Regulation of Acetyl-Coenzyme A Carboxylase and Acetyl-Coenzyme A Synthetase in Spinach Chloroplasts

In analogy to chloroplast fatty acid synthesis from acetate the key enzymes of acetate fixation, acetyl-CoA synthetase and acetyl-CoA carboxylase, in rapidly Triton X-100 lysed spinach chloroplasts show an activation by light and deactivation in the dark. The stim ulation of acetyl-CoA carboxylase by dithiothreitol in darkened chloroplasts points to an involvement of reducing equivalents in the light activation of this enzyme. But more than by alterations of the activation state per se, these enzymes appear to be effected by changes in their catalytic activity due to differences in the proton-, Mg²⁺- and adenine nucleotide levels of the chloroplast stroma. Thus the pH dependence of both enzymes, as immediately extracted from Triton X-100 lysed chloroplasts, resembles that recently found for lipid incorporation of acetate into intact spinach chloroplasts in the light with an identical pH optim um of about pH 8.5 for the acetyl-CoA carboxylase. Moreover, in the same extracts both enzyme activities show the already postulated requirement for MgATP and free Mg and are com petitively inhibited by free ATP and ADP with respect to MgATP. But on account of the fact, that the extractable acetyl-CoA synthetase as opposed to the carboxylase activities exceed by far the lipid incorporation rates of acetate by illuminated chloroplasts before disruption, acetyl-CoA synthetase will be excluded as rate limiting step in fatty acid synthesis from acetate. From key enzymes of acetate fixation only the carboxylase appears to be involved therefore in the light regulation of acetate incorporation into long-chain fatty acids