The chloroplast envelope is permeable for maltose but not for maltodextrins

Permeation of [14 C]maltose into the stroma (measured as the sorbitol-impermeable space) of isolated intact spinach (Spinacia oleracea L.) chloroplasts was studied using the silicone oil centrifugation technique. Maltose uptake showed Michaelis Menten-kinetics with a K(m) of 25 mM and a Vmax of 19.5 mumol maltose. mg Chl-1. h-1 at 15 degrees C. Lack of interaction of glucose and maltose uptake suggested the presence of individual translocators for maltose and glucose in the inner chloroplast envelope. Maltose uptake was markedly inhibited by maltodextrins (maltotriose up to maltoheptaose). The corresponding [14C]maltodextrins were prepared by degradation of [14C]starch with pullulanase and alpha-amylase and purified by high performance TLC. None of these maltodextrins, when administered at a concentration of 10 mM, was transported into the sorbitol-impermeable space of the chloroplasts. The results suggest that the transport system for maltose is also accessible to maltodextrins but that only maltose can be translocated across the inner envelope of spinach chloroplasts.