The inhibitory effect of 4,5-dioxovalerate on 5-aminolevulinate dehydratase and its implication in the regulation of light-dependent chlorophyll formation in pigment mutant C-2A′ of Scenedesmus obliquus

Abstract

Besides the known role of 4,5-dioxovalerate as an intermediate in the C 5 -pathway [1], a novel regulatory function in light-dependent chlorophyll biosynthesis is described. Considerable amounts of protochlorophyllide accumulate in dark grown cultures of the yellow mutant C-2A′ of Scenedesmus [2]. This accumulation is almost completely blocked in darkness by the addition of 4,5-dioxovalerate in vivo. Likewise, light-dependent chlorophyll biosynthesis is strongly inhibited by the addition of this compound during greening. The considerable increase of protochlorophyllide formation in darkness upon the addition of 5-aminolevulinate (Kotzabasis, K. and Senger, H. (1989) Z. Naturforsch., in press) is also drastically reduced by external 4,5-dioxovalerate. It is shown by in vitro experiments that concentrations of dioxovalerate, above the physiologically relevant level, inhibit 5-aminolevulinate dehydratase. The K i -value was determined to be 60±5 μM. From these results it is concluded that besides the predominant control of gli t-RNA-ligase by protochlorophyllide [4] a second regulatory mechanism is involved in chlorophyll biosynthesis. Under excessive concentrations of 5-aminolevulinate and 4,5-dioxovalerate further prophobilinogen and subsequently chlorophyll biosynthesis is inhibited.