Sulfakinin neuropeptides in a crustacean: Isolation, identification and tissue localization in the tiger prawn Penaeus monodon

The sulfakinin (SK) family of neuropeptides are characterized by a C‐terminal octapeptide sequence that begins with two acidic residues (most commonly DD), and ends with YGHMRF‐NH2, usually with the tyrosyl residue sulfated. So far, sulfakinins have only been identified in insects and the present study was initiated to investigate if the family is more widely distributed within the arthropods. Purification of an extract of the central nervous system of the giant tiger prawn Penaeus monodon has revealed three novel members of the sulfakinin peptide family. One of the peptides, Pem SKI, has the sequence <QFDEY(SO3H)GHMRF‐NH2, where <Q denotes a pyroglutamic acid residue, and is for all criteria typical of insect sulfakinins, several of which also have an N‐terminal pyroglutamic acid. Tyrosyl O‐sulfation was verified by mass spectrometry. The two other peptides have a hitherto unknown L/M variation at position three from the C‐terminus. One of these, Pem SKII, has a particularly glycine‐rich N‐terminus, AGGSGGVGGEYDDYGHLRF‐NH2. The other, Pem SKIII, is a truncated form of Pem SKII, with the sequence VGGEYDDYGHLRF‐NH2. Mass spectrometry of the latter two peptides indicated that only one of the two tyrosyl residues is sulfated. By analogy, it is suggested that the sulfation is located at the residue in position six from the C‐terminus. A small amount of a nonsulfated variant of Pem SKII was also present in the extract. Immunocytochemical studies with sulfakinin antisera show a sparse neuronal distribution pattern, similar to that of insects. A prominent pair of large (≈ 25 μm) cells and 6–8 pairs of smaller (≈ 10 μm) cells are present in the protocerebrum. The larger cells have prominent neurites that give rise to varicosities in the centre of the brain. Their axons exit the brain via the circumoesophageal connectives and continue along the intersegmental connectives. Each of the thoracic and abdominal ganglia has sulfakinin‐immunoreactive arborizations as a result of branching from the intersegmental nerves. This distribution pattern strongly suggests a role in neurotransmission or neuromodulation, although it remains to be elucidated what the exact role(s) is. However, on account of the conservation of peptide structure during the evolutionary period spanning the insect/crustacean lineage, especially between Pem SKI and insect sulfakinins, it may be assumed that the sulfakinins have a significant physiological role.