KINETIC-ANALYSIS OF THE TYPE-1 PROINSULIN ENDOPEPTIDASE BY A MONOCLONAL ANTIBODY-BASED IMMUNOADSORBENT ASSAY

A simple, rapid and sensitive assay for the type-1 endopeptidase (Arg-Arg cleaving) was developed by using an antiproinsulin monoclonal immunoadsorbent to separate reaction products from the substrate. The values obtained by this assay were identical with those obtained by an h.p.l.c.-based procedure and yielded similar values for the pH optimum (5.6) and Ca2+ activation (K0.5 = 2 mM). It was shown that the type-1 endopeptidase was readily solubilized by Triton X-114 (87 +/- 3 %, n = 12) and partitioned principally into the aqueous phase at 30-degrees-C (90.1 +/- 2.60 %, n = 12). Activity was lost on gel filtration, but could be restored by adenosine 5'-[gamma-thio]triphosphate (K0.5 = 6-mu-M), 50-mu-M-dithiothreitol or 50-mu-M-Ca2+-trans-1,2-diaminocyclohexane-NNN'N'-tetra-acetic acid (CDTA), indicating that the enzyme was particularly sensitive to heavy metal ions. The K(m) obtained with proinsulin as substrate (13 +/- 1.7-mu-M) indicated that the enzyme works at close to its V(max.) in the nascent secretory granule. The V(max.) of the enzyme prepared from insulin granules (0.6 % proinsulin converted/min) corresponded closely to the rate measured in vivo in rat islets. The type-1 endopeptidase also appears to be capable of binding to proinsulin in the region of the C-peptide/A-chain junction, since a peptide spanning this region was found to inhibit the I-125-proinsulin processing measured by this assay.