A Mg 2+-dependent ecto-phosphatase activity on the external surface of Trypanosoma rangeli modulated by exogenous inorganic phosphate

Abstract

In this work, we characterized a Mg 2+-dependent ecto-phosphatase activity present in live Trypanosoma rangeli epimastigotes. This enzyme showed capacity to hydrolyze the artificial substrate for phosphatases, p-nitrophenylphosphate ( p-NPP). At saturating concentration of p-NPP, half-maximal p-NPP hydrolysis was obtained with 0.23 mM Mg 2+. Ca 2+ had no effect on the basal phosphatase activity, could not substitute Mg 2+ as an activator and in contrast inhibited the p-NPP hydrolysis stimulated by Mg 2+. The dependence on p-NPP concentration showed a normal Michaelis–Menten kinetics for this phosphatase activity with values of V max of 8.94 ± 0.36 nmol p-NP × h −1 × 10 −7 cells and apparent K m of 1.04 ± 0.16 mM p-NPP. Mg 2+-dependent ecto-phosphatase activity was stimulated by the alkaline pH range. Experiments using inhibitors, such as, sodium fluoride, sodium orthovanadate and ammonium molybdate, inhibited the Mg 2+-dependent ecto-phosphatase activity. Inorganic phosphate (Pi), a product of phosphatases, inhibited reversibly in 50% this activity. Okadaic acid and microcystin-LR, specific phosphoserine/threonine phosphatase inhibitors, inhibited significantly the Mg 2+-dependent ecto-phosphatase activity. In addition, this phosphatase activity was able to recognize as substrates only o-phosphoserine and o-phosphothreonine, while o-phosphotyrosine was not a good substrate for this phosphatase. Epimastigote forms of T. rangeli exhibit a typical growth curve, achieving the stationary phase around fifth or sixth day and the Mg 2+-dependent ecto-phosphatase activity decreased around 10-fold with the cell growth progression. Cells maintained at Pi-deprived medium (2 mM Pi) present Mg 2+-dependent ecto-phosphatase activity approximately threefold higher than that maintained at Pi-supplemented medium (50 mM Pi).