Abstract
Summary
A cyclic pentapeptide amide, the disulfide of L-cysteinyl - L-tyrosyl - l-isoleucyl - L-glutaminyl - L-asparaginyl - l-cysteinamide, which represents the cyclic moiety of oxytocin has been prepared synthetically and some of its physical and chemical properties are given. This compound has been found to have low but significant oxy-tocic (3.3 U/mg) and milk-ejecting (1.1 U/ mg) activity but no detectable avian depressor activity. Moreover, this simplification of the oxytocin molecule appears to have resulted in a change in the ratio of the potencies of oxytocic, milk-ejecting, and avian depressor effects, the ratio being 3:1:< 0.03 for the cyclic pentapeptide amide compared to 1:1:1 for oxytocin. Some of the possible structural requirements for biological activity in oxytocin have been discussed in the light of these results.
The author wishes to thank Mr. Joseph Albert for carrying out the microanalyses, Dr. Paula Zimmering for the starch ohromatography, Miss Lorraine Silverman for her assistance, and Mrs. Sylvia Kirsi-magi White for the oxytocic and blood pressure assays reported herein. The stimulating discussions of the problem with Dr. Vincent du Vigneaud are acknowledged with pleasure.
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