beta-endorphin1-31 in the rat pituitary

Mass spectrometry was used to determine the molecular mass of rat pituitary beta-endorphin1-31 (BErat, 1-31). The measured molecular mass (3435 +/- 1 Da, n = 5) of endogenous BErat, 1-31 differed from the molecular mass of commercially available synthetic BErat, 1-31 (3465 +/- 1 Da, n = 9), but corresponded to the molecular mass of synthetic BEbovine, 1-31 (3436 +/- 3 Da, n = 3). Based on the combination of these ESIMS molecular mass measurements, HPLC retention time data, LSIMS measurement of the molecular mass of selected tryptic fragments, and consideration of codon sequences, we suggest that the amino-acid sequence of endogenous BErat, 1-31 differs from the DNA-deduced sequence of BErat, 1-31, and that endogenous BErat, 1-31 contains Ala instead of Val in position 26.