Species-Specific Differences in Acetyl Coenzyme A Synthesis of Chloroplasts

Acetyl-CoA and Fatty Acid Synthesis, Chloroplasts The present investigation indicates that photosynthetically active chloroplasts can synthesize acetyl-CoA either from acetate via acetyl-CoA synthetase (ACS) or from pyruvate via the pyru­ vate dehydrogenase complex (PDC). Both enzyme systems have been assayed in rapidly prepared extracts of chloroplasts isolated from spinach, peas and maize mesophyll. Their kinetic properties showed few species-specific differences. The differing pyruvate and acetate concentrations within the corresponding leaf tissues have been interpreted, therefore, as constituting a major factor determining the relative involvement of both acetyl-CoA synthesizing systems within the different types of chloroplasts. The idea that acetate originates from mitochondria and pyruvate from the cytosol has been supported by nonaqueous fractionation studies. Diffusion-mediated faster up­ take of acetate may indicate a predominant role of the ACS in spinach chloroplasts. Higher cellular pyruvate/acetate-ratios (2-5) in pea and maize leaves may enhance pyruvate uptake into chloroplasts and thus PDC-driven acetyl-CoA synthesis in pea and maize mesophyll chloroplasts. Maize mesophyll chloroplasts even show a light-driven pyruvate uptake accompanied by a stimulated acetyl-CoA and fatty acid formation. Assuming light-dependent increasing parameters in the stroma space, like Mg2+-concentrations, pH and ATP, as further control criteria in chloroplast acetyl-CoA formation, the ACS appears better adapted to the circumstances in illuminated chloroplasts because of the fact that 1. the ACS requires these cofactors altogether; 2. the PDC is stimulated by increasing pH (up to 8) and Mg-levels (up to 5 mᴍ) alone.