Occurrence of myo-inositol-1-phosphate phosphatase in pteridophytes: characteristics of the enzyme from the reproductive pinnules of Dryopteris filix-mas (L.) Schott

Abstract

Evident myo-inositol-1-phosphate phosphatase (MIPP) activity has been detected both in the vegetative as well as in the spore-bearing organs of some selected pteridophytes having wide phylogenetic diversity. The basic characterization of this enzyme was carried out using the cosmopolitan fern Dryopteris filix-mas. The enzyme was partially purified from the cytosol fraction obtained from the reproductive pinnules of the plant to about 41-fold over the initial homogenate following low-speed centrifugation, streptomycin sulfate precipitation, 25-70% ammonium sulfate fractionation, CM Sephadex C-50 chromatography and finally gel-filtration on Ultrogel AcA 34. The apparent molecular weight of the native MIPP was estimated to be 94 kDa. The enzyme activity increased linearly with respect to protein concentration to about 150 µg and with respect to time up to 75 min. The temperature optimum was found at 40ºC. However, the enzyme showed good activity over the temperature range of 30-50ºC. This enzyme used D/L-myo-inositol-1-phosphate as its principal substrate (95-100%), however, about 16% activity was recorded when D-myo-inositol-3-phosphate substituted as substrate. Furthermore, weak (3%) activity of this MIPP was observed with 2-glycerophosphate as substrate. The apparent Km for pteridophytic MIPP was 0.083 mM. The enzyme was functional in a narrow pH range of 7.5 to 8.5. The activity of this MIPP enzyme was remarkably inhibited by the presence of a monovalent cation, lithium, and even moderately so at a low concentration such as 1 mM. On the other hand, magnesium, a divalent cation, enhanced activity at least up to 10 mM. Calcium diminished MIPP activity at concentrations over 4 mM.