The Use of Radioimmunoassays for Cryptic Regions of Peptide Precursors, in the Study of Biosynthetic Mechanisms

Elucidation of gene sequences encoding peptide hormones and neurotransmitters has allowed prediction of the primary amino acid sequences of all the peptides that are likely to be produced by biosynthetic processing of the precursors for these substances. Thus, in addition to the known biologically active sequences, the precursors demonstrably contain sequences of peptides (‘cryptic peptides’) whose identity and function are so far unknown. We have raised Ab’s≠ to all the peptides expected to arise during biosynthesis of progastrin and proVIP, and to the C-terminal flanking peptide of CCK. All the Ab’s were produced by immunizing rabbits with small fragments or analogues of the precursor sequences, coupled to carrier protein in such a way as to leave the appropriate sequence free to promote Ab formation. We have used these Ab’s. in RIA to identify and characterize some novel peptides produced during gastrin and VIP biosynthesis, and to identify the cellular origins of the peptides by immunohistochemistry. The results have allowed elucidation of biosynthetic pathways and identification of cell-specific processing patterns. For progastrin at least, pathological changes in biosynthetic mechanisms have been identified.