A receptor binding
class of d-amino acid-containing peptides
(DAACPs) is formed in animals from an enzymatically mediated post-translational
modification of ribosomally translated all-l-amino acid peptides.
Although this modification can be required for biological actions,
detecting it is challenging because DAACPs have the same mass as their
all-l-amino acid counterparts. We developed a suite of mass
spectrometry (MS) protocols for the nontargeted discovery of DAACPs
and validated their effectiveness using neurons from Aplysia
californica. The approach involves the following three steps,
with each confirming and refining the hits found in the prior step.
The first step is screening for peptides resistant to digestion by
aminopeptidase M. The second verifies the presence of a chiral amino
acid via acid hydrolysis in deuterium chloride, labeling with Marfey’s
reagent, and liquid chromatography–mass spectrometry to determine
the chirality of each amino acid. The third involves synthesizing
the putative DAACPs and comparing them to the endogenous standards.
Advantages of the method, the d-amino acid-containing neuropeptide
discovery funnel, are that it is capable of detecting the d-form of any common chiral amino acid, and the first two steps do
not require peptide standards. Using these protocols, we report that
two peptides from the Aplysia achatin-like neuropeptide
precursor exist as GdYFD and SdYADSKDEESNAALSDFA.
Interestingly, GdYFD was bioactive in the Aplysia feeding and locomotor circuits but SdYADSKDEESNAALSDFA
was not. The discovery funnel provides an effective means to characterize
DAACPs in the nervous systems of animals in a nontargeted manner.