Fig. 6. Direct MALDI-FTMS analysis of midgut tissues. Data presented in this figure are from Cancer borealis, although identical peptide identifications were achieved from both Cancer magister and Cancer productus. Regardless of species or tissue, spectra were measured using DHB as the matrix, with conditions optimized for accumulation of m/z 1500. (A) A representative spectrum from a small piece of posterior midgut caecum (PMC). In PMC samples, an intense peak appearing at m/z 934.49 was consistently detected at a high relative abundance. This peak was identified as APSGFLGMRamide (CabTRP Ia), based upon the m/z value measured using internal calibration with poly(propylene glycol). Spectra of the PMC samples showed no indication of a peak corresponding to GYRKPPFNGSIFamide (Gly¹-SIFamide), i.e. m/z 1381.74, or any other known SIFamide isoform. (B) A representative spectrum from a small piece of anterior midgut caecum (AMC). In AMC samples, a peak at m/z 1381.74 (corresponding to the [M+H]⁺ ion for Gly¹-SIFamide) was detected in approximately 95% of the spectra measured; a peak at m/z 934.49 (corresponding to the [M+H]⁺ ion for CabTRP Ia) was detected in approximately 40% of the spectra. Because of the low intensities of these peptide peaks, only accurate mass measurements were used for peptide identification in this tissue.