Metabolism and degradation products of recombinant human insulin-like growth factor-I in lysosomes of rat kidney

1. The degradation of recombinant human insulin-like growth factor-I (rhIGF -I) by purified lysosomes of rat kidney was examined in vitro. The peptide structures of the 13 degradation products were deduced from the sequence an alysis and the molecular mass. Rat kidney lysosomal cathepsins efficiently cleave rhIGF-I to two chain peptides, like insulin. The cleavages mainly oc cur at the C-peptide/A-chain junction, D-peptide/X-chain junction and B21-2 2 or B22-23. 2. The effect of inhibitors on the lysosomal degradation of rhIGF-I was exa mined semiquantitatively by thr rate of formation of the degradation produc ts. The degradation of rhIGF-I was almost completely inhibited by the lysos omal cysteine protease inhibitors, leupeptin and leucine chloromethyl keton e, and a serine protease inhibitor, phenylmethylsulphonyl fluoride. On the other hand, the degradation was enhanced by the addition of a reducing agen t, glutathione.