Characterization of a bioactive 15 kDa fragment produced by proteolytic cleavage of chicken growth hormone

There is evidence for a cleaved form of GH in the chicken pituitary gland. A 25 kDa band of immunoreactive-(ir-)GH, as well as the 22 kDa monomeric fo rm and some oligomeric forms were observed when purified GH or fresh pituit ary extract were subjected to SIDS-PAGE under nonreducing conditions. Under reducing conditions, the 25 kDa ir-GH was no longer observed, being replac ed by a 15 kDa band, consistent with reduction of the disulfide bridges of the cleaved form. The type of protease involved was investigated using exog enous proteases and monomeric cGH. Cleaved forms of chicken GH were generat ed by thrombin or collagenase. The site of cleavage was found in position A rg(133)-Gly(134) as revealed by sequencing the fragments produced. The NH2- terminal sequence of 40 amino acid residues in the 15 kDa form was identica l to that of the rcGH and analysis of the remaining 7 kDa fragment showed a n exact identity with positions 134-140 of cGH structure. The thrombin clea ved GH and the 15 kDa form showed reduced activity (0.8% and 0.5% of GH, re spectively) in a radioreceptor assay employing a chicken liver membrane pre paration. However, this fragment had a clear bioactivity in an angiogenic b ioassay and was capable to inhibit the activity of deiodinase type III in t he chicken liver.