METABOLISM OF MOUSE GROWTH HORMONE-RELEASING FACTOR, MGRF(1-42)OH, AND SELECTED ANALOGS FROM THE BOVINE GRF SERIES IN MOUSE AND BOVINE PLASMA IN-VITRO

The presence of val(2) in mGRF(I-42)OH is unique and, as shown in this study renders this GRF resistant to plasma DPP-IV: the main enzyme re sponsible for rapid hydrolysis and inactivation of Ala(2)-containing G RFs from other species via cleavages between Ala(2)-Asp(3). The presen ce of DPP-IV activity in mouse serum, and mouse and bovine plasma has been demonstrated with Gly-Pro-p-nitroanilide and/or with two DPP-IV-s ensitive bGRF analogs, [Leu(27)]bGRF(I-29)NH2 and [Ala(15),Leu(27)/bGR F(I-29)NH2, which were effectively converted to their respective (3-29 ) fragments. During incubations of mGRF(I-42)OH in mouse serum or plas ma, as well as in bovine plasma in vitro, no major fragments were dete ctable, except for small amounts of metabolites with HPLC retention ti mes corresponding to those of mGRF(12-42)OH and mGRF(21-42)OH, indicat ive of possible trypsin-like cleavages between Arg(11)-Lys(12) and Arg (20)-Lys(21). Both mGRF(1-42)OH (tin 52-78.5 min) and [Val(2),Ala(15), Leu(27)]- bGRF(I-29)NH2 (t(1/2) 78.5 min) disappeared 5 to 7 times fas ter iii mouse than in bovine plasma, indicating much higher activity o f various degrading enzymes in mouse plasma. In summary, our data prov ide evidence that mGRF(I-42)OH, despite its resistance to plasma DPP-I V is degraded relatively fast in mouse plasma or serum because of tryp sin-like and other non-DPP-IV-related proteolytic cleavages.