tynorphin and spinorphin

Spinorphin has been isolated from the bovine spinal cord as an endogenous inhibitor of enkephalin-degrading enzymes (aminopeptidase, dipeptidyl aminopeptidase III, angiotensin-converting enzyme and enkephalinase), and tynorphin has been synthesized as a more potent inhibitor of dipeptidyl aminopeptidase III. In this study, the effects of spinorphin and tynorphin on synaptic transmission were studied in rat isolated hippocampal slices. Field potentials were recorded from the CA1 region after stimulation of Schaffer collaterals. Spinorphin (1 microM), which alone had no effect, potentiated the facilitatory effects of enkephalin on the filed potentials at a stimulation interval of 15 s. At a stimulation interval of 10--4 s, spinorphin alone frequency dependently inhibited the field potential. On the other hand, tynorphin (1 microM), which alone had no effect at any stimulus interval, tended to potentiate the facilitatory effects of enkephalin. Spinorphin blocked long-term potentiation induced by tetanic stimulation (100 Hz, 1 s), whereas tynorphin had no effect on long-term potentiation. These results suggest that, at a low stimulation frequency, spinorphin potentiates the facilitatory effects of enkephalin by preventing degradation of enkephalin, whereas at a high stimulation frequency spinorphin use dependently inhibits synaptic transmission independently of enkephalin. On the other hand, tynorphin tends to potentiate the facilitatory effects of enkephalin without use-dependent inhibition.

Topics: Animals; Enkephalins; Hippocampus; Long-Term Potentiation; Male; Naloxone; Narcotic Antagonists; Oligopeptides; Protease Inhibitors; Rats; Rats, Wistar; Synaptic Transmission

To find a more effective inhibitor than spinorphin (LVVYPWT), an endogenous factor derived from bovine spinal cord, we synthesized spinorphin analogues and assayed their inhibitory activity toward DPPIII among enkephalin-degrading enzymes. Tynorphin (VVYPW), an N-terminal and C-terminal truncated form of spinorphin, exhibited more potent inhibitory activity and an IC50 value of 0.086 +/- 0.05 microg/ml (n = 4), whereas structures smaller than four amino acid residues exhibited almost no or less activity, suggesting that a five amino acid structure containing a Tyr-Pro residue is essential for the inhibition. The inhibition of DPPIII by tynorphin was predominantly competitive and the Ki value was found to be 7. 50 +/- 1.19 x 10(-8) M on Lineweaver-Burk plotting. The inhibitory activity of tynorphin toward other enkephalin-degrading enzymes such as neutral endopeptidase, aminopeptidase, and angiotensin-converting enzyme was not as high as that toward DPPIII, suggesting that tynorphin is a specific inhibitor of DPPIII. In HPLC analysis, human serum cleaved tynorphin rapidly (38% of control at 2 h and background level at 4 h), but in the presence of leuhisitin, an aminopeptidase inhibitor, tynorphin was maintained at the original level for 24 h. These results indicated that tynorphin had a more effective structure for expression of inhibitory activity toward DPPIII.

Topics: Animals; Brain; Cattle; Dipeptidyl-Peptidases and Tripeptidyl-Peptidases; Drug Stability; Haplorhini; Humans; Oligopeptides; Protease Inhibitors; Structure-Activity Relationship