UPTAKE OF ISOPENTENYL DIPHOSPHATE BY PLASTIDS ISOLATED FROM VITIS-VINIFERA L CELL-SUSPENSIONS

The uptake of [1-C-14]isopentenyl diphosphate by intact plastids purif ied from cell suspensions of Vitis vinifera L. cv. Muscat de Frontigna n was investigated using vacuum-filtration and silicone-oil-filtering techniques. Transport across the plastid envelope which was stimulated by cations, such as Mg2+ and Mn2+, was characterized by a K(m) of app rox. 0.5 mM and a V(max) of 25 nmol.(mg protein)-1.h-1. The data showe d that isopentenyl diphosphate apparently accumulated in the plastid a gainst a concentration gradient. The involvement of a protein carrier was suggested by the strong inhibition of the uptake by compounds whic h are known to block SH groups. Thus, the saturation kinetics together with the pH optimum (7.5-8), the temperature dependence (maximum inco rporation at 37-degrees-C) and the competitive inhibition by a structu ral analogue of the substrate (aminophenylethyl diphosphate) provided evidence for a mechanism of uptake by facilitated diffusion. The carri er identified may thus play a major role in supplying the plastid comp artment with isopentenyl diphosphate for isoprenoid biosynthesis.