Dorota Napierska¹ and Edward F. Skorkowski^2
1Sea Fisheries Institute, ul. Kołłątaja 1, 81-332 Gdynia, Poland
²Gdańsk University Biological Station, 80-680 Gdańsk-Sobieszewo, Poland

Key words: malic enzyme, kinetic properties, thermostability, abdomen muscles, Euphausia superba

Abstract.
Malic enzyme (EC 1.1.1.40) was purified about 1800-fold from the abdomen muscle of Euphausia superba to a specific activity of 20.3 �mol � min^-1 per mg protein. The molecular weight of the native malic enzyme was determined to be 270,000. The K_m values determined at  pH 7.2 for decarboxylation reaction for malate and NADP⁺ were 0.229 mM and 10.6 �M, respectively. The K_m values for carboxylation reaction for pyruvate, NADPH and bicarbonate were 5 mM, 25.8 �M and 12 mM, respectively. The effect of temperature on apparent K_m for malate was studied. The minimum K_m appeared at 0�C, the ambient temperature of the species. The optimum pH for the decarboxylation reaction was between 7.0 and 8.0 and varied with the malate concentration. The enzyme showed substrate inhibition at a high malate concentration for the oxidative decarboxylation reaction at pH 7.0.  The  optimum  pH for the carboxylation  reaction  was between 6.5 and 7.0 and varied with the pyruvate concentration. The heat stability of the purified malic enzyme of Antarctic krill E. superba was determined and compared with heat stability of the purified malic enzyme from the Baltic shrimp Crangon crangon abdomen muscle. The enzymes lost their activities at about 34�C and 65�C, respectively.