Properties of Inosine-Decomposing Enzymes from Aquatic Animal Muscle

Activities of inosine (HxR)-decomposing enzymes (purine nucleoside phospholylase and HxR nucleosidase) in the muscle of twenty four species of aquatic animals were compared. The enzyme activities in HxR accumulating species were very low. On the other hand, the activities in hypoxanthine (Hx) accumulating species were generally higher in the absence than in the presence of phosphate. Some species of plaice, squid and octopus showed relative-ly higher activity. The activity of common squid enzyme was the highest of the species examined. In the absence of phosphate, the pH optima of the activity of sand dab and common squid enzymes were found to be around 8.5 and 6.5, respectively. The K_mvalues for HxR of sand dab and common squid enzymes were 5.72 xl0^-4and 1.59X10~4M, respectively. Although the sand dab enzyme was remarkably depressed by phosphate, the common squid enzyme was not affected. HxR nucleosidase of fishes was activated three times as high as the ordinary level by 0.5 mM Ca²⁺. while the molluscan enzyme was not affected by Ca²⁺.