Production of radiolabeled neuropeptide precursors by in vitro transcription and translation.

Bioactive peptide hormones and neurotransmitters are required for neuroendocrine regulation of cellular functions. Importantly, proteolytic processing of inactive neuropeptide precursors is required to generate physiologically active peptide hormones and neurotransmitters. Studies of the processing enzymes require authentic neuropeptide precursors as substrates, rather than peptide substrates. This study demonstrates an efficient method to general 35S-precursors from cloned cDNAs by in vitro transcription and translation. In vitro transcription of neuropeptide cDNAs with SP6 RNA polymerase generates large amounts (micrograms) of corresponding RNAs. Subsequent in vitro translation of RNAs with wheat germ extract and 35S-methionine generates large quantities of 35S-precursors (10-25 million cpm 35S-precursor protein per reaction) with high specific radioactivity. The radiolabeled precursor substrates offer a reliable, sensitive and accurate method for detecting the proteolytic activity. Importantly, specific detection of the primary proenkephalin processing activity in chromaffin granules by 35S-enkephalin precursor as substrate, but not by peptide methylcoumarinamide (MCA) substrates, illustrates the significance of using full-length precursor to detect appropriate processing enzymes. This study demonstrates that efficient production of radiolabeled neuropeptide precursors by in vitro transcription and translation will be useful for in vitro assays of relevant processing proteases.