Alternate reactions during neuropeptide biotinylation

The use of biotinylated neuropeptides has increased significantly over the past twenty years. Although peptides can be synthesized with biotin moieties in place, it is often more practical to chemically link biotin to a purified peptide with one of the many reagents that are commercially available. Of these reagents, N-hydroxysuccinimide esters of biotin (NHS- biotin) and paranitrophenyl esters of biotin are the most widely used. These esters are reported to react specifically with amino groups on peptides and proteins. However, we have found that these and other commonly employed biotinylating reagents can readily acylate non-amine functional groups in specific linear peptide sequences under relatively mild conditions. In particular, the hydroxyl groups on serine, tyrosine, or threonine will quickly react with NHS-biotin if these residues are located in a His +/- 2 position (e.g., His-Xaa-Ser). Such linear sequences are found in a wide variety of neuropeptides including amylin, galanin, gonadotropin-releasing hormone, angiotensin, kinetensin, and neurokinin A. Bath the reaction pH and the amount of organic solvent in the biotinylation reaction mixture can alter the relative amounts of N-biotinylation and O-biotinylation. Many, if not most, of the resultant O-acylated linkages are stable even under standard tissue culture conditions. Recognition of such alternate reaction products is critical when chemically modifying peptides with reactive esters and when interpreting experiments in which such peptide derivatives are used.