Physico-chemical properties of Alkaline Phosphatases released by a Planktonic Crustacean, Daphnia Magna (Cladocera)

Abstract

Our studies investigated the physico-chemical properties of alkaline phosphatase excreted by D. magna. This cladoceran mainly released alkaline phosphatase, though it also released a small amount of acid phosphatase. The alkaline phosphatase showed a broad pH optimum (8.05-10.0), and had a broad optimum temperature (30-35 • C) with a temperature coefficient (Q 10 ) of 2.45. The K m of the enzyme is 0.15 ± 0.02 mM when p-nitrophenyl phosphate is used as a substrate, and the V max is 0.43 ± 0.01 µM pNP mg −1 DW h −1 . Even though alkaline phosphatase had been incubated in chloroform saturated with WC medium for 13 days, its activity was 54% that of the original. The enzyme was strongly inactivated by EDTA, and appeared to be zinc dependent. The alkaline phosphatase activity remained constant when D. magna was fed different quantities of Chlorella sp. The sensitivity of D. magna phosphatase activity to phosphate was time-dependent. During the first 16 hrs, the enzyme was insensitive to phosphate addition, after 24 hrs incubation the enzyme became sensitive to phosphate addition. RÉSUMÉNos études ont porté sur les propriétés physico-chimiques de la phosphatase alcaline excrétée par Daphnia magna. Ce cladocère libère principalement une phosphatase alcaline, bien qu'elle excrète aussi une petite quantité de phosphatase acide. La phosphatase alcaline présentait un large optimum de pH (8,05-10,0) et une température optimum étendue (30-35 • C) avec un coefficient de température (Q 10 ) de 2,45. Le K m de l'enzyme est de 0,15 ± 0,02 mM quand le phosphate de p-nitrophényle est utilisé comme substrat, et le V max est de 0,43 ± 0,01 µM pNP mg −1 DW h −1 . Même lorsque la phosphatase alcaline était incubée dans du chloroforme saturé avec un milieu WC pendant 13 jours,3 )