RESUMEN
Arginine kinase (AK) was isolated from the body wall muscle of the primitive sea anemone Anthopleura japonicus by Ultrogel AcA34 gel filtration, DEAE-32 chromatography and elution on a Cosmogel-SP column. The denatured molecular mass as determined with SDS/PAGE was 80 kDa, twice that of the usual AK subunit, indicating that this AK has an unusual two-domain structure. The native form was eluted on a Superose 12 column with the same retention time as that of rabbit homodimeric creatine kinase, indicating that Anthopleura AK is a monomer of 80 kDa. The isolated enzyme gave a specific activity of 100-120 micromol of Pi/min per mg of protein in the pH range 7.9-9.1 for the forward reaction. The enzyme is fully activated by Ca2+, as it is with Mg2+. The cDNA-derived amino acid sequence of 715 residues of Anthopleura AK was determined. The validity of the sequence was supported by chemical sequencing of internal tryptic peptides. A bridge intron of 686 bp, which separates the two domains of Anthopleura AK, is present between the second and third nucleotide in the codon of Ala-364. This is the first two-domain AK to be sequenced. Anthopleura AK shows 48-54% amino acid sequence identity with known invertebrate AKs, and also shows a lower, but significant, similarity (39-46%) to marine worm glycocyamine kinase and rabbit creatine kinase.