Complete Amino Acid Sequence Analysis of a Peptide Isolated from the Thymus That Enhances Release of Growth Hormone and Prolactin

We have previously reported that thymosin fraction 5 (TF5), a partially purified calf thymus preparation, contains a peptide(s) that can enhance the production of GH and/or PRL from rat anterior pituitary cells in vitro. Using reverse phase HPLC, we have now isolated and chemically characterized from TF5 a peptide possessing this activity. This peptide, termed MB-35, is a highly charged basic molecule of 35 amino acid residues and a mol wt of 3756. A computer-assisted search of published protein sequences has revealed that this peptide has a 100% homology with a region of the histone H2A. Biological studies using rat pituitary cells have revealed that MB-35 is active alone or in combination with GH-releasing factor (GRF) or TRH and can increase the production of GH and/or PRL beyond that achievable with GH-releasing factor and TRH alone. The observation that histone H2A, the parent molecule, is without activity is of keen interest, since it suggests that nucleoproteins may have heretofore unknown physiological activities, perhaps related to cell cycle and/or other events associated with DNA activation events.