Immunochemical studies on big gastrin using NH2-terminal specific antisera

Methods are described for obtaining antisera specific for the NH2-terminal regions of human and porcine big gastrin (G34) that can be used in radioimmunoassays. Three antisera have been characterized in detail: one (L66) raised to human 1--15 (Tyr7-Pro8-Ser9) G34 has an antigenic determinant in the 1--6 region of human G34; a second (L107) raised to 1--19 hG34 has an antigenic determinant in the 1--12 region. Both these antisera react weakly with porcine G34. A third antiserum (L33) raised to porcine G34 has an antigenic determinant in the 1--12 region of this peptide, and reacts weakly with human G34. In human antral extracts fractionated on Sephadex G50, L66 and L107 revealed a minor peak of immunoreactivity corresponding to G34, and a major peak corresponding to the NH2-terminal tryptic peptide of G34. Concentrations of the latter peptide were closely similar to those of G17 (i. e.) the COOH-terminal tryptic peptide of G34), consistent with the idea that G34 is cleaved within G-cells by a trypsin-like enzyme to yield G17. Antiserum L33 revealed small amounts of immunoreactivity in antral extracts of dog and cat, but did not reveal significant immunoreactivity in rat antral extracts. In contrast, L66 reacted with rat antral extracts, but not dog or cat. The sequences of G34 in these species are not known, but the results suggest significant differences compared with human and porcine G34, and indicate a high degree of species-specificity with NH2-terminal G34 antisera.