A bombesin immunoreactive peptide in milk.

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Abstract
Immunoreactivity to the amphibian peptide bombesin was found in instant nonfat dry milk (.apprx. 0.7 ng/ml) and in the whey of whole or skim bovine milk (.apprx. 1.2 ng/ml) even after ultracentrifugation. The soluble immunoreactivity was associated with a peptide exhibiting the following characteristics: parallel displacement in an immunoassay using an antiserum recognizing bombesin amino acid residues 5-8; separation from both gastrin-releasing peptide and amphibian bombesin by gel filtration-the approximate MW was 3200; denaturation in urea, reduction by dithiothreitol, and acetylation by iodoacetamide had no effect on its elution profile by gel-filtration chromatography and the aggregation of added bombesin to milk proteins or peptides was not observed; reversed-phase high performance liquid chromatography separated milk immunoreactivity from gastrin-releasing peptide and bombesin; digestion by trypsin yielded a smaller immunoreactive peptide fragment, whereas nearly all immunoreactivity was lost by treatment with .alpha.-chymotrypsin; and the level of immunoreactivity was unaffected by boiling. Milk evidently is an exogenous source of bombesin-like immunoreactivity, which may account for the increase of gastric acid and gastrointestinal hormone levels after the consumption of milk.