Degradation of porcine relaxin by glutathione-insulin transhydrogenase and a neutral peptidase

Abstract The susceptibility of porcine relaxin and 125 I-polytyrosyl-porcine relaxin to degradation by 3 purified senzymes involved in the degradation of insulin and proinsulin was examined. Rat liver glutathione-insulin transhydrogenase (GIT), which cleaves disulfide bonds in insulin, catalyzed a time- and concentration-dependent increase in trichloroacetic acid (TCA)-soluble radioactivity of relaxin. The Sephadex G-50 profile of the reaction products revealed conversion to the A- and B-chains. Relaxin competitively inhibited the degradation of insulin by GIT; however, kinetic analysis revealed insulin to be preferred over relaxin as a substrate. Rat liver cytosol neutral thiol peptidase (NTP) catalyzed a time- and concentration-dependent increase in the TCA solubility of relaxin and a shift in the Sephadex G-50 radioactivity profile to low molecular weight products. Kinetic analysis revealed that insulin and B-chain are preferred over relaxin as substrates for NTP. A third enzyme, rat kidney neutral metalloendopeptidase, which degrades proinsulin and insulin C-peptide but not insulin, also did not degrade porcine relaxin.