Gastropod arginine kinases from Cellana grata and Aplysia kurodai. Isolation and cDNA-derived amino acid sequences

Arginine kinase (AK) was isolated from the radular muscle of the gastropod molluscs Cellana grata (subclass Prosobranchia) and Aplysia kurodai (subclass Opisthobranchia), respectively, by ammonium sulfate fractionation, Sephadex G-75 gel filtration and DEAE-ion exchange chromatography. The denatured relative molecular mass values were estimated to be 40 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The isolated enzyme from Aplysia gave a K_m value of 0.6 mM for arginine and a V_max value of 13 μmole Pi min⁻¹ mg protein⁻¹ for the forward reaction. These values are comparable to other molluscan AKs. The cDNAs encoding Cellana and Aplysia AKs were amplified by polymerase chain reaction, and the nucleotide sequences of 1608 and 1239 bp, respectively, were determined. The open reading frame for Cellana AK is 1044 nucleotides in length and encodes a protein with 347 amino acid residues, and that for A. kurodai is 1077 nucleotides and 354 residues. The cDNA-derived amino acid sequences were validated by chemical sequencing of internal lysyl endopeptidase peptides. The amino acid sequences of Cellana and Aplysia AKs showed the highest percent identity (66–73%) with those of the abalone Nordotis and turbanshell Battilus belonging to the same class Gastropoda. These AK sequences still have a strong homology (63–71%) with that of the chiton Liolophura (class Polyplacophora), which is believed to be one of the most primitive molluscs. On the other hand, these AK sequences are less homologous (55–57%) with that of the clam Pseudocardium (class Bivalvia), suggesting that the biological position of the class Polyplacophora should be reconsidered.