Functional characterization of leucine transport induced in Xenopus laevis oocytes injected with mRNA isolated from midguts of lepidopteran larvae (Philosamia cynthia)

The injection of poly(A)+ mRNA prepared from Philosamia cynthia midgut caused time- and dose-dependent increases of leucine transport in Xenopus laevis oocytes, with an increase in leucine uptake 1.5-3 times that of oocytes injected with water, When the NaCl concentration was reduced from 100 to 5 mmol l(-1), the difference between mRNA- and water-injected oocytes was greater and a fourfold increase of L-leucine uptake was measured, D-Leucine (10 mmol l(-1)) completely inhibited the induced uptake of 0.1 mmol l(-1) L-leucine, The newly expressed component of L-leucine uptake increased at alkaline pH and was abolished by incubation for 15 min with 15 mmol l(-1) phenylglyoxal, The mean K-m values, calculated using Na+ activation curves of leucine uptake, were 23.3+/-6.1 mmol l(-1) in water-injected oocytes and 0.4+/-0.2 mmol l(-1) for the newly expressed component of leucine uptake in mRNA-injected oocytes. On the basis of these results, we conclude that the increase of L-leucine uptake in mRNA-injected oocytes was due to the expression of a new transport system, which differs from the endogenous ones and shares many features with that found previously in Philosamia cynthia midgut.