Short Communication Arginine Kinase and Creatine Kinase Appear to Be Present in the Same Cells of an Echinoderm Muscle

Phosphagens are phosphorylated guanidine compounds which, via the catalytic action of corresponding phosphagen kinases, function as temporal and possibly spatial ATP buffers (Meyer et al. 1985) in cells capable of dramatic metabolic transitions such as muscle, neurons and spermatozoa. At least eight different phosphagen kinases have been identified and these enzymes are widely distributed throughout the animal kingdom (see excellent reviews by Van Thoai, 1968; Watts, 1968). One puzzling feature about the distribution of phosphagen kinases is the occasional presence of more than one phosphagen type in a given tissue, the socalled 'pluriphosphagen' phenomenon (Robin, 1964). For instance, many species of polychaetes, echinoids and tunicates contain two or more phosphagen systems in their muscles (Needham et al. 1932; Yudkin, 1954; Robin, 1964, 1980; Van Thoai et al. 1964; Watts, 1975). The situation in sea urchin lantern muscles is quite striking in that certain species contain both arginine kinase (AK) and creatine kinase (CK), while others, all relatively primitive echinoids, contain AK only (Yudkin, 1954; Morrison et al. 1967; Ratto et al. 1989). All asteroids and ophiuroids contain CK in muscle, while the more primitive echinoderm classes, holothuroids and crinoids, contain AK only (Ratto et al. 1989). The distribution of phosphagen kinases in the echinoderms shows that CK is present in the more advanced forms, implying some kind of evolutionary trajectory. As Ratto et al. (1989) put it '... echinoderms are displaying a transition from one enzymatic system to another, and that simultaneous expression of both enzymes is in fact a still picture of a long-term evolutionary process'. Recently, we showed that the phosphagen kinase reactions are not thermodynamically equivalent (Ellington, 1989). That is, the apparent equilibrium constant for the AK reaction (K'apk= [arginine][ATP]/[arginine phosphate][ADP]) is only 13 % of K'cpk. The K' values for the lombricine, glycocyamine and taurocyamine kinase reactions are intermediate (Ellington, 1989). These differences have