Choline oxidation by intact chloroplasts isolated directly from spinach leaves

Illuminated chloroplasts derived from spinach leaf protoplasts synthesize betaine from choline via the intermediate betaine aldehyde (BAL) (PNAS 82:3678). Photosynthetically active chloroplasts isolated directly from spinach leaves oxidized [14C]choline in the light at rates 10 times higher (25-80 nmol/mg chl b) than protoplast-derived chloroplasts. Up to 20% of the [14C]choline supplied during a 30 min incubation was oxidized in the light; the main product was [14C]BAL. Rates of [14C]choline oxidation in darkness were only 5-30% of rates in light. Light-dependent [14C]choline oxidation was abolished by DCMU and 5 mM DTT. Pre-illumination of the chloroplasts did not promote [14C]choline oxidation in darkness. The uncouplers nigericin and CCCP at concentrations which eliminated CO2-dependent O2 evolution did not affect [14C]choline oxidation in the light. They hypothesize that [14C]choline oxidation is not dependent upon light activation of an enzymatic system or upon the electrochemical proton gradient but requires an oxidant generated in the light