A large somatostatin-like polypeptide of apparent molecular weight 3000-4500 [4K somatostatin (SS)] was isolated from ovine hypothalamus. The polypeptide was obtained in the methionine sulfoxide form. Two microsequence analyses of 0.6 and 1.8 nmol of 4K SS were performed with a modified 890 C spinning cup sequencer. The sequencing data together with results of amino acid analysis and C-terminal end-group determination indicated that 4K SS was identical with somatostatin-28 (SS-28) isolated from procine upper small intestine and sequenced by Pradayrol et al. [Pradayrol, L., Jörnvall, H., Mutt, V., & Ribet, A. (1980) FEBS Lett. 109, 55-58]. No free cysteine sulfhydryl group could be detected, so that it was assumed that the two cysteine residues of ovine SS-28 formed an intramolecular disulfide bond. Besides the structure of SS-28, the N-terminal first 30 residues of an unknown polypeptide from ovine hypothalamus were sequenced as follows: H-Ile-Pro-Ile-Tyr-Glu-Lys-Lys-Tyr-Gly-Gln-Val-Pro-Met-Cys-Asp-Ala-Gly-Glu-Gln- Cys-Ala-Val-Arg-Lys-Gly-Ala-Arg-Ile-Gly-Lys. Trypsin cleaved the somatostatin (SS) entity less selectively from ovine hypothalamic SS-28 than from rat hypothalamic 12 000-dalton SS-like polypeptide (12K SS). Native ovine hypothalamic SS-28 was found to be highly potent in inhibit growth hormone release from cultured rat anterior pituitary cells. The results raised doubts that ovine SS-28 would be an SS precursor and indicated that SS-28 itself may possess regulatory functions.

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