Characterization of mevalonate metabolism in the sea bass (Dicentrarchus labrax L) liver

The activities of mevalonate kinase, mevalonate 5-phosphate kinase and mevalonate 5-pyrophosphate decarboxylase, were examined in sea bass (Dicentrarchus labrax L) liver. The activities of the three enzymes were studiedin vitro in relation to the influence of protein content, time of incubation, pH, temperature, mevalonate, ATP and Mg⁺⁺ concentration. Protein content in the assay medium affected the three enzymes differently. Mevalonate kinase, mevalonate 5-phosphate kinase, and mevalonate 5-pyrophosphate decarboxylase activities were linear up to 0.2, 0.4 and 0.8 mg protein, respectively. With respect to the time course studies, the enzymes also behaved differently. Mevalonate kinase activity increased over forty minutes, reaching a plateau thereafter, while mevalonate 5-phosphate kinase and decarboxylase increased over the entire assay period. All the three enzymes showed a maximum in activity at pH 7.5. The effect of reaction temperature showed that phosphorylation increased to maximum around 35°C for mevalonate kinase and 30°C for mevalonate 5-phosphate kinase while decarboxylation rates remained constant well until 30°C temperature decreasing afterwards. The enzymes behaved differently as a function of mevalonate concentration. Mevalonate 5-phosphate formed was maximal when the initial mevalonate concentration was 272 M, whereas mevalonate 5-pyrophosphate and CO₂ were formed maximally at mevalonate concentrations of 136 M and 68M, respectively. Optimal ATP concentration in the medium was 3 mM for decarboxylase and 6 mM for kinases, and Mg⁺⁺ requirements varied from 4 mM for decarboxylase to 6 mM for kinases.