Role of plastidial acyl-acyl carrier protein: Glycerol 3-phosphate acyltransferase and acyl-acyl carrier protein hydrolase in channelling the acyl flux through the prokaryotic and eukaryotic pathway.

In order to investigate whether the relative activities of the plastidial acyl-acyl carrier protein (ACP):glycerol 3-phosphate acyltransferase (EC 2.3.1.15) and acyl-ACP hydrolase play a role in controlling the acyl flux through the prokaryotic and eukaryotic pathway, we determined these enzymic activities in stroma fractions from 16:3- and 18:3-plants using glycerol 3-phosphate and labelled acyl-ACP as substrates. Several factors were examined which might influence the activities within plastids, such as leaf development, salts at physiological concentrations, stroma pH and substrates available to the enzymes. An appreciable alteration of the two enzymic activities was only observed with changes in the pH and substrate concentration, especially the concentration of glycerol 3-phosphate. An increase in pH from 7 to 8 resulted in a decreased ratio of acyltransferase versus hydrolase activity in stroma fractions from both pea (Pisum sativum L.) and spinach (Spinacia oleracea L.), whereas exogenously added glycerol 3-phosphate, which only influenced the acyltransferase, raised this ratio. On the other hand, the relative activities of the two enzymes stayed rather constant at oleoyl-ACP concentrations between 1 and 2 μM not only when it was offered alone but also in a mixture with palmitoyl-ACP. At pH 8, the stroma pH of illuminated chloroplasts, and at physiologically relevant substrate concentrations we observed clear differences between the 16:3-plants spinach and mustard (Sinapis alba ssp. alba L.) and the 18:3-plants pea and maize (Zea mays L.). In accordance with the different proportions of prokaryotic glycerolipids in the two groups of plants, pea and maize showed distinctly lower ratios of acyltransferase versus hydrolase activity than spinach and mustard. Consequently the relative activities of the plastidial glycerol 3-phosphate acyltransferase and acyl-ACP hydrolase can play a decisive role in controlling the acyl flux through the different pathways at least in these plants.