AN ASSAY OF DIPEPTIDYL PEPTIDASE-IV ACTIVITY IN HUMAN-SERUM AND SERUM OF PREGNANT-WOMEN WITH GLYCYL-L-PROLINE-1-NAPHTHYLAMIDE AND OTHER GLYCYL-L-PROLINE-ARYLAMIDES AS SUBSTRATES
A micromethod for measuring dipeptidyl peptidase IV activity in human serum with glycyl-L-proline-1-naphthylamide as substrate is described. The method requires < 20 .mu.l of serum. The pH optimum for cleaving glycyl-L-proline-1-naphthylamine by the enzyme in human serum in Tris-HCl buffer was 8.0 and Km value was established as 7.2 .cntdot. 10-4 mol/l. The advantage of this substrate is the absence of spontaneous hydrolysis during the assay of enzyme activity in contrast to glycyl-L-proline-4-nitroanilide. The Km values of the latter substrates and glycyl-L-proline-2-naphthylamide in the same buffer were 1.0 .cntdot. 10-4 mol/l and 2.4 .cntdot. 10-4 mol/l, respectively. Glycyl-D-proline-4-nitroanilide was not hydrolyzed by the dipeptidyl peptidase IV present in human serum. The activities of dipeptidyl peptidase IV in the sera from 30 healthy human subjects with glycyl-L-proline-1-naphthylamide as substrate were 176.1 .+-. 32.8 nkat[katal]/l [mean .+-. SD; 100.2-264.1 nkat/l of serum). In this group men had significantly (P < 0.01) higher activity of the enzyme than women. The cleaving of glycyl-L-proline-1-naphthylamide and glycyl-L-proline-4-nitroanilide by dipeptidyl peptidase IV in human sera was closely correlated (r = 0.86). During normal pregnancy the activity of dipeptidyl peptidase IV in human serum decreases markedly in the 1st half of pregnancy. After delivery, the serum enzyme activity returns progressively to initial levels. [Application to study of pathologic conditions is considered.].