Synthesis of plastoquinone-9 and phytylplastoquinone from homogentisate in lettuce chloroplasts.

Chloroform-soluble extracts of unpurified chloroplast preparations of lettuce, pea and spinach and of class I lettuce chloroplasts that have been incubated in the light with [methylene-3H]homogentisate contain 3H-labelled plastoquinones-9 and -8 (minor homologue), 2-demethyplastoquinones-9 and -8 (minor homologue), pytylplastoquinone and 2-demethylphytylplastoquinone. The absence of demethylquinols, the presumed precursors of the dimethylquinones, from the extracts to the fact that no precautions were taken in the extraction procedure to present their oxidation to the corresponding quinones. In unpurified lettuce chloroplasts the synthesis of these compounds from [methylene-3H]homogentisate is Mg2+-dependent and it is stimulated by light. The addition of isopentenyl pyrophosphate to the incubation mixtures increases the amounts of both groups of quinones (polyprenyl quinones and phytyl quinones) synthesised in the light and the amounts of polyprenyl quinones synthesised in the dark. Replacement of isopentenyl pyrophosphate with a source of preformed polyprenyl pyrophosphates brings about a marked rise in the amounts of polyprenyl quinones synthesized. This rise in polyprenyl quinone synthesis is further increased if the chloroplasts are subjected to osmotic shock. The presence of S-adenosylmethionine increases the amounts of dimethylquinones synthesized at the expense of the demethylquinones. The implied precursor-product relationships between 2-demethylphytylplastoquinone (quinol?) and phytylplastoquinone and between the 2-demethylplastoquinones (quinols) and plastoquinones were verified in a pulse-labelling experiment. Confirmation that these quinones, or their corresponding quinols, are synthesized in the chloroplast is provided by the fact that they are made in class I lettuce chloroplasts. In none of the many incubations carried out in the course of the study were any [3H]tocopherols produced.