Analysis by mass spectrometry of POMC-derived peptides in amphibian melanotrope subpopulations

We have previously shown that the melanotrope population of the pituitary i ntermediate lobe of Rana ridibunda is composed of two subpopulations, of lo w (LD) and high density (HD), that show distinct ultrastructural features a nd display different synthetic and secretory rates. To investigate whether LD and HD melanotrope cells also differ in proopiomelanocortin (POMC) proce ssing, we have analyzed the POMC-end products in single cells from both sub populations by means of matrix-assisted laser desorption/ionization mass sp ectrometry (MALDI-MS). The mass spectra-revealed the presence of 8 POMC-der ived peptides in HD and LD melanotrope cells, indicating a similar processi ng of the precursor in both subpopulations. However, the relative abundance of three POMC-end products (i.e. lys-gamma(1)-MSH, acetyl-alpha-MSH and CL IP fragment) was higher in the HD subset. Moreover, two peptides with molec ular weights of 1030 and 1818 Da, respectively, were detected that could no t be assigned to any product deduced from the frog POMC sequence. The relat ive amount of the 1030 Da peptide was higher in LD melanotrope cells. Taken together, our results suggest that POMC processing is differentially regul ated in the two melanotrope cell subsets.