cysteine-/J-naphthylamide, followed by S-benzyl-Z,-cysteine-p-nitroanilide and cystineamides. In hepatic disease, the enzyme activity with i-cysteine-jJ-naphthylamide was markedly increased, but that with other substrates was only slightly enhanced. In the placenta, brain, heart and liver, the highest catalytic activity was observed with S-benzyl-Z,-cysteine-p-nitroanilide and the enzyme activity pattern was similar to that in pregnant serum. However, in the kidney and small intestine the enzyme activity was highest withi-cysteine-s-naphthylamide, followed with S-benzyl-£-cysteine-p-nitroanilide and cystineamides. The enzyme activity with S-benzyl-Zrcysteine-p-nitroanilide in various sera was inhibited by Co2"1", but that with /,-cysteine^naphthylamide was stimulated by this ion. The purified oxytocinase has been reported to be inhibited by Co24". Therefore, the present observations suggest high efficacy of S-benzyk£-cysteine-p-nitroanilide for measuring oxytocinase activity for the assessment of placental function.