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Using a RIA, we have examined the character of the neurotensin-like (NT-like) peptides present in acid/acetone extracts of animal and human plasma. Plasma immunoreactivity was measured using four antisera with different specificities. Antisera which were directed against the COOH-terminus of NT measured higher levels of plasma immunoreactivity than did antisera with NH2-terminal or mixed specificity. Chromatographic fractionation of bovine plasma extracts revealed the presence of multiple substances, one of which was chromatographically and immunochemically indistinguishable from NT. We estimate the plasma level of this component to be about 15-25 fmol/ml, which is 30-50% of the measurement obtained on unfractionated extracts using antiserum HC-8. Several other components were also identified which behaved as though they were smaller than NT and seemed to share with NT four to eight of its COOH-terminal amono acids. One eluted from Sephadex G-25 in the region of the NT-like peptide previously identified in extracts of gastric mucosa. Infusion of synthetic NT into rats for 30 min did not result in the formation of these COOH-terminal relatives of NT. Our results argue strongly for the presence of NT in plasma and also indicate that other peptides, sharing COOH-terminal homologies with NT, appear in plasma, possibly from the stomach, liver, and other as yet unidentified source(s).

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