Quantitative neuropeptidomics of microwave-irradiated mouse brain and pituitary

In neuropeptidomics, the degradation of a small fraction of abundant proteins overwhelms the low signals from neuropeptides, and many neuropeptides cannot be detected by mass spectrometry without extensive purification. Protein degradation was prevented when mice were sacrificed with focused microwave irradiation, permitting the detection of hypothalamic neuropeptides by mass spectrometry. Here we report an alternative and very simple method utilizing an ordinary microwave oven to inhibit enzymatic degradation. We used this technique to identify brain and pituitary neuropeptides. Quantitative analysis using mass spectrometry in combination with stable isotopic labeling was performed to determine the effect of microwave irradiation on relative levels of neuropeptides and protein degradation fragments. Microwave irradiation greatly reduced the levels of degradation fragments of proteins. In contrast, neuropeptide levels were increased about 2-3 times in hypothalamus by the microwave irradiation but not increased in pituitary. In a second experiment, three brain regions (hypothalamus, hippocampus, and striatum) from microwave-irradiated mice were analyzed. Altogether 41 neuropeptides or fragments of secretory pathway proteins were identified after microwave treatment; some of these are novel. These peptides were derived from 15 proteins: proopiomelanocortin, proSAAS, proenkephalin, preprotachykinins A and B, provasopressin, prooxytocin, melanin-concentrating hormone, proneurotensin, chromogranins A and B, secretogranin II, prohormone convertases 1 and 2, and peptidyl amidating monooxygenase. Although some protein degradation fragments were still found after microwave irradiation, these appear to result from protein breakdown during the extraction and not to an enzymatic reaction during the postmortem period. Two of the protein fragments corresponded to novel protein forms: VAP-33 with a 7-residue N-terminal extension and β tubulin with a glutathione on the Cys near the N terminus. In conclusion, microwave irradiation with an ordinary microwave oven effectively inhibits enzymatic postmortem protein degradation, increases the recovery of neuropeptides, and makes it possible to conduct neuropeptidomic studies with mouse brain tissues.