Properties and possible significance of substance P and insulin fibrils

The neuropeptide substance P forms polymeric fibrils similar to those previously reported for the hormone, insulin. Structural and chemical aspects of these two fibrillary forms have been compared and their possible existence in vivo considered. Numerous substance P fibrils are readily formed in vitro in mM solutions under conditions which are physiological with respect to salt concentration, pH and temperature, whereas more severe conditions (heat and acid) are apparently required for the rapid formation of numerous insulin fibrils. Morphologically, both fibrils appear to be relatively long and unbranched and the neuropeptide fibrils are similar in size to such naturally occurring structures as neurofilaments. Disaggregation of the neuropeptide fibril follows dilution (1000-fold) whilst more stringent (alkaline) treatment is apparently necessary for insulin fibril dissociation. These observations are discussed in relation to the role of an insulin-like peptide in the formation of certain types of amyloid and the possibility that fibrillary of similar polymeric forms of substance P may exist in normal tissue.