Differential effects of N-terminal modifications on the biological potencies of growth hormone releasing factor analogues with varying chain lengths.
Coy. D H DH; Murphy. W A WA; Lance. V A VA; Heiman. M L ML
Key Findings
- GRF peptides shorter than 29 amino acids (e.g., 1‑27, 1‑24) lose most of their ability to stimulate growth hormone.
- Adding D‑amino acids or an acetyl group at the N‑terminus can boost the potency of the 1‑27 version, but the increase is modest compared to the full‑length peptide.
- Even the most potent 1‑27 analogue (with multiple D‑amino acids) is only about 20% as active as the unmodified 1‑29 peptide, and shorter versions (1‑22) remain essentially inactive.
Practical Outcomes
- For biohackers aiming to boost GH, the full‑length GRF‑1‑29 remains the most reliable choice; trying to save on peptide length with modifications offers limited benefit. If you do use a shortened version, expect much lower effects even with N‑terminal tweaks, so dosage adjustments may not compensate for the loss of activity.
Summary
The study shows that cutting off the tail end of the growth‑hormone‑releasing factor (GRF) peptide makes it much weaker, and only a few tweaks to the front end can partly rescue activity—but even the best‑modified short versions are still far less effective than the full‑length 1‑29 peptide.
Abstract
The excellent retention of biological potencies observed with human growth hormone releasing factor analogues with chains 29-44 amino acid residues long is suddenly lost when further amino acid residues are removed from the C-terminus. For instance, 1-27 and 1-24 exhibited little biological activity (greater than 1%) in vivo and in vitro in the rat. Studies were made to determine whether this was due to conformational changes rather than simply the loss of amino acids needed for direct receptor interactions. These involved the introduction of mild conformational restraint in the N-terminal region by the introduction of D-amino acid residues previously shown to increase the potency of the 1-29 peptide. D-Ala in position 2, responsible for a 40- to 50-fold increase in activity in the 1-29 species, resulted in little increase in the potency of 1-27 or 1-24 sequences. However, N-terminal acetylation, responsible for a 12-fold increase in 1-29 in vivo potency, caused greater than 50-fold increase in 1-27 potency but had little effect on 1-24 potency. Likewise, D-Asn in position 8 was far more effective in increasing the potency of the 1-27 sequence compared to the 1-29 ([D-Asn8]-GRF(1-29)NH2, 220% vs. [D-Asn8, Leu27]-GRF(1-27)NH2, 53%; in vivo]. This differential effect was even more clear in vitro. The highest in vivo potency in the 1-27 series was achieved with [D-Asp3,D-Asn8,Leu27]-GRF(1-27)NH2 (200%); however, this analogue was still far less potent than its 1-29 counterpart (3800%). None of the D-amino acid substitution strategies were effective in increasing 1-22 peptide potencies to detectable levels. The results indicate that the effect of N-terminal substitutions and resulting potencies of the GRFs is very much dependent on chain length, perhaps suggesting that C-terminal amino acids promote conformational effects at the N-terminus and/or vice versa.
Study Information
pubmed
1987
10.1021/jm00384a039